The 22nd international Santa Fe Symposium®

was held in Albuquerque, New Mexico, from18th–21st May 2008 (1), and attracted around 120delegates from thirteen countries in all continentsof the world. Once again, palladium jewellery man-ufacture featured strongly, with two presentationson investment (lost wax) casting studies that showhow this new jewellery metal can be successfullycast. The importance of the platinum group metals(pgms) for jewellery was further supported by apresentation on the investment casting of platinumjewellery and one on the use of laser welding inplatinum jewellery manufacture.

As noted at the 2007 Symposium (2), palladiumis attracting considerable interest as a jewellerymaterial in its own right. However, its success inthe market depends on an ability to manufacture itcost-effectively by the conventional technologies.Investment (or lost wax) casting is the major man-ufacturing technology for precious metal jewelleryand its application to palladium is seen as impor-tant. The relative lack of knowledge of the castingcharacteristics of 950 fineness palladium alloys,given that palladium is known to absorb hydrogenand oxygen, has been seen as a potential drawback.The current studies reported at the 2008Symposium address that concern.

Casting of Palladium AlloysThe first presentation, ‘Investment Casting

Behaviour of Pd-Based Alloys’, was given byProfessor Paolo Battaini (8853 SpA, Italy), whohas been involved in developing 950 palladiumalloys (see (3)). Battaini notes that palladium-silver-based dental alloys with palladium contentsbetween 40 and 60% have been developed sincethe 1970s and investment cast successfully. Hispresentation was concerned with reviewing the

chemical and physical characteristics of such den-tal alloys, and investigating how the experienceacquired in the dental field could be transferred tothe jewellery industry. He reviewed the effect ofalloying elements on the hardness of palladium,followed by a discussion of the hardness and melt-ing range of some common dental alloys. He notedthat the width of the melting range is larger (100 to150ºC) than that of common 950 platinum jew-ellery alloys (15 to 30ºC), a factor which affects thesolidification behaviour of these alloys. The castmicrostructure of the dental alloys, he pointed out,is often complex and multiphase, frequently con-taining lamellar eutectic constituents, as shown inFigure 1. Apart from the primary palladium phase,the second phases are often intermetallics contain-ing gallium. Segregation to grain boundaries orbetween dendrites is also common in high-palladi-um alloys. In contrast, 950 palladium jewelleryalloys are expected to be single phase, as they areless highly alloyed.

Battaini noted that high oxygen uptake in den-tal alloys is often due to internal oxidation of basemetals, and this could also occur in jewellery alloys

Platinum Metals Rev., 2009, 53, (1), 21–26 21

The 22nd Santa Fe Symposium on JewelryManufacturing TechnologyINVESTMENT CASTING OF PALLADIUM JEWELLERY ATTRACTS MUCH INTEREST

Reviewed by Christopher W. CortiCOReGOLD Technology Consultancy, Reading, U.K.; E-mail: chris@corti.force9.co.uk

DOI: 10.1595/147106709X395503

Fig. 1 As-cast microstructure of the dental alloyPd74.0-In5.0-Cu14.5-Ga1.6-Sn4.9, showing lamellar eutecticareas (Courtesy of Paolo Battaini, 8853 SpA, Italy)

if held in oxygen-containing atmospheres at hightemperatures. Such internal oxidation would bedetrimental to palladium jewellery, as it is with silver (where it gives rise to a dark coating knownas ‘firestain’), in terms of polishing. High oxygencontent also leads to gas porosity in both dentaland jewellery alloys. Carbon absorption is anotherproblem in dental alloys, and can also lead to gasporosity in castings, so palladium alloys are notmelted in graphite crucibles. Palladium is wellknown for absorbing large quantities of hydrogen.Further, as with the other precious metals, siliconcontamination causes embrittlement due to lowmelting point eutectic formation, usually located atgrain boundaries and so, unless coated with zirco-nia, silica melting crucibles should be avoided, asshould reducing conditions.

Palladium dental alloys are also susceptible tohot cracking due to the mismatch of expansioncoefficients with the phosphate-bonded investmentmould material. Such cracking has been observedin 950 palladium jewellery alloys after casting.

The above review led nicely into the next pre-sentation, ‘Challenges for Palladium CastingAlloys’, presented by Jörg Fischer-Bühner (LegorSrl, Italy). This was a report on work conductedlast year at the Research Institute for PreciousMetals and Metals Chemistry (FEM), Germany,sponsored by Palladium Alliance International, tostudy the investment casting of 950 palladium jew-ellery alloys. This work, in which Johnson MattheyNew York supplied the palladium alloys and indus-trial trials were carried out at TechForm and AuEnterprises, both in the U.S.A., is the first system-atic investigation of the investment casting of 950palladium alloys following the pioneering work byTeresa Fryé (TechForm Advanced CastingTechnology Inc, U.S.A.) (see (2)).

