Testing Precious Metals C.M Hoke

8/12/2019 Testing Precious Metals C.M Hoke

1/89

"Refining Precious Metals Wastes by CM. Hoke"Made available through the many generous donations of our members.

Come Join Us At http://goldrefiningforum.com/phpBB2/index.php

"Not for Re s a l e"

CM. Hoke has written this book in non technical terms with easy to followinstructions.

Notice Concerning Safety, First Aid, and Pollution ControlDue to the enormous changes in these fields over the past fifty years, you should not rely on the

information presented in this book but should consult current sources on these subjects

G o ld R e f i n i n g C o m m u n i t yGold Re f iners helping one a no the r!


2/89

TESTING PRECIOUS METALS

Gold, Silver, Platinum Metals

Identifying - Buying - Selling

A Handbook for the Jeweler, Dentist,

Antiquarian, Layman

By G. M. HOKE

Author ofRefining Precious Metal Wastes

THIRD EDITION

T H E JEWELERS' TECHNICAL ADVICE COMPANY

New York, N. Y.


3/89

COPYRIGHT 1946 BY C. M. HOKE

All rights reserved. This book, or parts thereof, may not be reproduced

in any form without permission of the publishers.

The First Edition was originally published inThe Brass WorldPlatingPolishingFinishing

of New York

Parts of the Third Edition were published in

The Jewelers* Circular-Keystone

of New York

PRINTED IN U. S. A.


4/89

THE JEWELERS TECHNICAL ADVICE COMPANY was founded in

1912, when platinum was first coming into use as a jewelry metal.Its manager, Sam W. Hoke, was a pioneer in the technology of

platinum. He patented a series of oxygen-gas torches, used formelting and welding platinum, soldering gold and platinum

jewelry, melting quartz glass, etc.In 1912 the melting of platinum was possible in only a few

plants in the whole world; today it is a commonplace in even thesmaller jewelry factories.

C. M. Hoke, the writer of this book, has a background of uni-versity training in chemistry and biology, as well as experience inthe teaching of chemistry. For years she has devoted her time toinstructing jewelers and others in refining, melting, salvaging andfinishing the precious metals, and in developing equipment forthe control of compressed gases.


5/89

Foreword

The precious metals are always interestingeven the prosaictasks of testing, refining, working, and selling them command aperennial interest. For many years the writer has been concernedwith these matters, and the following pages are designed to answerone group of questions that has arisen time and time again.

If you are handling precious metal articles, you will often needto distinguish, for example, between a piece of 18-karat and a

piece of 14-karat gold, or to decide whether a given article is whitegold or platinum. If you are buying or selling old jewelry or den-tal golds, you will often wish to know the approximate value of apiece, without taking time for an assay, and possibly without in-

juring the article itself.The purpose of this book is to describe methods, particularly

the touchstone method and its variations, that will give this in-formation.

The idea of testing gold with a touchstone is very old. We aretold that the Lydians used it in 500 B.C., rubbing the metal againsta smooth stone, then comparing the streak with similar streaksmade by metals of known composition. When during the MiddleAges men learned to make strong acids, the method became moreexact. During the last few years, with the introduction of themany new metals and alloys and combinations that now character-ize the precious metal industries, the method has been greatly am-plified. In the hands of a careful worker it yields quickly a largeamount of useful information.

These chapters will first describe the touchstone method as used

on the ordinary yellow gold alloys that have been in vogue sincethe days of our grandfathers. Then we shall cover silver, and thewhite golds and platinum alloys that came into use at about thetime of World War I.

Some other methods of identification that do not employ thetouchstone will also be described, and full attention will be givento those new alloys and combinations, including the ruthenium al-loys, which appeared during World War II.


6/89

The book does not discuss assaying (which is the chemical an-alysis of a small weighed sample) and it gives only a few paragraphsto the problems of the prospector. Its purpose is to assist the

jeweler, antiquarian, metal buyer and layman to identify preciousmetal articles and estimate their value. A knowledge of chemistryis not required.

One chapter is devoted to quality stamps and karat marks. An-other contains advice not only for the layman with some jewelryto sell, but also for the jeweler or refiner who might buy it. Itoutlines the evolution of the gold-buying industry, calls attentionto the laws under which it operates, and suggests the problems,

profits, and responsibilities that accompany it.C. M. HOKE.

Palisade, New Jersey, 1945.


7/89

Contents

Foreword 7

Chapter I. The Old Touchstone Method and the Yellow

Golds 11

Chapter II. Silver and Some Other White Metals 18

Chapter III. The Quality Stamp"Let the Buyer Beware!" 29

Chapter IV. The Platinum-Group Metals and the White

Golds 39

A. New Metals; Old Tests

B. New Metals; New Tests

C. Some Other Tests

Chapter V. Buying and Selling Old Precious Metals 63

Chapter VI. Some Paragraphs for the Prospector 78

Appendix 83

A. A List of Equipment.

B. When Handling Strong Acids.

C. How to Determine Specific Gravity.D. A Table of Metals, their Melting Points and Specific

Gravities, and their Responses to Acids and to the

Oxy-Gas Flame.

E. Some Definitions and Formulas.

Index 91


8/89


9/89

CHAPTER I

The Old Touchstone Method and the YellowGolds

THE equipment for testing ordinary golds is shown in thefrontispiece, though most workers will add one or two moreacid bottles. There is the smooth, flat stone, of slate or fine-grainedbasalt; there are the so-called needlespointed bits of yellow goldof various finenesses, each marked with its quality; a small triangu-

lar file; and the acid bottles. The gold buyer will need in additiona scale and a set of weights.

Our first task is to make sure that the article to be tested reallyis gold. We then determine its quality by rubbing it upon thestone so as to make a mark or streak, and comparing this streakwith streaks made by the standard needles. This in one paragraphis the whole story of the touchstone tests.

Let us first acquaint ourselves with the ordinary, old-fashionedgold alloys of a golden color, postponing our examination of whitegolds, silver and the platinum metals until later. For your firststep, obtain several articles of different types, but of whose qual-ity you are sure; for example, a genuine gold coin or a piece or twoof high-grade jewelry made and stamped by a reputable manufac-turer; some moderately-priced articles; a handful of very cheapnovelty jewelry that is finished to look like gold; and finally, forcomparison, a piece of clean brass. The more articles, and thelarger their variety, the more quickly you will learn to identify andappraise the "unknown" articles that will come to you.

THE FILE

The experienced gold-buyer always begins by filing a deepnotch in the article, in order to penetrate any outer layers, and hemay learn immediately that the gold is only skin deep. Medium-priced jewelryrolled-gold or gold-filled goodsconsists of a coreof inexpensive metal to which an outer layer of karat gold has


10/89

been affixed. The core is usually brass; occasionally it is a gold al-loy of lower karat; and during World War II use was made of asterling silver core, instead of brass, because wartime regulations

forbade the use of brass for jewelry manufacture. Low-pricednovelties are apt to be electroplated and their surface film of pre-cious metal is very thin indeed.

Most high-grade articles are flash-finished with a light electro-deposit of pure gold, and when new may be further protected bylacquer. While these latter films can be removed by a few strokesof the file, the heavier coatings, such as those found in gold-filledor rolled-gold goods, are pierced only by a deeply-filed notch. Ac-

cordingly, as we said, the experienced buyer always begins byfiling a deep notch.

NITRIC ACID

The first acid bottle contains chemically pure (C.P.) nitric acid,full strength, which can be bought from a drugstore or supplyhouse. This acid attacks the majority of metals, and will destroyskin, clothing, woodwork, and so on, and therefore must be han-dled with care. If you should get acid on your skin or clothing, im-

mediately wash it off with much waterhold your hand under thefaucet and let the water run on it freelyand very little harm willbe done. If no running water is nearby, provide a basin of waterfor immediate use if needed.

Note that the glass stopper of the acid bottle is extended into along tongue. With this tongue, apply a small drop of nitric acidto each of your metal articles, on a clean surface or in a freshly-cutnotch, and watch the results, noting the color changes, if any. Aftera half minute, rinse the acid off with plenty of water, dry, and seeif the metal has been attacked.

Brass or copper boils up instantly and the acid turns green. Goldof 6-karat#or lower will be attacked almost as promptly, and willshow a green color, due to the copper with which it is alloyed; 10-kwill darken; ordinary gold of 12-k or better will show little or noreaction.

* The term karat means a twenty-fourth part, and expresses the proportionof gold in an alloy. Thus pure gold is 24-k; 6-k gold is 6/24ths (ori/4)gold, the

remaining i8/24ths being some other metal or metals. Pure gold is also de-scribed as "fine" gold, or as being "1000 fine," and 6-k gold is sometimes spokenof as "250 fine."


11/89

In general, any metal of the yellow color of gold that will standthis nitric acid test, may be assumed to be gold or a gold alloy.Note that we sayof yellow color,for there are several white metals,

such as platinum and stainless steel, that resist nitric acid.If possible, get a friend to hand you some unstamped articles ofwhose quality he is sure; examine these "unknowns" and reportto him regarding their character, repeating the tests until you havelearned how the various metals and alloys respond to the acid test.

