Gypsum / dental lab technology courses

GYPSUM PRODUCTSGYPSUM PRODUCTS
INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com
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IntroductionIntroduction
Desirable PropertiesDesirable Properties
Chemical & Physical nature of gypsum productsChemical & Physical nature of gypsum products
Manufacture of dental plaster & stoneManufacture of dental plaster & stone
Types of gypsum productsTypes of gypsum products
Setting ReactionSetting Reaction
Theories of Setting ReactionTheories of Setting Reaction
Setting TimeSetting Time
Control of Setting TimeControl of Setting Time
Setting ExpansionSetting Expansion
Control of Setting ExpansionControl of Setting Expansion
Hygroscopic Setting ExpansionHygroscopic Setting Expansion
StrengthStrength
Surface Hardness & Abrasion ResistanceSurface Hardness & Abrasion Resistance
ViscosityViscosity
Reproduction of DetailsReproduction of Details
Proportioning, Mixing & Caring for Gypsum ProductsProportioning, Mixing & Caring for Gypsum Products
Special gypsum productsSpecial gypsum productswww.indiandentalacademy.comwww.indiandentalacademy.com

INTRODUCTIONINTRODUCTION
Gypsum is a naturally occurring whiteGypsum is a naturally occurring white
powderypowdery mineralmineral mined in various parts of themined in various parts of the
world, with chemical name calcium sulfateworld, with chemical name calcium sulfate
dihydrate ( CaSOdihydrate ( CaSO44.2H.2H22O ).O ).

FORMSFORMS
1. ROCK :- Dull colored rock.1. ROCK :- Dull colored rock.
2. ALABLASTER2. ALABLASTER :- Fine grained variety.:- Fine grained variety.
3. GYPCRETE (gypcrust) :- Hard layer3. GYPCRETE (gypcrust) :- Hard layer
formed on soil.formed on soil.
4. SELENITE4. SELENITE
5. SATIN SPUR :- Fibrous with silky luster.5. SATIN SPUR :- Fibrous with silky luster.

USESUSES
1. For construction purposes.1. For construction purposes.
2. Used in industry for making pottery, moulds etc.2. Used in industry for making pottery, moulds etc.
3. Used in orthopedics to make plaster casts.3. Used in orthopedics to make plaster casts.
APPLICATION IN DENTISTRYAPPLICATION IN DENTISTRY
1. For cast preparation.1. For cast preparation.
2. Models and dies.2. Models and dies.
3. Impression Material.3. Impression Material.
4. As Investment Material.4. As Investment Material.
5. Mounting of Casts.5. Mounting of Casts.
6. As a mold material for processing of complete dentures.6. As a mold material for processing of complete dentures.

CHEMICAL & PHYSICAL NATURE OFCHEMICAL & PHYSICAL NATURE OF
GYPSUM PRODUCTSGYPSUM PRODUCTS
Gypsum – Calcium Sulphate Dihydrate
CaSO4. 2H2O on heating CaSO4. ½H2O + 1½H2O
(Gypsum) (Calcium Sulphate (water)
Hemi hydrate)
CaSO4.½H2O + 1½H2O CaSO4. 2H2O + 3900
(plaster of paris) (water) (gypsum) (cal)
Reaction is exothermic

MANUFACTURE OF DENTAL PLASTER &MANUFACTURE OF DENTAL PLASTER &
STONESTONE
Formed by calcining of gypsumFormed by calcining of gypsum
110-130ºC 130-200ºC 200-1000ºC110-130ºC 130-200ºC 200-1000ºC
CaSOCaSO44. 2H. 2H22O (CaSOO (CaSO44).1/2 H).1/2 H22O CaSOO CaSO44 CaSOCaSO44
Gypsum plaster/stone (hexagonal (ortho-rhombicGypsum plaster/stone (hexagonal (ortho-rhombic
(Calcium Sulphate (Calcium Sulphate anhydrite) anhydrate)(Calcium Sulphate (Calcium Sulphate anhydrite) anhydrate)
Dihydrate) HemihydrateDihydrate) Hemihydrate
α or β)α or β)

