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Ed. by Harsini,

drg 2010

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Dental Applications :

Gypsum products are used mainly formaking positive reproductions or

replicas of oral structures. Casts, dies, or models

Other uses: impression, mounting,

part of investment

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Artikulator mounting

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Desirable properties(1) Accuracy(2) Dimensional stability(3) Ability to reproduce fine detail

(4) Strength and resistance toabrasion(5) Compatibility with the impression

material(6) Color(7) Biological safety

(8) Ease of use

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Manufacture

Chemical and Physical

properties

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For dental purposes in form of

Calcium sulfate dihydrate(CaSO4.2H2O)

Gypsum is a common name of calcium sulfate dihydrate.

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Dental gypsum products

Primary constituent = Calcium sulfate

hemihydrate,CaSO4.H2O

From a process called CalcinationCalcination: heating process

performed by the manufacturer to

create the hemihydrate powder formof the product

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CaSO42H2O CaSO4H2O +100-130oC

Calcination

Dehydration

H2O

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This process is reversible.

CaSO4H2O+ H2o CaSO42H2O+

Rehydration

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Types of Dental Gypsum

Plaster

Stone

High-Strength or Improved Stone

Other types of gypsum: impression

plaster, mounting plaster,..etc

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Plaster

Manufacturedby grindingthe

gypsumrock toa fine powder

andthenheatingthe powder in

anopencontainer eliminate

water &shatter the crystal

Porous, irregular powder particles

Sometimes is referredtoas -

hemihydrate or Type II stone.

Weakest andleast expensive

Usually white incolor. Used

mainly where strengthis not a

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Made fromgypsumby carefully

controlledcalcinationunder steam

pressure inaclosedcontainer.

Powder particle ismore regular,

more uniforminshape, andlessporous

Is oftenreferredtoas -

hemihydrate, Type III stone, or

Hydrocal.

Various colors. Usedmainly for

makingcasts for diagnostic

purposes andcasts usedfor

complete andpartial denture

Stone

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High-Strengthor ImprovedStone

Made fromgypsumbycalciningthe gypsum

inacalciumchloride solution.

Very dense powder particlein cuboidal

shape witha reduce surface area

Strongest andmost expensive

Oftenreferredtoas Type IVstone,

die stone, densite, andmodifiedhemihydrate

Usedmainly for makingcasts or dies

for inlay andcrownfabrication

Anewly developedhigh-strength stone

withahigher compressive strength

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WorkingWorkingCast withCast withRemovableRemovableDiesDies

RemovableRemovableDie withDie with

Waxed InlayWaxed Inlay

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Forms of hemihydrate

-hemihydrate

(plaster of Paris)

Sponginess,

porous,

irregular

-hemihydrate

(dental stone)

Well-formedcrystals,

regular, more dense

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Setting Reaction

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Somethingtoremember!

All gypsumproducts set by the same reaction.

CaSO 4 H 2O+1H 2O CaSO 4 2H 2O+Heat

After calcinationprocess, some calciumsulfate

dihydrate remaininthe powder.

Bothhemi- anddihydrate are water soluble but

the hemihydrate is four times more soluble.

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Hemihydrate + water suspension

(fluidandworkable)

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Dihydrate

(supersaturated)

Dihydrate precipitate out in

formof needle-like crystals.

(some calciumsulfate dihydrate

remaining

inthe powder are

nuclei for precipitation.)

Solnnolonger sat. withhemihydrate

dissolution&precipitationor

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Formation of needle like crystals Mass

thickens

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Water/Powder ratio

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Theoretically, if 100 gof any hemihydrate were

combinedwith19 ml of water, all the hemihydrate wouldbe convertedtothe dihydrate.

(19 ml = sufficient water for 100 ghemihydrate toreact)

However inpractice, this amount of water will

not produce a mass that canbe manipulated

andpouredintoanimpression. (toodry and

crumbly mix) Therefore, inpractice, excess

water must be addedwhenone is mixingto

produce a workable mass .

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Various gypsumproducts require different amount of

water tomix.

- hemihydraterequireslesswater than hemihydrate hemihydrate=>Veryporous, irregular, donot pack together verywell

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Dependsonthe physical characteristics of the powder

particles

Plaster (Type II) 45-50 ml/100g(0.45-0.50)

Stone (Type III) 28-30 ml/100g(0.28-0.30)

Improvedstone(Type IV) 19-24 ml/100g(0.19-0.24)

different consistencies of aworkable mix

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The excess water does not react but is simply trappedin the mass whenit sets.

porosities or voids inthe mass

Set plaster has the lowest

density (most porous) because

it hadthe most excess water

andthe most voids inthe mass.

Set high-strengthstone has the

highest density.

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The water/powder ratiohas a direct effect onthe

properties of eachgypsumproduct andmust be

controlledfor optimumresults.

