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

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GYPSUMGYPSUMProduct Use in DentistryProduct Use in Dentistry

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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

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• 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

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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

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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

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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

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Physical properties

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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.

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• 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.

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• 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

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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

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• 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

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• 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

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• 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

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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?

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Manipulation (cont.)

• Mixing:

– Manual: Rubber bowl and spatula. Powder is sifted into water to ensure good wetting and avoid clumps, and avoid air bubbles

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Mechanical (also used with vacuum)

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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.

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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

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• 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

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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.

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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

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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

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Ready for use criteria

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• 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.

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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

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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.

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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:

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1. Double-pour method

1 2

3

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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?

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3. Boxing method

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• 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.

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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

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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?

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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

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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

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References

• Dental materials, clinical applications for

dental assistants and dental hygienists.

Chapter 12

• Phillips’ science of dental materials.

Chapter10

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