The laboratory work was conducted by meltingeither under an argon cover on the crucible orunder vacuum followed by backfilling with argonin the closed casting chamber of a centrifugal cast-ing machine, using zirconia-coated silica crucibles.Melting time was controlled and casting carried outon a tree size between 90 and 180 g, using mouldsmade in either a two-part platinum investmentpowder with acid binder or a one-part platinum

investment powder with a water binder. Flask(mould) temperatures were in the range 650 to850ºC. Two alloys were selected: 950 palladium-ruthenium-gallium and 950 palladium-silver-gallium-copper, for casting trials of a ball-on-ringtest pattern, sprued either on the ball or on theshank opposite the ball.

The drying time of the flask before burnoutinfluenced the incidence of porosity in the cast-ings, with shorter times of 1.5 h yielding littleporosity. For the one-part investment, optimisingthe powder :water ratio gave good castings, where-as excess water resulted in some porosity. It wasconcluded that 950 palladium alloys can be veryreactive with the investment mould material, lead-ing to gas porosity. They are especially sensitive topoor investment preparation and degradation ofthe investment powder over its shelf life. For thetwo-part investment, short but effective drying andbinder removal are essential.

Using a flask temperature of 650ºC, meltingwith just an argon cover on the crucible led toporosity in the casting, whereas use of a vacuumfollowed by back-filling the closed chamber withargon resulted in good castings. When the flasktemperature was raised to 850ºC, pore-free cast-ings were obtained with just an argon cover on thecrucible, where the feed sprue system allowedrelease of gas. The flask is full of air when just anargon cover on the crucible is used in melting, andthis allows the melt to take up oxygen after pour-ing, so tailoring the feed sprue to allow gas releaseis important. In both gas atmosphere conditions,formation of inclusions within the alloy wasobserved and analysis showed these to be galliumoxide or silica.

The industrial trials at TechForm using theirshell mould casting process resulted in castingswith cracks and hot tears, typically at the junctionof the feed sprue to the ring shank. Scanning elec-tron microscope/energy dispersive X-ray(SEM/EDX) analysis of the fracture surfacesshowed gallium enrichment and traces of silicon,leading to the conclusion that hot tears were due tolow melting phases. The number of silica inclusionparticles found in the castings increased with theamount of scrap used in the melt charge.

Platinum Metals Rev., 2009, 53, (1) 22

It was concluded that overheating of the meltand extended melting times should be avoided,as these allow the melt to take up silicon, phos-phorus and oxygen. This leads to gas porosityand hot tears after the chamber is evacuated, cre-ating reducing conditions, and back-filled withargon.

In subsequent work carried out by Fischer-Bühner and colleagues at Legor Group Srl, Italy,the development of palladium alloys containinggallium was studied. This showed alloys with galli-um, silver and indium, with lower melting ranges(below 1500ºC), to be preferable over other com-positions. Systematic casting trials of such an alloy(known as ‘Pd950G’) were reported. Good cast-ings could be obtained, with low levels of gasporosity and no cracks or hot tears, see Figure 2.By comparison, use of a 950 palladium-niobium-gallium alloy cast under similar conditions led tocracks, hot tears and significant gas porosity in thecastings. This alloy had a higher melting range(above 1500ºC), and it was concluded that thisresulted in increased reactivity with the crucible

and mould material. Casting trials on the Pd950Galloy conducted at jewellery manufacturers’ work-shops showed that melting in air resulted in largeamounts of gas porosity in the castings, but nocracks or hot tears. When an argon cover on thecrucible was used, both gas and shrinkage porosity were observed, the latter due to non-opti-misation of the feed sprue. The finished surfacequality of the ring was satisfactory. Using themethod of melting under a vacuum followed byback-filling with argon, good castings with lowporosity and no cracks or hot tears were obtained.Trials in which Pd950G alloy scrap was recycledover several cycles showed little impairment ofsurface quality, no degradation of melt fluidity, alack of cracks and hot tears and little increase ofgas microporosity. From this study of theirPd950G alloy, it was concluded that it is possibleto obtain good castings in 950 palladium alloysunder typical industrial conditions, although strictprocess control is essential.