β-hemihydrateβ-hemihydrate((plasterplaster)-)-
- Gypsum is heated in a kettle, vat or rotatory kiln open- Gypsum is heated in a kettle, vat or rotatory kiln open
to air.to air.
- Crystals – spongy & irregular.- Crystals – spongy & irregular.
Powder particles of
plaster of paris
Scanning electron micrograph of
dental plaster powder particles

α-hemihydrate (stone)-α-hemihydrate (stone)-
- Gypsum is- Gypsum is heated to 125heated to 12500
C under steam pressure in anC under steam pressure in an
autoclave or boiled in a solution of CaClautoclave or boiled in a solution of CaCl22 ..
- Crystals- more dense & prismatic.- Crystals- more dense & prismatic.
Powder particles of
dental stone
Scanning electron micrograph of
dental stone powder particles

DESIRABLE PROPERTIES:-DESIRABLE PROPERTIES:-
1. Accuracy1. Accuracy
2. Dimensional Stability2. Dimensional Stability
3. Ability to reproduce fine detail.3. Ability to reproduce fine detail.
4. Strength & resistance to abrasion.4. Strength & resistance to abrasion.
5. Compatibility with the impression materials.5. Compatibility with the impression materials.
6. Color6. Color
7. Biological safety7. Biological safety
8. Ease of use8. Ease of use
9. Cost.9. Cost.

HYDRATES OF CALCIUM SULPHATEHYDRATES OF CALCIUM SULPHATE
Mineral source By product of otherMineral source By product of other
IndustriesIndustries
Calcium Sulphate dihydrate (Gypsum)Calcium Sulphate dihydrate (Gypsum)
Heat in an open Heat in autoclave Heat ground gypsum Heat in a boilingHeat in an open Heat in autoclave Heat ground gypsum Heat in a boiling
vessel at120ºC under steam in H2O with small aqueous solution 30%vessel at120ºC under steam in H2O with small aqueous solution 30%
pressure, 120-130ºC quantity of organic of CaClpressure, 120-130ºC quantity of organic of CaCl22/ MgCl/ MgCl22
acid or salt, in anacid or salt, in an
Autoclave, 140ºCAutoclave, 140ºC
Calcined CaSOCalcined CaSO44 Autoclaved CaSOAutoclaved CaSO44 Autoclaved CaSOAutoclaved CaSO44 CaSOCaSO44 hemihydratehemihydrate
hemihydrate hemihydrate hemihydrate ( DENSITE)hemihydrate hemihydrate hemihydrate ( DENSITE)
(β- hemihydrate) (HYDROCAL or (α- hemihydrate)(β- hemihydrate) (HYDROCAL or (α- hemihydrate)
α- hemihydrate)α- hemihydrate)
Heat < 200º CHeat < 200º C
Hexagonal CaSOHexagonal CaSO44 (sometimes called ‘soluble anhydrite’)(sometimes called ‘soluble anhydrite’)
CaSOCaSO44
Heat > 200º CHeat > 200º C
Ortho-rhombic CaSOOrtho-rhombic CaSO44
(sometimes called ‘insoluble anhydrite’)(sometimes called ‘insoluble anhydrite’)
Calcium sulphate anhydrite

TYPES OF GYPSUM PRODUCTSTYPES OF GYPSUM PRODUCTS
Classification-Classification-
According toAccording to ADA specification No.25-ADA specification No.25-
Type 1- Impression plasterType 1- Impression plaster
Type 2- Lab or model plasterType 2- Lab or model plaster
Type 3- Dental stone ordinary or model stoneType 3- Dental stone ordinary or model stone
Type 4- Dental stone with high strength and low expansionType 4- Dental stone with high strength and low expansion
Type 5-Dental stone with high strength and high expansionType 5-Dental stone with high strength and high expansion
Type 4 and type 5 dental stones are known as die stonesType 4 and type 5 dental stones are known as die stones