DETERMINING THE KARAT

When the acid test has convinced you that an unknown is indeeda gold alloy, your next step is to determine its karat, using the

standard needles and the touchstone. Rub first one needle andthen another upon the stone, thus making a series of streaks uponthe smooth surface. Each streak is a thin layer of metallic mole-culesmolecules of gold and molecules of base metals. Now withthe stopper of your acid bottle, draw a little nitric acid across eachstreak.

As you would expect, the base metal molecules, thus exposed tothe acid, will dissolve promptly, while gold molecules remain un-changed; hence, streaks made by the lower karat needles will al-

most disappear, but those of higher quality will show little or noresponse.

Returning then to your unknown, rub it hard against the stone,making a streak. Suppose you suspect that it is about 10-k quality,maybe less. Beside the first streak make two others, one with the10-k needle, another with the 8-k needle. With the stopper ofyour acid bottle, draw nitric acid across the three streaks of metal.


12/89

Watch the way in which the acid works. As soon as you find astandard streak whose response is the same as that of your un-

known, then you have found the approximate fineness of the un-known. By "response" we mean the speed and completeness withwhich the streak is attacked.

But suppose your unknown is of such high quality that it is notaffected by plain nitric acid. We must now turn to the secondbottle, which contains chemically pure hydrochloric acid, and weshall make up some aqua regia.

AQUA REGIA

Aqua regia is a mixture of nitric acid and hydrochloric acid.The name means royal water, and was used by the ancients be-cause the mixture dissolves gold, the noble metal. Practically thesame results are obtained by adding a little table salt to diluted ni-tric acid. When aqua regia is first made up, chlorine is evolved, anoxious gas which attacks metals and should not be permitted toreach machinery, balances, and so on. Nor should the mixturebe kept in a stoppered bottle, for the evolving gas might break thecontainer.

Because of this (and also because it spoils on standing) aquaregia should be made up only as needed. Mix it the same wayevery time; the exact proportions are not important, but get ac-customed to a certain mixture and continue to use it.

It is possible to mix the two acids right on the stone, after mak-ing the streaks; that is, draw a little nitric acid across the streaks,then add a little hydrochloric acid, letting the two acids run to-gether. This is not good practice, however, because each stopperbecomes contaminated with the other acid, creating confusion.

A better plan is this: With a medicine dropper measure out tendrops of nitric acid into a tiny bottle; add ten drops of water, pre-ferably distilled; then using a clean dropper add two drops of hy-drochloric acid. This gives you enough aqua regia for about adozen tests. Wash your medicine droppers after every usage.

Returning then to the streak that was not affected by plain nitric

acid, wash and dry the stone, and apply aqua regia with a smallglass rod or a clean medicine dropper. Even fine gold is attackedby aqua regia. By comparing the response with first one standard


13/89

needle, then another, you can determine the quality of your un-known.

This test differs slightly in principle from the nitric acid test, inthat aqua regia dissolves the gold molecules as well as those of most

base metals.Some workers make up their aqua regia with even more water

than above, because the reactions proceed more slowly with thedilute mixture and therefore are easier to compare. Some workersuse a different proportion of hydrochloric acid.

Note the color changes. Fine gold when dissolved gives a yellowcolor, but this is usually masked by the green color of the copperthat is almost always present in gold alloys. Nickel, used in mostwhite golds, also gives a green color. Silver when treated with

aqua regia, forms a cheesy white substance on the stone which maywell confuse a beginner. Because of the influence of the alloyingelements, it is well, if possible, to use yellow gold standard needleswhen testing yellow gold unknowns, green gold needles with greengold unknowns, and so on.

GREEN GOLDS

Green gold alloys, especially those of high quality, contain con-siderable silver and little or no copper. The response of silver to

aqua regia is peculiar, as we shall find in a subsequent chapter.Green golds respond more slowly to aqua regia than yellow goldsof the same karat, and may lead you to think that they are more

Standard needles for testing the quality ofwhite golds and green golds.


14/89

valuable than they are. Therefore, we repeat, when testing greengolds, use standard needles made with green gold points.

RED GOLDS

These alloys contain more copper and less silver than the yel-low golds of the same karat, and respond slightly more rapidly toaqua regia.

WHITE GOLDS

A white gold is an alloy that contains enough of some whitemetal to destroy the yellow color. There are two whiteners in gen-

eral usenickel and palladium.Most inexpensive white golds consist primarily of gold andnickel, to which copper and zinc may be added, sometimes othermetals. When testing them, use the same procedure as with yel-low golds, but it is wise to use standard needles made with pointsof white gold.

Many better quality white golds consist of the same elements,gold, nickel, and small amounts of other base metals. However,

many white golds of especially fine quality, including many dentalalloys, are whitened withpalladium.Now there is considerable difference in the value of gold-nickel

and gold-palladium alloys, assuming that the proportion of goldis the same. Hence your concern, after deciding that a given ar-ticle is white gold, is to learn what kind of white gold it isnickel-gold or palladium-gold. This takes us to a later chapter of thisstory, in which we cover palladium and nickel.

DENTAL ALLOYS

There are dozens of dental alloys in use, ranging in value fromiridio-platinum pins, through the wrought and casting golds andhigh-karat solders, down to the amalgams, base metal "technic"alloys, and occasional pieces of stainless steel and aluminum thatmay find employment in dental work.

Some dental fillings are almost pure gold. If a piece of yellow

metal has been in use in the mouth for some time and still presentsa tarnish-free surface, it probably is gold of good quality, andshould respond to the acid and touchstone tests in much the samemanner as the jewelry alloys. Do not be deceived by the word


15/89

solderas used in dentistry; it may refer to a gold alloy of high value,used to join together the parts of a denture.

The tendency today is away from the conspicuous yellow goldsand toward the white alloyswhite golds and alloys containing

platinum-group metalswhich will be discussed fully in a laterchapter.

Dental golds do not carry the quality stamps that are commonlyfound on jewelry, hence gold buyers who distrust their own abilityto appraise metals often refuse to quote on dental alloys. For thatreason the buyer who can appraise properly, will find excellent op-portunities in this field.

Clean the stone frequently to remove all marks, perhaps by rub-bing it with fine pumice, or by covering the spots with a little aquaregia. Wash it free of acids before putting it away, or the traces oftoday's tests may confuse you tomorrow.

A list of the equipment used in these tests will be found in theAppendix.

The Appendix also contains a table of metals with their meltingpoints and specific gravities, as well as their responses to nitricacid, to hydrochloric acid, and to the oxy-gas flame.


16/89

CHAPTER II

Silver and Some Other White Metals

T ^ H IS is the group that offers the greatest challenge to the pre-J- cious metal buyer, and can offer him the greatest profit. It

includes on one hand the silver alloys, the white golds and theplatinum metals, and on the other hand a vast array of alloys likestainless steel, which, though handsome and useful, are not pre-cious metals. The purchaser wants to be able to separate out,from a trayful of white metal articles, precisely those that are valu-able to him, and to do it with speed and assurance. These chapterswill present him with tests that should give him this assurance.

There are about seventy metals known to science (the numberis uncertain because some are on the borderline between metaland non-metal), and of this number all except gold and copper are

described as white. When the student contemplates all the possi-bilities of composition and value that are presented by the words"a white metal" he must realize that the task calls for care, knowl-edge, and patience.

Fortunately for our purpose, most of the seventy-odd whitemetals are quite unsuited to jewelry making. Thus mercury isliquid at ordinary temperatures; tin is much too soft; potassiumreacts violently with plain cold water; radium gives off rays that

destroy the flesh; and so on.Our attention therefore will be placed primarily upon thosewhite metals that are precious, and upon those that are associatedwith them, or are apt to be confused with them.

THE MAGNET AS A DETECTIVE

Gold buyers often use a magnet to locate such things as steelsprings in bracelets. If a piece of metal is strongly attracted to amagnet it is probably iron or steel. However, certain nickel andcobalt alloys and some kinds of white gold also respond to the mag-net, which therefore should not be relied upon too implicitly, es-


17/89

pecially as some of the stainless steels are attracted only feebly ornot at all.

Other metals besides iron respond tothe magnet; some stainless steels do not.

THE FLAME TEST

If you can turn the flame of an air-gas or oxygen-gas blowpipeon a piece of suspected metal, you can, within a few seconds, ob-tain an excellent idea of its nature. Nickel, chromium, brass, andmost other base metals promptly turn black. Most base metalswill melt, forming oxides of characteristic color and form. White

gold alloys will melt promptly in the oxy-gas flame; more slowly inair-gas. Or, if the flame is removed before actual melting occurs, adefinite darkening is visible. This is also true of sterling silver.

Fine silver when molten absorbs oxygen, and on cooling expellsit with violent spitting and "crabbing". This tendency is less con-spicuous with sterling and coin silver.

Stainless steel soon shows a darkening; if heated further it willignite and burn with a hissing and sparkling flame; the final resultwill be a shapeless lump of black oxides.