PlatinumDespite the quick advent of computer aided

design/computer aided manufacturing (CAD/CAM) and rapid prototyping technologies in thejewellery industry, there has been a dearth of infor-mation on casting behaviour of the resultingpatterns (or models) in the various plastic/poly-mer materials that result from the differenttechnologies on offer. That was the subjectaddressed by Teresa Fryé (TechForm AdvancedCasting Technology Inc, U.S.A.). This companyspecialises in shell-mould casting of platinum jew-ellery. In her presentation, ‘A Study of the Effectof CAD/CAM-Derived Materials in the Castingof Platinum Alloys’, Fryé observed that experiencehad shown all these plastic materials performeddifferently in the casting process. The study wasundertaken to understand these differences. Anumber of plastic model materials were studied:two light-curing photopolymers, a thermoplasticproduced by jetting, four grades of polymericmilling wax and one of a conventional hydrocar-bon injection wax.

A test model was created, with a geometrydesigned to encourage failures typically seen in real

Platinum Metals Rev., 2009, 53, (1) 23

Fig. 2 (a) Castings in an alloy of palladium with gallium,silver and indium, known as ‘Pd950G’; (b) the finishedalloy surface, showing no cracks or hot tears (Courtesyof Jörg Fischer-Bühner, Legor Group Srl, Italy)

(a)

(b)

castings; it was based on a heavy torus shape withdeep holes, sharp angles and deep recesses. Thefirst set of tests were burnout experimentsdesigned to assess the amount of residual ash afterburnout under conditions of both restricted air andfree flow of air. These showed that, surprisingly,there was little ash residue in any case, and thatrestricting air flow resulted in less ash for all mate-rials; the lowest ash was found in one milling waxand the highest with one photopolymer. Thermalexpansion was also measured, using a simpledilatometric technique. The milling waxes and thephotopolymers showed substantial expansions (upto 200ºC and 450ºC, respectively), whereas theinjection wax and thermoplastic had low expan-sions. These results have implications for themould in the burnout cycle. The mould materialexpansion is complex and its compressive strengthrises with temperature in the case of phosphatebonded materials.

Casting tests on platinum-10% iridium alloywere undertaken using the TechForm ceramicshell system with a phosphate-bonded platinuminvestment. These showed that injection wax, withlow expansion and low melting temperature, per-formed well and produced good castings. Incontrast, the milling waxes performed less well,with investment failure evident in the blind hole(Figure 3(a)) and the thin walled areas. The ther-moplastic, with low expansion, gave good castings(Figure 3(b)), whereas the photopolymers pro-duced mixed results. Some ceramic corerelocation was also found with the milled waxes

and one photopolymer. These results support theview that thermal expansion is at the root of theobserved casting defects, whereas ash residue didnot appear to be significant, contrary to perceivedwisdom.

In his presentation, ‘Platinum and Lasers: TheNatural Solution’, Jurgen Maerz (Platinum GuildInternational, U.S.A.) discussed the application oflasers to platinum jewellery manufacture and repairthrough several practical examples. All were con-cerned with laser welding, and the advantage forplatinum of its low thermal conductivity, whichmeans, for example, that gemstones are not dam-aged in the process. The examples presenteddemonstrated the versatility and design potentialthat laser welding confers, and serve as model casestudies for those new to platinum.

Jewellery Properties andManufacturing

The property of hardness and ‘The Role ofHardness in Jewellery Alloys’ were discussed byChris Corti (COReGOLD, U.K.), in which he con-cluded that all precious metal jewellery should havea hardness value of at least 100 HV for satisfacto-ry performance. Low hardness is a problemsometimes found with cast platinum jewellery dueto poor alloy selection, for example; see (4). Somecommercial palladium alloys also tend to have lowhardness values in the as-cast or annealed condi-tion, although this is being addressed by the alloysuppliers. The inconsistency of hardness data inthe open literature was discussed in a subsequent

Platinum Metals Rev., 2009, 53, (1) 24

Fig. 3 (a) Investment failure in ablind hole in platinum-10%iridium casting, cast from a modelin milled wax produced on a CAMmilling machine; (b) Good castingquality in a blind hole inplatinum-10% iridium, cast from amodel in thermoplastic materialproduced on a rapid prototypingmachine (Courtesy of Teresa Fryé,TechForm Advanced CastingTechnology Inc, U.S.A.)

(a) (b)

roundtable discussion at the 2008 Symposium.The need for an agreed standard testing procedurewas identified, and it was noted that some alloyscan show differing hardness values from cast sur-face to bulk alloy.

‘Designing for Rapid Manufacturing and OtherEmerging Technologies’, was presented by GayPenfold (Jewellery Industry Innovation Centre,Birmingham City University, U.K.). Her presenta-tion was a stimulating look into the future in termsof the design potential afforded by CAD/rapidprototyping-based technologies. The ability toproduce a range of related designs from one basicdynamic CAD program, each unique and differentfrom the others, was astounding.

