1
Introduction
• Gypsum is a mineral naturally found in
nature.
• Dental applications:
– Impression materials
– Constructing casts and dies
– Making replicas
2
Ideal properties of die materials
• Dimensional accuracy.
• Ease of use.
• Accurate reproduction of fine detail.
• Inert.
• Good strength.
• Inexpensive.
• Abrasion resistant.
• Good color contrast
3
GYPSUMGYPSUMProduct Use in DentistryProduct Use in Dentistry
4
GYPSUMGYPSUM == calcium sulfate =calcium sulfate =naturally occurs as dihydratenaturally occurs as dihydrate
Heat removes water and converts Heat removes water and converts dihydrate to hemihydratedihydrate to hemihydrate..
[Gypsum Powder] + [H[Gypsum Powder] + [H22O]O] [Gypsum] + [Heat][Gypsum] + [Heat]Calcium Sulfate HemihydrateCalcium Sulfate Hemihydrate Calcium Sulfate DihydrateCalcium Sulfate Dihydrate
DIRECT versusDIRECT versus INDIRECT PROCEDURESINDIRECT PROCEDURES
Dental AmalgamDental Amalgam Inlays, Onlays, Crowns, and BridgesInlays, Onlays, Crowns, and BridgesDental CompositesDental Composites Partial and Full DenturesPartial and Full Dentures
Temporary AppliancesTemporary Appliances
Properties and behaviour
5
• Chemical properties: (exothermic reaction)
Ca sulfate dihydrate heat water loss ground Ca sulfate
hemihydrate
• The production of various types of gypsum is basically the
same:
– If the heating process occurs in atmospheric pressure at
115°C, the powder particles are porous and irregular (b
hemihydrate, plaster).
– if heating process is under pressure, powder particles
are regular and less porous (a hemihydrate, or dental
stone).
– A further increase in pressure and refining of
the powder by grinding results in denser
stone, high-strength or die stone
– When stone is mixed with silica: dental
investment, a material that can withstand high
heat and stress produced when molten metal
is forced into molds to form indirect
restorations
6
GYPSUMGYPSUMSetting ReactionSetting Reaction
[Gypsum Powder] + [H[Gypsum Powder] + [H22O]O] [Gypsum] + [Heat][Gypsum] + [Heat]
[[CaSOCaSO44--(1/2)H(1/2)H22O]O] + [(3/2)H+ [(3/2)H22O]O] [[CaSOCaSO44--(2)H(2)H22OO] + [Heat]] + [Heat]AcceleratorsAccelerators
RetardersRetarders
Calcium SulfateCalcium Sulfate HemihydrateHemihydrate Calcium SulfateCalcium Sulfate DihydrateDihydrate
7
GYPSUM PRODUCTSGYPSUM PRODUCTS::
•• PlasterPlaster•• StoneStone•• High strength/low expansion stoneHigh strength/low expansion stone•• High strength/high expansion stoneHigh strength/high expansion stone
•• Specialty StonesSpecialty Stones•• Investment MaterialsInvestment Materials
WorkingWorkingCast withCast withRemovableRemovableDiesDies
EdentulousEdentulousCastCast
OrthodonticOrthodonticModelModel
WorkingWorkingCastCast
RemovableRemovableDie withDie withWaxed InlayWaxed Inlay
8
Gypsum productsP
last
er o
f P
aris
Den
tal
sto
ne
Heated under pressure 125°C
Heated in open kittle (110-120°C)
Hig
h s
tren
gth
Made fromdensite, boiled in 30% CaCl2, thenwashed with water and groundst
one
9
Physical properties
10
Type W/P Porosity Compressiv
e strength
Abrasion
resistance
Setting
expansion
plaster 0.45 High 8.8 MPa Low High
Stone 0.30 moderate 20.6 MPa Moderate Moderate
High strength
stone
0.23 Low 34.3 MPa High Low
High strength
High-
expansion
stone
0.20 Low 48.0 MPa High High
Physical properties (cont.)
• Strength and hardness: Affected by
– Porosity
– Shape of particles (large irregular particles
don’t condense well leading to decreased
density).
– Amount of water mixed with the powder.