Tungsten, tantalum, and molybdenum change color at low tem-peratures, and soon begin to burn in the oxy-gas flame, though theywill not become actually molten.

The response of platinum and its high grade alloys to a flameis highly characteristic. (By high grade alloys we mean iridio-platinum, or others in which only precious metals are present.)Suppose you bring the metal to a brilliant red heat, then removethe flame. There will be no darkening whatever. Heat it still fur-ther using an oxy-gas flame, and melt it; it melts smoothly andcleanly, without forming any oxide or crust. When the button

cools, it will be white and smooth. Base metals, treated in thatway, become a mass of clinkered oxides.

Palladium and alloys rich in palladium show colored oxides atabout 4000 C, but when heated further these disappear, and ifthe metal is cooled quickly they will not have time to form againand the cooled button will be free from tarnish.


18/89

Molten palladium absorbs very large volumes of gases, and ifthe flame is removed suddenly the gases are expelled violently. Thebutton that remains will be distorted and honeycombed withbubbles.

Low grade platinum alloys when heated strongly will darken,in proportion to the base metal present.

Fine gold, heated to redness, will cool without changing color.But if even small amounts of base metal are present, the surfaceafter cooling will show a film of oxide.

The oxy-gas flame, if properly handled, is thus one of the mostilluminating of all quick tests, and the air-gas flame is almost as

useful. This test will be discussed again in Chapter IV, and achart showing the responses of several metals to the oxygen flame isgiven in the Appendix.

SPECIFIC GRAVITY

In general the precious metals are heavier than base metals, andthe experienced worker can obtain a hint as to the value of anarticle merely by "hefting" it in his hand. This ratio between the

bulk and the weight of a substance, called its density or specificgravity, is often helpful in identification. The student soon ob-serves that platinum and its high grade alloys are somewhatheavier than the white golds; while steel, nickel, silver, and mostof the base metals are so much lighter than platinum that thereis small excuse for a mistake.

Tungsten and tantalum are two base metals of very high specificgravity, comparable with that of platinum. However their leaden

color, and the fact that they ignite under the oxy-gas flame andform colored oxides, reduce the chances of confusion.This method of identification, which has both its advantages

and its limitations, will be discussed again in Section C of ChapterIV, and in the Appendix.

How SILVER REACTS

As in Chapter I, the first step is to provide yourself with several

articles of whose composition you are sure, then apply to them thevarious tests, in turn, and observe the results. Obtain a piece ofgood quality sterling silver, something made recently and stamped


19/89

by a reputable manufacturer; a silver coin; perhaps a bit of somelower-grade silver alloy; and some stuff that you know to be silver-plated.

First remove any surface coat, such as lacquer, and to each ar-ticle apply a drop of nitric acid. Let it remain for thirty secondsor so, then rinse it off and see if the surface of the metal were at-tacked. You will find that nitric acid reacts with silver, even thehighest grade, turning dark and making a gray spot on the metal.

Fine silver, when dissolved in nitric acid, gives a colorless solu-tion that darkens after exposure to light. Sterling silver and coinsilver show some green color, the green being due to the copperwith which they are alloyed.

To confirm silver, place a fresh drop of nitric acid on a cleansurface, let it react for a half minute, then with the point of a pen-knife drop in a single small grain of table salt. A white substancewill appearsilver chlorideof a cheesy consistency. This is char-acteristic of silver. Instead of the grain of salt you could use a tinydrop of dilute hydrochloric acid.

In effect this is almost the same as applying a drop of aqua regia.

We think of aqua regia as a powerful solvent since it will dissolvegold. However, it is surprisingly slow to attack silver. Make thetest and see; note that the nitric acid in it will eat into the surface alittle, but very soon the white cheesy stuff forms, and protects themetal from further attack. Wash the metal and you will find thata whitish spot remains, difficult to remove.

This will help to explain why green gold, which contains muchsilver, responds more slowly to aqua regia than does yellow gold


20/89

of the same karat. Some of the high-karat green golds are almostinsoluble, even in hot aqua regia.

Here is another way to establish silver: In another bottle mixup nitric acid and a few crystals of potassium dichromate. Placea drop of this solution on the suspected article (after getting ridof lacquer, etc.) and note the color effect. Silver will show a verystrong, definite red, through the formation of silver dichromate.

SlLVERPLATED GOODS

The recognition of silverplated ware is usually easy. File a deep

notch and apply nitric acid to the cut, and note the difference inappearance and behavior of the silver surface and the base-metaltains no silver at all. Sometimes a brass core is found. Both ofthese core materials react quickly to nitric acid, causing it to bub-ble and turn a deep green.core. The favorite core material is a copper-nickel-zinc alloywhich is called "nickel silver" or "German silver/' but which con-

Ordinary silverplated ware is of such small value that refiners

and gold buyers normally refuse to buy it, so it is important to beable to recognize it every time.

GOLD-ON-STERLING

In Chapter I we mentioned "gold" jewelry which was found toconsist of asterling silver core to which a thin surface layer ofgold has been applied. The wearer may think of such jewelry asgold, but to the buyer it is silver. At present silver prices it is not to

Thin sheets of karat gold are welded to one or more surfaces of a thickbillet of less expensive metalusually brass or a nickel alloy, sometimessterling silverand the whole is then rolled very thin. The resulting thin

sheet is used in makinggold filledorrolled gold platejewelry. In order tomeet U. S. standards the gold layer must be of at least 10-k quality. If theweight of the karat gold is more than 1/20 of the total weight, the jewelrymay be stamped "gold filled" The term "rolled gold plate" is used when

the gold layer is thinner.


21/89

be ignored, and sometimes there is enough gold present to add alittle to the buyer's price.

This combination is normally recognized in the preliminarytests involving nitric acid and a deeply-filed notch. Fresh, well-made goods present a handsome gold-like appearance, but if thegold film is thin, the silver soon tarnishes underneath the gold.

This combination attained special vogue during World War IIat the time when silver and fine gold were available, while copperand nickelthe metals so generally used in the cores of inexpensive

jewelrywere subject to wartime restrictions. The fact that silverresponds only feebly to aqua regia has led some careless buyers tomisjudge the value of this combination, to their loss. Fortunatelyfor them, it is apt to be clearly stamped "Sterling." Sometimes

the outer layer of gold is of sufficient thickness and quality to classthe goods as "rolled gold" or "gold filled"; but these goods aregenerally stamped with a quality mark; e.g., "Rolled Gold Plate onSterling" or "Sterling -f- 1/20 12K."

THE VARIOUS SILVER ALLOYS

Pure unalloyed silver, called "fine silver," is so soft that it hasfew practical applications. The most important alloy issterlingsilver, which contains 925/ioooths fine silver, the remainder us-

ually being copper. The word "sterling" goes back to the twelfthcentury; it seems that five towns in eastern Germany were bandedtogether in the so-called Hanseatic League; they were free citiesand maintained their own currency. The British soon learnedthat their coins, called the coins of the Easterlings, were depend-able; hence the term "sterling" as a stamp of quality.

Coins of the United States of America are 900/ioooths silver, theremainder being copper. Much jewelry and tableware used to bemade of this alloy, and may be stamped "Coin" or "Coin silver."

Alloys of lower silver content are often encountered, not onlyin the coinage of several foreign countries, but also in articles ofcommerce, but the designation "silver" cannot now be used legallyin connection with them in the United States. Imported goodsmay be encountered, stamped "Silver," which on assay may proveto be of very poor quality indeed.

It is sometimes desirable to make simple tests to distinguish be-


22/89

tween sterling, coin and lower grade silver alloys, but most ob-servers have found that it cannot be done with the same assurance

and satisfaction as with the gold alloys. Sterling silver and coinsilver differ by only 25 parts per 1000, or2]/2percent, and when werecall that both the silver and the copper are soluble in nitric acid,we can see why the acid test has its limitations. However, by hav-ing both surfaces clean and smooth, and by applying equalamounts of acid to both surfaces and exposing both together tothe light for the same length of time, and noting the color changes,the difference is detectable.

Silver of lower grade, if alloyed with copper, can easily be dis-tinguished from sterling silver in the same way. Unfortunately,these lower grade silvers may contain a variety of alloying elementsmore or less nickel, or zinc, or cadmiumto fit them for differentpurposes, and unless you have some knowledge of the alloying ele-ments, these simple spot tests can be misleading, and experiencedworkers advise against their use.

SILVER SOLDERSSilver solders, also called silver brazing alloys, are of many

formulas, containing from five percent to about eighty percentsilver, the balance being copper and zinc and perhaps some cad-mium. Large amounts of these alloys have been consumed inrecent years, in dozens of applications, not only in the manufac-ture of jewelry, but also in such jobs as the assembling of incendi-ary bombs, the repair of ice-cream freezers, the construction of

equipment for the chemical industries, and many others. Whilenot of high intrinsic value, these silver brazing alloys should notbe ignored by the metal buyer, especially as they may often befound in large quantities.