IMPRESSION PLASTER Type 1IMPRESSION PLASTER Type 1
--They are composed of Plaster of Paris to which modifiers have been added toThey are composed of Plaster of Paris to which modifiers have been added to
regulate the setting time & setting expansion. Crystalline form of hemihydrate isregulate the setting time & setting expansion. Crystalline form of hemihydrate is
known as beta-hemihydrate. The particles are irregular in shape and porous. W/Pknown as beta-hemihydrate. The particles are irregular in shape and porous. W/P
ratio is 0.5 i.e 50ml/100gm.ratio is 0.5 i.e 50ml/100gm.
- Hemihydrate plaster is produced by dry calcination methods and are powders- Hemihydrate plaster is produced by dry calcination methods and are powders
with a low apparent density. A relative high surface area and poor packingwith a low apparent density. A relative high surface area and poor packing
ability.ability.
- Microscopically the beta form is a fibrous aggregate of fine crystals with- Microscopically the beta form is a fibrous aggregate of fine crystals with
capillary pores.capillary pores.
-- ModifiersModifiers such as:-such as:-
-Potassium sulphate-Potassium sulphate
-Borax-Borax
-- Advantage:Advantage: - Records excellent fine details.- Records excellent fine details.
-- Disadvantages: -Disadvantages: - Small dimensional changes.Small dimensional changes.
Fracture on removal from undercutsFracture on removal from undercuts
Separating media is requiredSeparating media is requiredwww.indiandentalacademy.comwww.indiandentalacademy.com

2. MODEL PLASTER2. MODEL PLASTER
( Type 2):-( Type 2):-
- β – hemi hydrate- β – hemi hydrate
- Powder particles are porous & irregular.- Powder particles are porous & irregular.
-It is usually white in color.-It is usually white in color.
-- Use: -Use: - For primary cast for complete dentures.For primary cast for complete dentures.
- For articulation purposes.- For articulation purposes.
- For flasking in denture construction.- For flasking in denture construction.
-- Advantage: -Advantage: - InexpensiveInexpensive
- Disadvantage: -- Disadvantage: - Low strengthLow strength
High PorosityHigh Porosity

3. DENTAL STONE (Type 3):-3. DENTAL STONE (Type 3):-
Medium strength stoneMedium strength stone
- Discovered in 1930- Discovered in 1930
- Gypsum is calcined under steam pressure in an autoclave at a- Gypsum is calcined under steam pressure in an autoclave at a
temperature of 120 C to 130 C for 5-7 hours.temperature of 120 C to 130 C for 5-7 hours.
- α – hemihydrate or Hydrocal- α – hemihydrate or Hydrocal
- Powder particles are more uniform, dense & prismatic in shape.- Powder particles are more uniform, dense & prismatic in shape.
- Comes in different colors like yellow, green, etc.- Comes in different colors like yellow, green, etc.
- Accelerator – K- Accelerator – K22SOSO44
- Retarder – Sodium Citrate- Retarder – Sodium Citrate
-- Uses: -Uses: - Making casts for diagnostic purposes & forMaking casts for diagnostic purposes & for
complete or partial denture construction.complete or partial denture construction.
-- Advantages: -Advantages: - Greater strength & surface hardnessGreater strength & surface hardness
-- Disadvantage: -Disadvantage: - More expensive than plaster.More expensive than plaster.

4. DENTAL STONE,4. DENTAL STONE, HIGHHIGH
STRENGTH (Type 4):-STRENGTH (Type 4):-Die stoneDie stone
- Modified α – hemi hydrate, Densite or Die stone.Modified α – hemi hydrate, Densite or Die stone.
- Gypsum is calcined by boiling it in 30%calcium chloride orGypsum is calcined by boiling it in 30%calcium chloride or
MgClMgCl22 and autoclaved in presence of sodium succinate 0.5%and autoclaved in presence of sodium succinate 0.5%
- Powder particles are very dense, cuboidal in shape &- Powder particles are very dense, cuboidal in shape &
has reduced surface area.has reduced surface area.
-- Use: -Use: - For making casts or dies for crown, bridge &For making casts or dies for crown, bridge &
Inlay fabrication.Inlay fabrication.
-- AdvantagesAdvantages -High strength-High strength
-Surface hardness-Surface hardness
-Abrasion resistant-Abrasion resistant
-Minimum setting expansion-Minimum setting expansion

5. Dental stone ,5. Dental stone , High strength,High strength,
High expansion (Type v);High expansion (Type v);
-- Most recent gypsum productMost recent gypsum product
- Higher compressive strength than Type 4- Higher compressive strength than Type 4
- Lower the W/P ratio more strength is- Lower the W/P ratio more strength is
attainableattainable
- Higher setting expansion- Higher setting expansion
- They are expensive, easy to use and- They are expensive, easy to use and
compatible to all impression materials.compatible to all impression materials.