Higher porosity requires more water to be
mixed to produce a cast which will be weaker.
11
• Dimensional accuracy:
– The higher the setting expansion, the
lower the accuracy.
– Setting expansion results from the growth
of crystals as they join.
• Solubility:
– Set gypsum is not highly soluble
– The greater the porosity the greater the
solubility.
12
• Reproduction of detail:
– Greater porosity decreases surface detail
production
– The impression material should be compatible
with the gypsum to improve detail
reproduction. The best compatibility is
between silicon and polyethers and gypsum
13
Classification of gypsum
• Impression plaster (Type I) *seldom used*
• Model plaster (Type II): used for
– Diagnostic cast
– Articulation of stone cast
– Art portion of working cast
– Flasking procedure for acrylic dentures (cast end)
The mix produces a weak cast compared to dental stone. It’s available is fast and regular sets.
Impression (negative replica), poured
into gypsum to make a cast (positive
replica
14
• Dental stone (Type
III): used for making
– Full or partial denture
models
– Orthodontic models
– Flasking procedure for
acrylic dentures (teeth
end)
It requires less water,
stronger than plasterDental stone casts
15
• Dental stone, high strength
(Type IV):
– Known as die stone, used
in fabricating wax patterns
of cast restorations (crown
&bridge)
– Implants
– Precision attachments
work
– Often colored pink or
green
– Almost 2 times stronger
than type III stone
Die stone used in the fabrication
of cast crown restoration
16
17
• High strength, high expansion dental stone
(Type V)
– Colored blue or green
– Most costly of all gypsum materials
– Lowest W:P ratio, higher compressive
strength
– The need for higher expansion was to
compensate for the solidification shrinkage of
some alloys used as base metals used for
dental casting
Manipulation
• Selection: based on the desired properties
and dental application. e.g.:
– For a diagnostic cast, plaster can be used.
– For a working cast, strength is required and
accuracy, dental stone is the gypsum of
choice
– Working models for cast restorations require
die stone
18
Manipulation (cont.)
• Proportioning (W:P ratio)
– Golden rule: Manufacturer instructions
should always be followed.
– Variations in W:P ratio affect the set materials’
properties such as strength and accuracy.
• Too much water?
• Too little water?
19
Manipulation (cont.)
• Mixing:
– Manual: Rubber bowl and spatula. Powder is sifted into water to ensure good wetting and avoid clumps, and avoid air bubbles
20
Mechanical (also used with vacuum)
21
Manipulation (cont.)
• Initial setting time-working time
– Working time start after mixing for 1 minute
– Initial setting time: time elapsed from the start
of mix until loss of gloss (8-10 minutes)
– 6-10 minutes of working time are available to
pour the gypsum.
22
GYPSUMGYPSUMSetting MechanismSetting Mechanism
Dissolution of hemihydrateDissolution of hemihydrate
Precipitation of dihydratePrecipitation of dihydrate
23Crystal expansion and interlockingCrystal expansion and interlocking
MANIPULATIONMANIPULATIONof Gypsum Productsof Gypsum Products
Proportion P and LProportion P and L
Transfer to impressionTransfer to impression
Bulk PBulk P
Microstone
PrePre--packaged Ppackaged P
24
• Final setting time: is reached when the materials
can be safely handled, the gypsum is cool
(exothermic reaction is over).
• Setting expansion: Results from crystal growth
during setting. Can be decreased by the addition of
potassium sulfate, sodium chloride, borax.
• Hygroscopic expansion. If gypsum soaked during
setting, water fills pores and increases volume
• It’s recommended to separate the cast from
impression after 1 hour.
• Strength increases 2-3 times after 24 hours
25
Tests for initial setting, and final
setting times
• Loss of gloss test for initial setting time: loss of
gloss occurs as water is taken up by gypsum to
form the dihydrate. The materials does not have
measurable compressive strength.
• Initial Gillmore test for initial set: needles are
used to indent the material until no indentation
can be seen = initial setting time.