SOME WHITE BASE METALS

Many base metals are attacked and dissolved by nitric acid, butby no means all. Obtain scraps of various metals, such as lead,

tin, pewter, Britannia metal, tungsten, stainless steel, aluminum,chromium-plated and nickel-plated ware, and so on. Clean themwell to remove any surface grease or lacquer, then touch each witha drop of nitric acid. Let it act for a half-minute or so, while you


23/89

observe any color changes, then wash well and notice if the surfacewere etched or spotted. To describe all the effects fully wouldtake more space than is here available; moreover, a few minutesspent at such tests will teach you more than hundreds of words. Sowe repeat: practice with pieces of metal of whose nature you aresure, exposing them to various tests and comparing results.

For instance, we suggest that you try the dichromate mixturementioned above, on other white metals beside silver. Lead showsa yellow color; Britannia metal turns dark. Platinum is not affectedin the least, nor is high-grade white gold, but palladium will bedarkened and will show a spot.

NICKEL-SILVER

The terms "nickel-silver" and "German silver" are applied toan important series of white alloys in which copper, nickel andzinc are the principal components. Both terms are highly mislead-ing, since no silver at all is present, and many other names havebeen suggested, including "nickel-brass" and synthetic words like"Cunizin" and "Nicuzin," none of which have received generalfavor.

Tableware and hollow-ware made of nickel-silver and electro-plated with silver, nickel, or chromium, have been made in enor-

mous quantities. Rolled and filled gold jewelry, especially thatwhose outer layer is white gold, is generally made on a nickel-silverbase; and there are dozens of other applications.

Accordingly it is important that the gold buyer be able to recog-nize these nickel-silver alloys wherever found. They are attackedvigorously by nitric acid, showing a strong green color because ofthe copper and nickel content. When heated strongly theydarken; under the oxygen flame they ignite and burn to a blackclinker, meanwhile conferring a green color to the flame.

STAINLESS STEEL

This handsome but inexpensive alloy, stainless steel, has hadquite a vogue for sports jewelry, men's belt buckles, wrist watches,etc. In appearance it resembles white gold or platinum. Oddlyenough it is not readily attacked by nitric acid nor by aqua regia,


24/89

and for that reason it has occasionally deceived unwary appraisers.As we have indicated, some kinds are attracted to the magnet, some

are not.It is considerably lighter in weight than either white gold orplatinum, and most jewelers will at once notice this lack of "heft".As we mentioned above, it darkens under the oxy-gas flame, thenignites and burns to a dark clinker.

But if you are in doubt about any article of white color andnoticeable hardness, which resists the action of nitric acid and ofaqua regia, pause a moment and then test it with plain hydro-chloric acid.

If possible, heat either the article or the acid somewhat; hydro-chloric acid attacks the stainless steels promptly, making a definitespot or dissolving the streak in a short time. Sulphuric acid also at-tacks stainless steel; so does a solution of ferric chloride. None ofthese affects white gold or platinum.

RESISTANCE ALLOYS

There are dozens of more-or-less white alloys on the market,which though not stainless steels by definition (since they containlittle or no iron) are often confused with them. We refer to thoseheat- and corrosion-resisting alloys of which Stellite, Nichromeand Illium are only three examples of a long list. Chromium,cobalt, nickel, tungsten, silicon, manganese and other elementsmay be present, and the number of formulas is legion. Occasion-ally such alloys present an appearance that might confuse themetal buyer, and many of them resist nitric acid surprisingly well.But mostly they are lighter in weight than platinum or white gold,and their crystalline structure and their "feel" under the file givesufficient warning. Their melting points are high, but under thestrong heat of the oxy-gas flame they will ignite and burn, after themanner of other base metals.

SOME LESS COMMON METALS

Tungsten, tantalum and molybdenum are three of the semi-raremetals that have found growing commercial importance during


25/89

recent years. In color they are somewhat dark, and are tough,heavy, strong and hard. Their carbides are extremely hard, andare compacted and sintered into points or blades for drills, cutting

tools and the like, for which purpose they rival the diamond.These metals all possess remarkable resistance to nitric acid,

aqua regia, and most other reagents, and accordingly have sometimes been confused with the platinum metals. But above redheat they all oxidize readily, and under the oxy-gas flame they ig-nite and burn to form colored oxides.

CONTACT POINTS

Electrical contact points must have high heat and electricalconductivity, hardness, strength, and resistance to corrosion atthe high temperatures of the electric arc. Many metals and alloysare being used in their manufacturesilver, copper, platinum-group metals, tungsten, tungsten carbide, cobalt, and others. Some-times a point consists of two alloys welded together, the combina-tion then being brazed or welded to the device of which it is apart, and many of the alloys involved are quite complex.

Old contact points can be quite a problem to the metal buyer.

Knowing that much platinum and iridium go into this market, heis tempted to buy the things, even after experience has taught himthat he is more apt to lose than to profit when handling them.The task of appraising them and recovering the precious metals,if any, is difficult, and many professional refiners refuse to buythem. Accordingly the beginner is advised to approach thismarket with caution.

CHROMIUM PLATE

Chromium is a hard white metal, unusually resistant to mostcorrosive agents. Chromium plate, when properly applied, is ahandsome finish and sometimes is used on cheap white gold

jewelry, as well as on many base metal articles.It resists nitric acid, and therefore is sometimes mistaken for

white gold or platinum. However, it is attacked readily by hydro-chloric acid, and by sulphuric acid. When heated under the air-gas or oxy-gas flame, it blackens promptly.


26/89

RHODIUM PLATE

Rhodium plate is also deceptive. Rhodium is a metal closely

related to platinum, costing more per ounce than platinum itself.It can be deposited electrolytically in a very thin layer, on silver orbase-metal articles, to give them a handsome appearance, free fromtarnish. Rhodium is not attacked by nitric acid, aqua regia, norany other single acid. It is fairly hard to the filealmost as hard aschromium plate. But the deposit is always so thin that a fewstrokes of the file will expose the metal below. For that reason itshould not cause any great confusion to the buyer of preciousmetals.

WHITE GOLDS

The tests described so far, when applied to most white golds, willbe suggestive, but not always conclusive. You may still be uncer-tain as to whether the unknown is white gold or a platinum metalalloy of some kind. We shall therefore return to the white goldsin a later chapter, with conclusive tests.

THE PLATINUM METALSThese metals and their alloys are so important, not only in

jewelry and dentistry but in many other applications, that an en-tire chapter will be given over to them.

For the moment we shall content ourselves with rememberingthe facts we have recently noted: that platinum is not attacked bynitric acid nor by hydrochloric acid; that its melting point is veryhigh; that it melts cleanly under the oxy-gas flame and coolsagain without the formation of visible oxide; and that it is notice-ably heavier than most other white metals.

Palladium is the one metal of the platinum group that is at-tacked by nitric acid. It dissolves promptly togivea deep brownsolution. It is much lighter in weight than platinum. Palladiumelectroplate is sometimes used to give a handsome non-tarnishingfinish to jewelry or scientific instruments. Alloys in which goldand palladium are the main constituents are important in dentis-try, also they form one kind of white gold. All of these, and others,

will be discussed more fully in Chapter IV.


27/89

CHAPTER III

The Quality Stamp"Let the Buyer Beware!"

Since gold and silver have been used in coinage for many cen-turies, it is natural that their stamping or marking should be reg-ulated by law. Such laws are not only a protection to the pur-chaser, but are of equal value to the manufacturers because theysustain public confidence in the industry. Regulations for theplatinum metals are of more recent date.

Everyone who handles precious metal articles should under-stand these laws thoroughly. Thus the manufacturer must keephis alloys high enough to meet the law, but not so unduly high asto jeopardize his profits. The retail jeweler, who is equally liablebefore the law, does well to check the goods he sells and to giveattention to the reputation of the manufacturers from whom hebuys. The metal buyer, in his turn, has good reasons for observ-ing and interpreting the various stamps. For example, if an ar-ticle is marked "10-k," he need not waste time testing it againstthe 12-k needle. Also, he should familiarize himself with thetrademarks of the various manufacturers, and observe which, ifany, are associated with sub-standard goods. Finally, all groupsmust understand the meaning of "tolerance" in marking, whichwill be explained shortly.

BRITISH HALL MARKS

The marking of gold jewelry began in England in the four-

teenth century. The Goldsmiths' Company, incorporated in 1327,and certain other Guilds, found it necessary to organize for theprotection of their craft and of the public against fraud. Theyhad, among other functions, that of testing gold and silver articlesat their several Halls. A small sample was cut from each pieceand assayed, and the article then received four or more stamps,including the quality mark, a town mark, a date letter (changedeach year) and a maker's mark. Various symbols were used, such


28/89

as a leopard's head, a crown, a lion, and the like. Goods madebetween 1784 and 1890 also carried a duty mark indicating that acertain tax had been paid. Stamping was not compulsory, and

small articles were not always marked. It should be noted that

13 CARAT GOLD.

9 CARAT GOLD.

STANDARD SILVER.