‘Basic Metallurgy of the Precious Metals – PartII’ by Chris Corti (COReGOLD, U.K.) was anupdate on Part I, presented at the 2007Symposium (2). In Part II, he reviewed the devel-opment of microstructure through solidificationand working. The importance of controlling grainsize was central to this presentation. The problemof hard spots in carat golds, which lead to polish-ing problems, was analysed by Damiano Zito(ProGold Srl, Italy), in his presentation, ‘HardSpots: A Trip through Ambiguity’. Many of thesewere found to be due to insoluble pgms that canoccur in the fine gold bars used to make thealloys.

Andreas Zielonka (FEM, Germany), presentedhis research on ‘Incorporation of GoldNanoparticles in Metal Matrix Systems’. This isachieved by a novel electroplating approach inwhich gold nanoparticles are dispersed during theelectroplating of nickel. This approach to nanopar-ticulate metal matrix materials has considerablepotential. The more conventional particulateapproach to making jewellery was discussed byJoseph Strauss (HJE Company Inc, U.S.A.), whoupdated the Symposium on ‘Powder Metallurgy inJewelry Manufacturing’. He reported on severalinstances of commercialisation of metal injectionmoulding (MIM) that are now emerging inEurope, the U.S.A. and the Far East, not only forjewellery but also for precious metal watch casesand medical components. On a more theoreticallevel, Boonrat Lohwongwatana (Chulalongkorn

University, Thailand) spoke about thermodynamicmodelling in alloy design in his talk ‘Alloys byDesign’, an approach which can save considerabletime and expense in alloy development.

On a more general theme, there were severalpresentations concerned with jewellery technologyand practice: Klaus Wiesner (Wieland GmbH,Germany) discussed the practical aspects of ‘Wireand Bar Manufacturing’ for precious metal jew-ellery and Hubert Schuster (consultant, Italy)spoke about ‘Stone-in-Place Casting for High-EndJewelry’. The problem of defects in casting waxpatterns was described by Patrick Sage (NeutecUSA, U.S.A.), in his eye-opening presentation‘Casting the Perfect Defect’. In particular, the mal-functioning of wax injectors was shown tocontribute to the problem.

On the health and safety front, Samuel Davis(Stern-Leach, U.S.A.) spoke about ‘Methods forReducing Ergonomic Risk’, describing severalexamples of improvements to working conditionsimplemented at his company. In a similar vein,Alexandre Auberson (Cartier, U.S.A.) discussed‘Handling and Shipping at the Manufacture Stage’,in which he described how packaging of jewelleryis a critical part of Cartier’s product offering inorder to preserve the top quality finish and condi-tion of the piece delivered to the retailer andcustomer.

A Lifetime Achievement Award was presentedto John McCloskey, recently retired from StullerInc, U.S.A. John presented at the original Santa FeSymposium, back in 1987, and has made many sig-nificant contributions over subsequent years tofurther our knowledge and understanding in jew-ellery manufacture, see also (3).

Concluding Remarks Palladium as a jewellery metal was again the

centre of attention at this Symposium. In particu-lar, the good progress being made inunderstanding the technology of casting palladiumjewellery will contribute strongly to its future suc-cess in this application sector. Furthermore, thetechnology for platinum continues to be devel-oped and underpins its continued attraction in themarketplace. There is no doubt that the Santa Fe

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Symposium is the important technology forum forthe jewellery industry worldwide.

The Santa Fe Symposium proceedings are pub-lished as a book and as PowerPoint® presentationson CD-ROM. They can be obtained from theorganisers (1). The 23rd Santa Fe Symposium®

will be held in Albuquerque on 17th–20th May2009 (1).

References 1 The Santa Fe Symposium:

http://www.santafesymposium.org/2 C. W. Corti, Platinum Metals Rev., 2007, 51, (4), 1993 C. W. Corti, Platinum Metals Rev., 2007, 51, (1), 194 M. Grimwade, ‘Choosing the Correct Platinum

Alloy’, Goldsmiths’ Company Tech. Bull., Spring 2005,(1), 10

Platinum Metals Rev., 2009, 53, (1) 26

The AuthorChristopher Corti holds a Ph.D. in Metallurgy from the University of Surrey(U.K.) and is currently a consultant for the World Gold Council and theWorshipful Company of Goldsmiths in London. He served as Editor of GoldTechnology magazine and currently edits Gold Bulletin journal and theGoldsmiths’ Company Technical Bulletin. A recipient of the Santa FeSymposium® Research Award, Technology Award and Ambassador Award, heis a frequent presenter at the Symposium.