26
GYPSUMGYPSUMSetting StagesSetting Stages
MixingMixingTimeTime
00:000:00:000:00 00:000:01:001:00WorkingWorking
TimeTime00:000:07:007:00
SettingSettingTimeTime
00:00:10:0010:00
TIMETIME
MixingMixingIntervalInterval
WorkingWorkingIntervalInterval
SettingSettingIntervalInterval
Final Set =Final Set =Setting TimeSetting Time
Initial Set =Initial Set =Working TimeWorking Time
LOSS OF GLOSSLOSS OF GLOSS
LargeLargeGilmoreGilmoreNeedleNeedle
SmallSmallGilmoreGilmoreNeedleNeedle
27
A
• Vicat penetrometer (A) for
setting time: used for the next
stage of setting (refer to figure)
after loss of gloss, the plunger
rod is released onto the mix.
Time elapsed until the rod no
longer penetrates is the setting
time.
• Gillmore test (B) for final
setting time: a heavier
Gillmore needle to determine
final setting time.
BB
28
29
Ready for use criteria
30
• The ability to judge readiness of gypsum
to be handled improves with experience.
• Technically, the material is considered
ready when compressive strength reaches
80% of the strength attained after 1 hour.
• Most products are ready to use in 30
minutes.
• Clinical tip: before separating cast from
impression, ensure that no part of the tray
is connected to the gypsum
• Clinical tip: if alginate impression dried
before cast separation, soak in water for
15 minutes.
31
How to control setting time
1. Changing water : powder ratio
Increasing water Decreasing water
Retarded setting Shorter setting time
Weaker model or cast Mix difficult to manipulate
Inaccurate model Bubbles inclusion in mix
-------- Inaccurate model
32
2. Spatulation: rapid and prolonged
spatulation accelerates setting and also
increases setting expansion.
3. Temperature: increasing water
temperature to a certain level will
accelerate setting.
33
34
4. Accelerators and retarders:
– Manufacturers add accelerators or retarders
to gypsum. These chemicals increase or
decrease gypsum solubility respectively
which will alter setting time accordingly.
– Clinicians can add accelerators such as
potassium sulfate or set gypsum (slurry
water), they act as sites for crystallization.
– Setting reaction retarders: blood, saliva,
alginate. If left on impression, can affect
surface details of impression. Impression
surface need to be properly rinsed before
being poured.
• Pouring of the cast: the cast is composed of
two parts which are prepared separately
– The anatomical part (hard and soft tissue),
impression poured using a vibrator
– Art portion or base, which is important to aid in
handling and articulating the casts. Can be poured
in different ways:
35
1. Double-pour method
1 2
3
36
2. Single step
• Both anatomical and art portions of the
cast are prepared at the same time. This
method requires skill and accurate timing.
Difficulty encountered:
– If mix is too runny?
– If mix started to initially set?
37
3. Boxing method
38
• A strip of wax is fitted around the impression
then gypsum is poured. The wax border should
extend at least 0.5 inch above the highest point
of the impression.
Storage and clean up
• Gypsum should be stored in airtight dry
area. Prolonged exposure to moisture can
retard setting due to decreased solubility
of powder.
• Relevant equipment should be kept clean
to avoid unwanted acceleration of setting
by set gypsum.
39
Infection control
• Casts should have set for 24 hours before
being disinfected if necessary.
• Spray rather than immerse
• Disinfectants commonly used:
– Sodium hypochlorite
– iodophors
– Chlorine dioxide
40
Trimming
• Plaster bass are recommended since
trimming them is easier than dental stone.
• If base is made from stone, it should be
soaked in water for 5-10 minutes to soften
it before trimming. Important
considerations when trimming?
41
Trimming considerations
• Proportion of base to
anatomical part
• Parallelism
• Use of wax bite
registration
• Outer border of cast
• Shaping of anterior part
of upper and lower
arches
42
Other types of Die stone
• Metal plated die stone: silver or copper
plated to increase abrasion resistance
• Epoxy die stone: resin is used as a
hardener
• Resin reinforced die stone: resin is
incorporated into the gypsum material to
increase abrasion resistance.43
Investment materials
• Used to form metal
casting through the
lost wax technique
44
References
• Dental materials, clinical applications for
dental assistants and dental hygienists.
Chapter 12
• Phillips’ science of dental materials.
Chapter10
45