Some British Hall marks. These were usedby the Birmingham Assay Office.

these marks were impressed not by the maker, as in the UnitedStates of America, but by the Guild Halls, after assay. Hall mark-ing has been the subject of a considerable literature, which is wellworth the study of the antiquarian and historian.

AMERICAN LAWS AND STANDARDS

In this country the manufacturer himself, subject to law, is per-

mitted to affix quality marks and trademarks to his goods. TheUnited States National Stamping Law, covering falsely or spur-iously stamped articles made of gold or silver or their alloys, wasenacted June 13, 1906. The text may be found in almost any lawlibrary or big public library. (Ask for Rev. Stat. U. S., vol. 34, pt.1, p. 260, 59th Cong., 1st Sess., Public Law 226.)

Handy and Harman, silver dealers at 82 Fulton Street, NewYork, sell a reference book calledHandy Book for Manufacturers,

which contains, among other useful facts, the full text of the lawjust mentioned, as well as summaries of the laws affecting plati-


29/89

num, of the several Commercial Standards which now have theeffect of law, and of the Canadian law applying to the marking ofprecious metals. Another most useful volume is Trademarks of

Jewelry and Kindred Trades,published by the Jewelers' Circular-

SomeAmerican stamps. They consist of the quality stampand the maker's registered trademark.

Keystone, 100 East 42nd Street, New York. This book illustratesseveral hundred trademarks, and in addition summarizes thestamping laws and explains their application.

The manner in which the various silver alloys may be stampedhas already been discussed in this bookseeChapter II.

"TOLERANCE"

Our lawmakers have assumed that jewelers and silversmiths aresubject to human error, so they allow a "tolerance" between thequality indicated by the stamp and the actual quality as deter-mined by an assay. The law also allows for solder, and requires

that the article, solder and all, must approach within a certainpercentage of the stamp. Thus the law of June 13, 1906, as sum-marized in theHandy Book,provides that:

"If an article is made of gold and is stamped gold, it must also bear aquality mark such as *io karat' (10-K), '14 karat' (14-K).

"If an article of gold is given a quality mark, the fineness by assay mustnot be lower than:


30/89

Watch Cases and Flatware .003 less than stamped quality.Other articles, not including solder 0208 (i/2karat) less than the

stamped quality.

"However, the assay of a complete article, including solder, must not bemore than .0417 (1 karat) under the stamped fineness per karat."For example,the gold in a 14-karat watch case, free from solder, must

be at least .5803 by assay. The entire case, including solder, must assay atleast .547 (13 karat). A gold ring, not soldered, stamped '14-K' must assayat least .5625 (13I/2 karat). The gold in a brooch stamped '10-K' must as-say at least .3958 (gi/2karat) and the entire brooch, solder and all, mustassay at least .3750 (9 karat).

"The silver in any article stamped 'Sterling Silver' should assay .925,and the silver in an article marked 'Coin Silver' should assay .900. The

silver in an article, not including solder, must not be less than this bymore than .004.For example,an article marked 'Sterling Silver,' free fromsolder, must assay at least .921.

"Soldered parts must not reduce the assay of the entire article, includ-ing solder, by more than .010 under the standard assays of .925 and .900,respectively, for sterling silver and coin silver. For example, an articlemarked sterling silver when melted, including solder, must assay at least

Most manufacturers make their goods as close to the limit of

tolerance as they dare. Many of them, either wittingly or unwit-tingly, go below this tolerance. The buyer must keep this pos-sibility in mind when he is calculating the value of a precious metalarticle.

LAWS FOR STAMPING PLATINUM

For some years after the introduction of platinum as a jewelrymetal there was confusion regarding its marking, and much mis-

branding, adulteration and fraud took place. Three of the Statesin which considerable platinum jewelry was manufacturedNewJersey, New York, and Illinoispassed laws regulating the stamp-ing of platinum and its alloys. And finally, on June 20, 1938, theNational Bureau of Standards made effective a series of regula-tions, based on these State laws, that cover the entire nation. Thefull text of the New York State law will be found in the Handy

Book.Copies of the new National Standard, known as CommercialStandard 66-38, may be obtained from the Superintendent of Doc-

uments, Washington, D. C, for 5c. Briefly its main provisionsare as follows:


31/89


32/89

letin TS-1942, of July, 1933, defines the terms "Gold filled" and"Rolled gold plate."

Marking of jewelry and novelties of silverCS 118-44.

ENFORCEMENT

While the marking of precious metals has, as we see, been sub-ject to law for centuries, obedience to these laws is not yet perfect.But the fight for honesty in marking and in advertising is beingcarried on actively by a number of organizations, some of themmaintained directly by the precious metal industries. The readerwho finds instances of fraud or misrepresentation would reportthem at once to the Better Business Bureau of his city; or, he or his

jeweler should communicate with the Jewelers Vigilance Commit-tee, Inc., New York 19, N. Y. These groups, in co-operation withthe Federal Trade Commission and the National Bureau of Stand-ards, have accomplished much, not only in the enforcement ofpenal laws, but also in obtaining official condemnation of variousborderline cases. Also the American Gem Society of Los Angeles,through its members, has done a great deal toward clarifying the

advertising and labeling of diamonds and other gem stones.

STANDARDS IN FOREIGN COUNTRIES

Each nation has its own standards, not only for the alloys usedin coinage, but also for silverware and jewelry. In Chapter II weobserved that the word "silver" on a piece of jewelry does not meanthe same thing in all lands. To give the details of all these varyingstandards would not be profitable here, inasmuch as the buyerrarely is sure of the origin of the old metal he buys. The wise prac-tice is to confirm all stamps by one or another of the methods de-scribed in these chapters.

"LET THE BUYER BEWARE!"

If every article made of precious metal was truthfully stamped,there would be little need for a book like this. But many articles

are never marked at alldentures and chemical ware, for example.And the antiquarian handles articles made before the present lawswere framed. An article can be truthfully marked when made,


33/89

then, perhaps because its thin outer layer is worn off, or becausesome repair job added considerable solder or even an additionalpart of a different composition, the old mark may have becomemisleading. Finally there is always the possibility of fraud. Thus

it is clear that the buyer of old precious metals must indeed bewary. One well-known buyer says, "Never believe a karat markunless it is accompanied by a reputable trademark, and sometimesnot even then." If there is no trademark, the stamp may well bequite meaningless.

Chains and mesh are probably the worst offenders. It is recog-nized that considerable solder is needed in making some kinds ofchains, and the wise buyer will assume that even more than that ispresent. Links that test say 12-k on the stone, when melted down

and assayed may turn out to be 10-k or less; chains that test 10-kon the stone may assay 8-k. And so on.

SOME PRECAUTIONS

If an article consists of more than one part, like the old-fashionedwatchcase with front, back, bezel and bow, test each piece sep-arately, as those less exposed may be of lower value. Lockets andbig cuff links sometimes are re-inforced by a base-metal disk inside.Examine the pin and safety catch on brooches. Do not hesitate to

file deep notches, maybe two or three, on different surfaces of eachpiece. Remember that an old article may have been repaired,with the addition of much solder or even a new low-karat segment.

Articles such as candlesticks are often made of a hollow metalshell which is filled or loaded, sometimes with pitch, sometimes


34/89

with lead which has been melted and poured in. Sometimes thebase alone is loaded. This same scheme has been used withheavy link bracelets, etc., and has occasionally deceived the in-experienced observer.

Rolled or filled gold requires special care. It consists largely ofbase metal such as brass, with a thin layer of karat gold on theoutside. Usually this outer layer is 10-k or 12-k. You may find astamp reading "1/10 12-k." Analyze this stamp and you willrealize that this articlewhen newassayed only one-twentiethfinegold, as the 12-karat alloy is only half fine gold, and the karat goldshell is only one tenth of the total weight of the article. Afteryears of usage the outer gold layer, originally very thin, may beworn down to almost nothing. Therefore, when estimating itsvalue, "let the buyer beware." Some professional gold buyers re-fuse to handle this material.

ANTIQUES

Very old gold jewelry is sometimes worth more than you would

think. Years ago when platinum was cheaper than gold, it wassometimes used as an alloy. It cheapened and stiffened the gold,without increasing its tendency to tarnish, and in rare cases wasused in sufficient amount to increase the value of the article. Onthe other hand, much old jewelry is dishonestly marked, and some-times you will find that an antique with a handsome exterior isnothing but soft solder inside.

FRAUD

Deliberate fraud occurs too often to be ignored. The Jewelers'Circular-Keystone, in its issue of September, 1943, reports one in-stance. A customer complained that a certain ring, stamped andsold as 14-k gold, blackened his finger. The retailer tested ithastily (by rubbing an edge on the stone and testing the streak)and it seemed to be a full 14-k. But further examination dis-closed that about nine tenths of the ring was silver, lightly gilded.Thin circles of 14-k gold wire had been soldered to the top and bot-tom edges of a heavy silver ring, so that if a touchstone test weremade in haste, only gold would rub off. The moral of this is: file


35/89

a deep notch if possible, and test more than one surface. Inci-dentally, the buyer might have been warned by the fact that whilethis ring bore a karat stamp, there was no maker's trademarkal-ways a suspicious circumstance.