Higher W:Pratio(Thinner mix)

Longer settingtime

Weaker product (more free water more porositieswhenset)

Lower hardness

Reduceexpansion(inthesametypeof gypsum)

Lower W:Pratio(Thicker mix)

Broader crystals stronger andmoresolidmass

Shorter settingtime

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Setting Time

Types of settingtime

Factors tocontrol settingtime

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TIMETIME

GYPSUMGYPSUMSetting StagesSetting Stages

MixingMixingIntervalInterval

WorkingWorkingIntervalInterval

SettingSettingIntervalInterval

MixingMixing

TimeTime00:000:00:000:00 1:001:00

WorkingWorkingTimeTime

0707SettingSettingTimeTime

11:0011:00

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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Workingor Initial Setting

Time

Represents the available

time for manipulating

Indicates partial progress

of thesettingreaction

Loss of gloss

Vicat needle does not

completely penetrates.

8to16 minutes

Final SettingTime

Represents the timeuntil the

settingmass becomes rigidand

canbe separatedfromthe mold

Indicates the major completion

of the hydrationreaction

Failure of penetrationof a

fingernail or knife.

Dissipationof

the heat of reaction

30 to45 minutesStart mixing Material starts toset. Material sets.

Mixingtime

Workingtime/Initial settingtime(8-16min)

Final settingtime

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Control of the SettingTime

Controlledby the manufacturers particular

formulation

3 methods

Control solubility of the hemihydrate

Control number of nuclei for crystallization

Control rate of crystal growth

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Accelerators

Increase the solubility of the hemihydrate

without increasingthe solubility of the dihydrate

inorganic salts

SodiumChloride upto2%conc., Potassiumsulfate >2%

conc., Sodiumsulfate upto3.4%conc.

Powderedgypsum(CaSO4.H2O) or slurry of

groundgypsumcasts

Increase nuclei of crystallization

Increase spatulation(time andspeed)

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GYPSUMGYPSUMSetting MechanismSetting Mechanism

Dissolution of hemihydrateDissolution of hemihydrate

Precipitation of dihydratePrecipitation of dihydrate

Crystal expansion and interlockingCrystal expansion and interlocking

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Control of SettingExpansion

Expansioncanbe advantage or disadvantage. In

general, minimal settingexpansionis desirable.

accurate dimensional reproduction

Less W:Pratioand/or Longer mixingtime (within

practical limit) expansion (inthe same type of stone) Higher W:P

ratio

fewer nuclei/unit volume

space betweennuclei less growthinteractionof the dihydrate

crystals andless outwardthrust

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t

What will happenif

the settingprocess

occurs under water?

Expansionbecause additional

crystal growthis permitted

Hygroscopic SettingExpansion

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Hygroscopic SettingExpansionStageI: initial mix

StageII: crystalsof dihydrateare

formed

Water aroundtheparticlesis

reducedbythehydrationandthe

particlesaredrawnmoreclosely

together bythesurfacetension

actionof thewater

hydratedwater isreplaced

StageIII: crystalsgrow

Water isdecreased, particles

aredrawntogether

water isreplaced, crystalsgrow freely

StageIVandV

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Reproductionof Detail

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Reproductionof detail of animpression

depends onthe compatibility and

interactions betweenthe model material

andthe impressionmaterial.

The ability of the material toflowintoand

register finer detail andfiner lines is used

torate its detail of reproduction.

Factors: Compatibility + Wettability

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Tensile Strength

Important whenseparatingdies froman

impression.

Fromdiametral tensile tests, ingeneral:

1 hr strengthis 1/2 the dry strength.

Plasters tensile strengthis 1/2 that of die

stone.

Tensile strengths are usually 1/5 the

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MixingThe preferredmethodof

mixingis toaddthe measured

water first, followedby gradual

additionof the powder.

The use of anautomatic

vibrator, of highfrequency and lowamplitude, is helpful.

The guessworkof repeatedly

addingwater andpowder to

achieve theproper consistency

must be avoided

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Pouringthe Model

Themixedgypsumis placedinto

theimpressioninincrements,

withuse of vibrationtoenhance

theflowof the material intothe

impression.

Allowthematerial toflowacross

theimpression, withavoidance

of entrapment of air.

Alternatively, the teethandsoft tissues may be

pouredinstone or die stone andallowedtoset, and

the base canbe made fromplaster later.

The impressionshouldnot be removedfromthe

gypsumfor 45 to60 minutes toallowthe final set to

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MANIPULATIONMANIPULATIONof Gypsum Productsof Gypsum Products

Proportion P and LProportion P and L

Transfer to impressionTransfer to impression

Bulk PBulk P

Microstone

Pre-packaged PPre-packaged P

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Desirable Properties

Types of gypsum: Plaster, Stone, Improved stone

SettingReaction

W:PRatio

SettingTime

SettingExpansion, Hygroscopic Expansion Reproductionof Detail

StrengthandHardness

Summary