SETTING REACTIONSETTING REACTION
(CaSO(CaSO44))22.1/2H.1/2H22O + 3HO + 3H22O 2CaSOO 2CaSO44.2H.2H22O + Heat (3900 cal)O + Heat (3900 cal)

THEORIES FOR SETTINGTHEORIES FOR SETTING
REACTION:-REACTION:-
1. Colloidal/ Gel Theory.1. Colloidal/ Gel Theory.
2. Hydration Theory.2. Hydration Theory.
3. Crystalline Theory3. Crystalline Theory
4. Dissolution- precipitation Theory.4. Dissolution- precipitation Theory.

Colloidal/ Gel Theory:-Colloidal/ Gel Theory:-
-- Originated in 1893 by M. Michaelis.Originated in 1893 by M. Michaelis.
- When plaster is mixed with water, plaster enters- When plaster is mixed with water, plaster enters
into the colloidal state through a sol- gelinto the colloidal state through a sol- gel
mechanism.mechanism.
- In the sol state, hemi hydrate particles are hydrated- In the sol state, hemi hydrate particles are hydrated
to form dihydrate, thereby entering into an activeto form dihydrate, thereby entering into an active
state.state.
- As the measured amount of water is consumed, the- As the measured amount of water is consumed, the
mass converts to a solid gel.mass converts to a solid gel.www.indiandentalacademy.comwww.indiandentalacademy.com

Crystalline Theory:-Crystalline Theory:-
-- Originated in 1887 by Henry Louis Le ChatelierOriginated in 1887 by Henry Louis Le Chatelier
- In 1907, supported by Jacobus Hendricus van’t Hoff - TheIn 1907, supported by Jacobus Hendricus van’t Hoff - The
difference in the solubilities of calcium sulphate dihydrate &difference in the solubilities of calcium sulphate dihydrate &
hemi hydrate causes setting differences. -hemi hydrate causes setting differences. -
Dissolved CaSO4 precipitates as calcium sulphate dihydrate,Dissolved CaSO4 precipitates as calcium sulphate dihydrate,
since it is less soluble than hemi hydrate. - X-raysince it is less soluble than hemi hydrate. - X-ray
diffraction studies – not all hemi hydrate is converts todiffraction studies – not all hemi hydrate is converts to
dihydrate.dihydrate.
In a setting mass of plaster 2 types of centers are there:-In a setting mass of plaster 2 types of centers are there:-
a) Dissolution center- around CaSO4 hemi hydratea) Dissolution center- around CaSO4 hemi hydrate
b) Precipitation center- around CaSO4 dihydrateb) Precipitation center- around CaSO4 dihydrate

Dissolution- PrecipitationDissolution- Precipitation
Theory:-Theory:-
Based on dissolution of plaster & instant re-crystallization of gypsumBased on dissolution of plaster & instant re-crystallization of gypsum
interlocking of crystalsinterlocking of crystals
The Setting Reaction is as follows:-The Setting Reaction is as follows:-
1. When the hemi hydrate is mixed with water, a suspension is formed1. When the hemi hydrate is mixed with water, a suspension is formed
that is fluid & workable.that is fluid & workable.
2. The hemi hydrate dissolves until it forms a saturated solution.2. The hemi hydrate dissolves until it forms a saturated solution.
3. This saturated solution of hemi hydrate, supersaturated in dihydrate,3. This saturated solution of hemi hydrate, supersaturated in dihydrate,
precipitates out dihydrate.precipitates out dihydrate.
4. As the dihydrate precipitates, the solution is no longer saturated with4. As the dihydrate precipitates, the solution is no longer saturated with
the hemi hydrate, so it continues to dissolve. Dissolution of hemithe hemi hydrate, so it continues to dissolve. Dissolution of hemi
hydrate & precipitation of dihydrate as either new crystals or furtherhydrate & precipitation of dihydrate as either new crystals or further
growth to the already present. The reaction continues until no furthergrowth to the already present. The reaction continues until no further
dihydrate precipitates out of solution.dihydrate precipitates out of solution.
The reaction rate is followed by the exothermic heat evolved as shown inThe reaction rate is followed by the exothermic heat evolved as shown in
fig.fig. www.indiandentalacademy.comwww.indiandentalacademy.com