A poorly disguised fraud. Circles of thin gold wire were soldered to the

edges of a heavy silver ring, and the combination was gold-plated. The

quality stamp was not accompanied by a trademark. Part of the silver ring

and part of one gold circle have been cut away.

DENTAL ALLOYS

Metals that have been used in dentistry carry no stamp, andtheir purity and suitability depend upon the integrity and knowl-

edge of the dental technician. Much dental gold is of high qual-ity, especially inlays and crowns, but in the construction of a den-ture it is often necessary to use considerable solder, which may be16-k, 14-k, or even lower. Parts of metal that are covered by vul-canite or porcelain may be of low grade gold or even of base metal,and sometimes rivets of copper or silver are used, then coveredover with gold solder.

Old fashioned false teeth were, in many cases, provided with twosmall pins of high-grade iridio-platinum. Much of the work donetoday, while more satisfactory to the patient, may contain no

precious metal at all, so each job must be considered individually.

SCIENTIFIC APPARATUS

Enormous amounts of precious metals have been made up intoinstruments and equipment for the various scientific industriesand professions. The laws applying to jewelry apply equally well


36/89

to these instruments, and quality stamps and makers' trademarksshould always be looked for. In other chapters we learn that in

these fields the precious metals may be alloyed with or combinedwith each other, or with the base metals, in such a profusion offorms that the beginner may well be discouraged. However, noother field is potentially more profitable to the buyer of metals.

TREASURE HUNTING IS STILL FASCINATING

These paragraphs may have suggested that this business of buy-ing and selling old precious metals may be as interesting as it is

profitable. To find value in a piece of unattractive, unwantedmetal brings a thrill of satisfaction over and above the mere gratifi-cation of the profit motive. To solve the question of its worthmay be as full of unexpected twists as any other puzzle. For in-stance, we once had occasion to buy a heavy old-fashioned watch-chain. Its appraisal seemed to be as simple a task as could befound. We exposed the metal to the oxygen flame; all the linksglowed, but three of them glowed with a difference. On closer

examination we found that those three links were silvercarvedexactly like the othersapparently the result of some old repair

job, long since forgotten. On another occasion one section of adiscarded penholder, when scraped clean of encrusted ink, turnedout to be 18-k gold. . . . Incidents like these help to make thiswork a constant adventure.


37/89

CHAPTER IV

The Platinum-Group Metals and the White Golds

SECTION A. NEW METALS; OLD TESTS

WHEN platinum first came into vogue, it was natural to applyto it the same tests that we use on gold and silver. As we havelearned, the old acid and flame tests, described in Chapters I andII, are extremely useful, but they do not always tell the observer asmuch as he wants to know. Accordingly it is our purpose now to

expand these tests, then later to add some new ones, to permit therecognition of many of the alloys of the platinum group that arenow in use in the arts and industries.

It is only fair to point out while some of these white metals arepromptly and easily identified, this is not true of all of them. How-ever, all these tests are well within the powers of the layman whowill follow instructions, who is willing to obtain indubitablesamples of the various metals, and who will practise with thesesamples until he learns their characteristics.

THE SIX SISTER METALS

Platinum, palladium, and iridium are the more plentiful mem-bers of the platinum group, and the ones of greatest general inter-est. The other three, osmium, rhodium, and ruthenium, are muchrarer but are finding increased usefulness as time goes on.

These six metals share certain characteristics, notably rarity,white color, density, resistance to corrosion, high melting points,and many chemical peculiarities; but, like human sisters, each has

an individuality of its own.Many combinations of two or more of these six, with or withoutadditional metals from other groups, have found employment in

jewelry, in many industries, and in the sciences. To explore allsuch combinations would require much more space than is hereavailable. These chapters, therefore, will concern themselvesmainly with those alloys that are of interest to the jeweler and the


38/89

old jewelry buyer. These include the two or three kinds of "hard"platinum that form the foundation of the platinum jewelry in-

dustry; those alloys of palladium that have found favor in jewelry,including the white golds, and a few of the alloys in which basemetals are present by accident or design. Because of their resem-blance to the alloys used in jewelry, a number of dental alloys willalso be included.

WHAT WARS DO TO JEWELRY METALS

When platinum jewelry first came into fashion, early in thiscentury, the alloys generally used were the simple iridio-plati-nums. (Pure platinum is almost as soft as fine gold, and must behardened and stiffened for most purposes. The addition of 5percent to 10 percent iridium gives an ideal alloy for jewelry pur-poses.) Careful assay of jewelry made at that time may show otherelements, but these probably got there by accident because knowl-edge of how to purify these metals was then far from complete.

World War I clamped an interesting economic pincer on plati-num. Russia was the main source of supply, and it was cut off bywar. At the same time demands increased hugely, both becauseplatinum is used in making chemicals for the munitions industries,and also because the public wanted jewelry made of platinum anddid not care how much it cost. Accordingly prices skyrocketed.

This situation so stimulated the ingenuity of metal workers that

many substitutes and new alloys were devised. Some of these havefound honored places in the world of metals, for example, someof the palladium and ruthenium alloys. Some others, in whichnickel and other base metals were used, had poor working quali-ties, and in addition were economically unsound since their com-plexity caused enough trouble in refining and remelting the scrapto counterbalance the original saving.

The fact that we then had no regulations to cover the platinum

group served to increase the confusion and to encourage fraud.One of the by-products of the great demand for white jewelry

was white gold. The first white golds were gold-palladium alloys,followed shortly by a variety of alloys in which nickel served aswhitener. The fact that these alloys came under the gold stamping


39/89

laws tended to reassure the careful buyer, and doubtless con-tributed to their popularity.

In England a considerable quantity of palladium jewelry wasmade during World War I, even though at that time its price washigher than that of platinum.

World War II, in its turn, placed restrictions upon most of ourmetals, and again challenged the ingenuity of the precious metalmetallurgist. One expedient, rolled gold on a silver base, wasmentioned in Chapter II. During the long Armistice, our chem-ists and refiners had learned much about the properties and puri-fication of the six members of the platinum group, and supplies ofmost of them had increased considerably. As a result, alloys andcombinations that had been standing unnoticed in the laboratory

were escorted forth to make their debut upon the stage of fashion.Thus, when iridium went to war, ruthenium came forward,

and the useful ruthenio-platinum was introduced to the jewelryworld. In working qualities it so nearly resembles the classic iridio-platinum that its future as a jewelry alloy seems assured. How-ever, "Ruth-Plat," as it is designated by the commercial standardwhich by this time had been formulated for the platinum group,was in its turn a war casualty when platinum came under restric-tion. Rhodium also was called to the colors. Finally certainpalladium alloys in which ruthenium serves as hardener, receivedtheir opportunity, and "jewelry palladium" came into use.

When in our mind's eye we review this parade of alloys acrossthe stage of history, we realize that the task of identifying them hasbecome more and more complex. We see why the itinerant goldbuyer became confused and decided that it was better for him notto bother with the white metals at all. We see why identification,though more difficult, is far more interesting, and when properlycarried out is correspondingly more profitable. (Incidentally, thisreview brings the practical suggestion that a hint as to the compo-

sition of a piece of "platinum" jewelry may sometimes be found inthe date at which it was made.)

SAMPLES NEEDED FOR TESTING

When we were examining silver and some other white metals inChapter II, we provided ourselves with samples of as many differ-


40/89

ent metals as possible. The serious student will now provide him-self with as many samples of platinum-group metals and their al-loys as possible. Sets of standard platinum needles, much like thestandard gold needles, are on the market, and are useful. Theseare brass points, tipped with bits of pure platinum, pure palla-dium, several of the platinum-palladium alloys and an iridio-platinum alloy.

Standard needles for testing platinum-group alloys.

In addition to these, however, or in place of them, you shouldobtain from a reputable source several sizable pieces of metalapennyweight or so of each will be enough for the careful workerwhich you will feel free to heat to redness, and from which you cancut off portions to be dissolved in acids, and to which you can applythe various reagents. By all means have pieces of pure platinum,pure palladium, and fine gold. If you plan to distinguish between

iridio-platinum and ruthenio-platinum, buy samples of both al-loys. The highest grade white golds, which consist of about 85per cent gold with 15 per cent palladium, more or less, are usuallythought of as gold alloys, though they can with equal propriety becalled palladium alloys. It is well to have samples of one or twoof these. The more samples you have to compare, the greater willbe your skill and assurance in identifying unknowns.

Mark each sample carefully by stamping or engraving on itsome symbol or number. One plan is to have each piece a differentshapesquare, or oblong, or triangular, or the likeand to makecareful record of the composition of each piece.


41/89

THE ROUTINE OF TESTING

Let us assume that we have some white metal articles and wish topick out those made of precious metals and to determine as much

as we can of their composition.First we employ the tests described in Chapter II. If the student

is not already familiar with these, he should read that chapteragain, noticing carefully the references to the platinum metals andthe white golds. Thus we use the magnet; we use the air-gas or theoxy-gas flame; we observe the specific gravity or "heft" of the ar-ticles; we file deep grooves and apply nitric acid: sometimes we ap-ply plain hydrochloric acid, or plain sulphuric acid, or a grain oftable salt. Probably by this time we have separated out the base

metals and discarded them, and quite possibly we have formedexcellent guesses as to the composition of the more resistant arti-cles.

If in doubt, we subject our samples to the same tests, and observeresults.

THE AIR-GAS OR OXY-GAS FLAME

All manufacturing jewelers, all jewelers who do repair work,and all dental technicians, have air-gas or oxy-gas torches of one

type or another. Such a torch properly used is one of the best, aswell as one of the quickest devices for the identification of preciousmetals. As we have suggested in Chapter II, a few seconds spent inbringing a suspected metal to red heat may answer all your ques-tions. The flame will spot the base metal articles for you, and maygive you valuable clues to the composition of the precious metalarticles.

The use of oxygen from a tank, instead of compressed air, has be-come increasingly common not only for making platinum jewelry,where it is essential, but also for making gold or silver jewelry.For our purposes the oxy-gas flame is preferred. Oxy-acetyleneflames are almost as good, but are so hot that they must be usedwith caution.

We have learned in Chapter II that platinum and its preciousmetal alloys, if brought to white heat and then allowed to cool inair, will show no tarnish whatever, differing therein from mostwhite golds, from sterling silver, and from all the base metals. For


42/89

that reason the professional metal buyer normally makes thistest the first order of business. For that same reason the estab-

lished jeweler or dental technician, who has a torch and knowshow to use it, is better equipped to buy old precious metals thanis the most energetic house-to-house buyer.

One kind of small oxygen-gas blow-pipe.

Much can be learned about a piece of metal by heating it to itsmelting pointby making it actually molten. For example, iridio-platinum is slower to melt than soft platinum; the more iridiumthe higher the melting point. If base metals are present, even insmall amount, the button that forms on cooling will show a dark-ened surface and probably will be brittle. An experienced meltercan identify the impurities by the stains that form on the crucible.

Palladium responds to the flame rather oddly. If you start with

cold metal and heat it gradually, you will see films of peacock-colored oxides play across the surface when the metal reaches about4000C. At about 8oo C. these disappear, and if you quench thehot metal in water at the right moment, it will cool before theoxides have time to form again, and the button will be clean andwhite. The melting point of palladium, 15540C, is higher thanthat of gold, lower than that of platinum. Molten palladium ab-sorbs large volumes of gas, and the button swells and puffs; then

when it solidifies again the gas is expelled with much spitting and"crabbing."But it is not always possible or convenient to heat your unknown


43/89

metals to the molten stage. Sometimes, in fact, you will wish todamage their appearance as little as possible. That brings us tothe second section of this chapter, to a series of tests in which aminute quantity of the unknown metal is dissolved in a drop ofaqua regia, then treated with some chemical that will reveal itsnature.

SECTION B. NEW METALS; NEW TESTSIf the student has not already done so, he should at once assem-

ble his samples of platinum, platinum alloys, palladium alloys,white golds, and so on, and apply to them the traditional acid testsfirst nitric acid, then aqua regia.


44/89

So, let us now with our samples of metals and alloys of knowncomposition, make metallic streaks in the cavities of the spot plate,rubbing hard with the hard metals, more gently with the soft ones.

Next we shall treat the streaks with nitric acid, then with aquaregia. Following that, whenever such tests do not tell us all wewant to know, we shall add some new and additional chemicals,thus carrying out the new tests that have been developed in stepwith the development of these new alloys.

NITRIC ACID AND PALLADIUM

Palladium is the only member of the platinum group that dis-

solves in nitric acid. Make a streak in a cavity of the spot plate,add a drop of nitric acid, and observe the deep brown color of thesolution.

Pure palladium is too soft for most commercial purposes. Alloysstiffened with a little ruthenium and rhodium have working qual-ities suitable for jewelry, and attained a mild vogue during thelong Armistice. When, during World War II, restrictions wereplaced on rhodium, platinum, and ruthenium, the so-called

"jewelry palladium" came into quite general use. Several formulaswere used, in most of which ruthenium had the role of hardener(with or without the addition of other elements), and some hand-some palladium jewelry was made. In most of these alloys theproportion of palladium is so high that nitric acid attacks themat once, showing the brown color. Later in this chapter confirm-atory tests for palladium will be described.

The inclusion of even a little platinum in a palladium alloygreatly reduces its solubility in nitric acid. Thus the alloy 98%palladium with 2% platinum reacts on the stone like 14-k gold;and the alloy 90% palladium with 10% platinum resists the coldacid completely.

Clean the spot plate after each using, dissolving any stain withnitric acid or aqua regia, then rinsing well with plenty of water.

AQUA REGIA AND THE PLATINUM METALS

Again make streaks in the cavities of your spot plate, using your

platinum-group metals and alloys, your samples of white golds anddental golds, and if possible including several samples of low-


45/89

grade platinum alloysalloys containing copper or nickel or silver,with or without gold or palladium.

Mix up some fresh aqua regia. For this work a good mixture isone part nitric acid to four parts hydrochloric acid, and the bestcontainer is a dropping bottle. The sketch shows one type of drop-ping bottle. Notice the grooves on the stopper and in the neck ofthe bottle; when these coincide you can easily pour out one drop,or as many as you wish, without fumbling or waste.

One kind of dropping bottle.The stopper is grooved, andthere is a channel in the neck

of the bottle.

Never close tightly any bottle that contains aqua regia. Keepthe stopper turned so that the grooves coincide and the gases thatevolve may escape. If dropping bottles are unobtainable you canmanage with ordinary glass-stoppered bottles and a handful ofmedicine droppers or small glass rods; but the dropping bottlesare much the better arrangement.

Add about four drops of aqua regia to each metallic streak, and

await results. With some streaks the acid goes to work at once.With others the action is so slow that the hasty observer will con-clude that they are not dissolving at all. But sooner or later, de-pending upon the nature of the alloy and the temperature of theplate, the aqua regia will take on a deeper color and the metallicstreaks will disappear.

In a notebook write down the order in which the streaks are at-tacked.


46/89

To hasten matters, heat the plate until it is uncomfortably hotto the hand, possibly by placing it on steam pipes, or on an asbestospad resting on an electric hotplate; or grasp it with tongs and slip

it into a pan of hot water.We spoke just now of lower-grade platinum alloysthose con-

taining base metals. Compared with iridio-platinum and ruth-enio-platinum, these may dissolve readily in aqua regia, thereforemay be confused with certain high-grade alloys in which palladiumor gold is present. On the other hand, low-grade alloys containingmuch silver may be as slow to react as the very valuable "hard"platinums. Thus we see that the mererate of solutiongives only

partial information as to the value of an alloy; thus copper or pal-ladium hastens action, while silver, iridium, or ruthenium slows itdown, and observations based on speed alone can be quite mislead-ing.

This brings us, then, to the modern extensions of this method,whereby it is easy to detect palladium or gold (or both) in a plati-num alloy; also to detect platinum, palladium or nickel in a whitegold or dental alloy; and to distinguish between iridio-platinum

and ruthenio-platinum. First we make a streak with our unknownmetal and dissolve it in aqua regia. Then we add certain chem-icals to the drop, and by noting the color changes we learn thecomposition of the unknown. That is the whole story in one par-agraph.

STANNOUS CHLORIDE TESTING SOLUTION

This solution, often called "Testing Solution A," is extremelyuseful. Rightly handled it reveals the presence of gold, silver,platinum, iridium and palladium in solution, and suggests theproportions in which they are present. It is easy to prepare andthe ingredients are inexpensive.

From your supply house purchase an ounce of stannous chloridecrystals, and an ounce or less of pure tin metalmossy, granular, orfoilbut it must be pure tin. You will also need some hydro-chloric acid, and by far the best container to use is a droppingbottle, similar to that mentioned above. These quantities willprovide several hundred tests.

Make up only a little of Testing Solution A at a time, as it doesnot keep well. Take about a pennyweight or less of the stannous


47/89

chloride crystals (also called tin salts) in the dropping bottle, adda half pennyweight or so of tin metal, and fill the bottle three-fourths full of water. Tap water will do. Now add about 20 to30 drops of hydrochloric acid, more or less, to a 30 cc. bottle. Thisgives a milky liquid that is ready to use. The tin metal will dis-solve very slowly, and it serves to keep the solution in good condi-tion. Label the bottle "Testing Solution A."

As we said, Testing Solution A when properly used shows thepresence of precious metals in solution. In order to get acquaintedwith the color-changes involved, you should first make up somesolutions containing these precious metals. You should have asolution containing gold, one containing platinum, and other con-taining palladium. This method is so useful and fascinating that

most users wind up with a whole series of standard solutions, soperhaps you might as well get a half-dozen dropping bottles in thefirst place.

STANDARD SOLUTIONS OF GOLD, PLATINUM, PALLADIUM

To make up a standard solution, simply dissolve a small pieceof metal in a little aqua regia, then add water. For instance, takeexactly a grain of pure platinum wire; dissolve it in a little aquaregia, using a small porcelain dish and heating gently until all the

metal dissolves. Use as little aqua regia as will do the work. Washthe solution with water into a glass-stoppered two-ounce bottle,and fill the bottle up to the mark with water. Label this bottle"ONE GRAIN PLATINUM IN 2 FLUID OUNCES OF SOLUTION."

(When your only object is to become acquainted with the vari-ous solutions, it is not necessary to use exact measurements. Butlater on, when trying to approximate the amount of preciousmetal in a solution, it will be extremely helpful to have standardsolutions made up with a definite weight of precious metal in adefinite volume of liquid. Therefore it saves time to make upyour solutions in the beginning according to a definite plan.)

STANNOUS CHLORIDE TESTING SOLUTION WITH PLATINUM

Let us become acquainted with Testing Solution A. Take thespot plate and drop one drop of the standard platinum solutioninto a cavity. Notice the pale yellow color. Add a drop or more


48/89

of Testing Solution A. If properly prepared the two will react in-stantly to give a deep yellow or brown color. If too concentrated,the color will be almost black; in that case, dilute the platinumsolution with an equal volume of water. This deep yellow colorwith Solution A is a characteristic of platinum and iridium.

WITH GOLD

In another cavity, place one drop of gold solution, and add adrop of Solution A. After several moments add several more dropsof Solution A. Note the first intense dark color, deep purple or

black. This is characteristic of gold. After it stands a few minutes,notice the purple stain on the white porcelain.Do not let the liquids dry on the plate. Wash it promptly after

each test, removing any stains with a drop of aqua regia and rins-ing well.

Now, in another cavity, take just one drop of your gold solution,and dilute it with five drops of plain water. Take one drop ofthis dilute gold, in another cavity, and add a drop of Solution A.Note that the color is still definite. Dilute with five more drops ofplain water, and try again. See how dilute this gold solutionmust be before it becomes so weak that you cannot detect a changewith Testing Solution A. If you figure this out, you will find thatthis is a delicate test, one that will reveal the presence of a verysmall percentage of gold.

WITH PALLADIUM

In the same way, learn the color-changes shown when mixingstandard palladium solution with Testing Solution A. This color-change is even more interesting than the others. When the twodrops are first admixed, you see a deep yellow, not unlike the ef-fect produced by platinum. After some minutes the yellow turnsblue-green. This blue-green color is characteristic of palladium.

WITH SILVER

Silver solutions, such as silver nitrate, do not give any color-reac-tion with Testing Solution A. What you will see when the twoare mixed is a white cheesy precipitate of silver chloride, similar


49/89

to that obtained when table salt is added to a silver nitrate solu-

tion.

WITH BASE METALS

Solutions containing only such base metals as iron, copper, zinc,nickel and cadmium give no color change with stannous chloride.Lead may give a white precipitate that looks like silver chloride,but if you employed the dichromate test in Chapter I this willcause you no confusion.

STANNOUS CHLORIDE TESTING SOLUTION WITH UNKNOWNS

You should now be ready to examine metals of whose composi-tion you are ignorant. Take an article that you suspect of being

platinum or some platinum alloy. Rub it hard to make a goodstreak in a clean cavity of your spot plate. Dissolve the streak inaqua regia, noting whether or not it is necessary to heat the plate;and making up for evaporation if you do heat it.

Add a drop of Solution A and note the color change.Repeat with a piece of what you believe to be a good gold alloy.

Repeat with something you believe to be palladium. Finally asksome friend to hand you pieces of metal, preferably pure metalsor simple high-grade alloys, test them and check your reports with

him.(Testing Solution A must be made up freshly from time totime. It loses its virtue completely in a few days. Therefore, be-gin the day's work by checking your Solution A against a drop ofstandard gold solution; if it fails to respond, throw it away at once.)

DETECTING GOLD, PLATINUM, AND PALLADIUM, IN THE PRESENCE

OF EACH OTHER

The next step is to detect palladium in metal that is mostly

platinum. This is especially valuable when testing dental alloysor buying metal that may be contaminated or of low grade. If youhave a standard needle of a platinum-palladium alloy, rub it onyour spot plate, warm the plate, dissolve the streak in aqua regia,and test the solution with Testing Solution A.

Can your eye detect the difference between that effect and theeffect produced by pure platinum?


50/89

Next, rub pure platinum in a cavity, and make a few rubs in thesame cavity using a bit of fine gold. Suppose you make fifteenrubs with platinum, and three rubs with gold. Again warm theplate, dissolve the streaks in aqua regia, and test with Solution A.

Can your eye detect the presence of that small amount of gold?Also, can your eye detect the presence of all three metalsgold,

platinum, and palladiumat the same time, in a single drop ofsolution?

Your eye may not be able to do this the first time. But after alittle experience, you will know which metals are present androughly the proportion of each.

Skill in appraisal comes with practicepractice in studying thebehavior of alloys of whose composition you are certain, exposingthem to the various tests and comparing them with each other andwith unknowns handed to you by some friend who can check yourreports.

If a spot plate is not obtainable, it is possible, though not con-venient, to use other plans. Thus, get a minute amount of yourunknown metal into solution in some other way, perhaps by

cutting off a scrap with a file or saw and dissolving it in aqua regiain a tiny test tube or small watch glass. Soak up the solution inclean white blotting paper or filter paper. Now drop one drop ofTesting Solution A onto the stain. Colors will appear and spreadthrough the paper handsomely. If two precious metals are pres-ent, say gold and palladium, the characteristic colors of both willappear.

IF THE TESTS ARE NOT CONCLUSIVE

The beginner sometimes gets confusing results. Sometimes the

colors refuse to appear. This may be due to the fact that one solu-tion or another has lost its potency. More likely it is because youhave used too much acid.

Remember that aqua regia weakens on standing. Rememberthat Testing Solution A spoils on standing. Both must be mixedafresh from time to time. The standard solutions do not spoil.If properly kept in glass-stoppered bottles they will keep for years.But when you make them up, do not use an excessive amount of

acid to dissolve your bits of metal. If too much aqua regia is used,the tests will be weakened or even destroyed.


51/89

The excess acid can be driven off by evaporating the solutionsgently until sirupy, then adding a little water. In dissolving thestreaks made on the spot plate, you sometimes use more aqua regia

than is wise. There again you can remove the excess by warmingthe spot plate gently; if the drop should go entirely dry, add plainwater to bring your substances again into solution.

Another situation that may confuse the beginner is to find analloy containing much platinum and very little palladium; ormuch gold and very little palladium. He finds that the palladiumcolor is obscured by the intense reactions of the platinum or thegold. As his eye becomes skilled he can detect smaller and smallerproportions; however, he will be glad to know that there is another

solution that is especially valuable in detecting small amounts ofpalladium.

DIMETHYL GLYOXIME SOLUTION

This solution has the added virtue of showing up nickel, even insmall amounts. It will show up nickel in a platinum alloy; in awhite gold alloy; in a dental alloy; or in a solution. It will showup palladium and nickel when both are present in small amounts

in an alloy that is largely platinum or gold.Purchase a gram of dimethyl glyoxime. One gram will be

enough for several hundred spot plate tests. Be sure to get a goodquality product. It is a white or pale yellow powder. The nameis pronounced "dye-methyl glyoxeem," but no one will blame usif we refer to it as DMG.

Dissolve this gram of DMG by bringing it to a boil in about 100cc of waterabout 4 fluid ounces. The powder dissolves ratherslowly. Let it cool and if possible let it stand overnight; thenfilter. It is important that the solution be clear and free fromsediment or crystals. It is now ready to be placed in a droppingbottle, which should be labeled DMG. It keeps quite well for years,except that you may have to filter it again.

Let us get acquainted with DMG. Its most interesting charac-teristic, as we said, is to show up palladium and nickel, in the pres-ence of other metals and in the presence of each other.


52/89

DMG AND PALLADIUM

First, place a very small drop of the standard palladium solution

in a cavity of the spot plate. Or, better, take one drop and diluteit with several drops of water to obtain a pale yellow solution;then place one drop of the pale solution in a clean cavity.

Do the same thing with a drop of your standard platinum solu-tion, and with your standard gold solution; a drop to a cavity. Allthree have a pale yellow color.

Now add a drop or two of DMG to each cavity. Note thePRECIP-ITATE that forms, and its color. Note that the pale yellow colors

do not change, but that in the cavity containing palladium youwill see a PRECIPITATE, or sediment. Note carefully that whileTesting Solution A gives prompt changes in color, DMG distin-guishes between palladium and the other metals by forming a pre-cipitate.

You can see this more clearly by making similar tests in small testtubes. Make one

Date post:	03-Jun-2018
Category:	Documents
Upload:	scott-ludwig
View:	550 times
Download:	58 times

Testing Precious Metals C.M Hoke

Documents