Stages During CrystallizationStages During Crystallization

DissolutionDissolution

SuspensionSuspension

SaturationSaturation

Super saturationSuper saturation

Nuclei formationNuclei formation

Growth of nucleiGrowth of nuclei

CrystallizationCrystallization

Spherulite formationSpherulite formation

The particles or crystals of hemi hydrate are first dissolved in water toThe particles or crystals of hemi hydrate are first dissolved in water to
form a suspension that soon becomes saturatedform a suspension that soon becomes saturated
In this saturated solution nuclei of crystallization appear and crystalsIn this saturated solution nuclei of crystallization appear and crystals
of dihydrate begin to form until time the whole mass becomesof dihydrate begin to form until time the whole mass becomes
supersaturated with dihydrate.supersaturated with dihydrate.
Since dihydrate is less soluble than hemi hydrate, the dihydrateSince dihydrate is less soluble than hemi hydrate, the dihydrate
crystals precipitate out leaving the remaining solution to dissolvecrystals precipitate out leaving the remaining solution to dissolve
more hemi hydrate crystals into it.more hemi hydrate crystals into it.
The precipitated dihydrate crystals begin to grow in size throughoutThe precipitated dihydrate crystals begin to grow in size throughout
the mix as needle like crystals.the mix as needle like crystals.
This process is repeated until all the hemi hydrate is converted toThis process is repeated until all the hemi hydrate is converted to
dihydrate.dihydrate.
The needle like crystal grow long, intermesh and intertwine with eachThe needle like crystal grow long, intermesh and intertwine with each
other. This stage is called Spherulite formation- and it indicates thatother. This stage is called Spherulite formation- and it indicates that
material has set.material has set. www.indiandentalacademy.comwww.indiandentalacademy.com

Clusters of needle like
crystals
Dark- field photomicrograph
of set plaster

SETTING TIMESETTING TIME
Measured by “Penetration Test” (time taken from the start of mix until theMeasured by “Penetration Test” (time taken from the start of mix until the
needle no longer penetrates to the bottom) with the help of Vicat &needle no longer penetrates to the bottom) with the help of Vicat &
Gillmore needles.Gillmore needles.
There are number of stages in the setting of a gypsum product:-There are number of stages in the setting of a gypsum product:-
MIXING TIME:-MIXING TIME:- The time from the addition of powder to the waterThe time from the addition of powder to the water
until the mixing is completed.until the mixing is completed.
Mechanical mixing – 20-30 secs.Mechanical mixing – 20-30 secs.
Hand spatulation - 1 min.Hand spatulation - 1 min.
WORKING TIME: -WORKING TIME: - The time available to use a workable mix.The time available to use a workable mix.
Working time – 3min.Working time – 3min.

VICAT
NEEDLE
SET OF GILLMORE NEEDLES

-- LOSS OF GLOSS TEST FOR INITIAL SET:-LOSS OF GLOSS TEST FOR INITIAL SET:- Some of the excess water isSome of the excess water is
taken by forming the dihydrate so that the mix loses its gloss.taken by forming the dihydrate so that the mix loses its gloss.
-- INITIAL GILLMORE TEST FOR INITIAL SET:-INITIAL GILLMORE TEST FOR INITIAL SET:- The mixture is spread out, &The mixture is spread out, &
the needle is lowered onto the surface. The time at which it no longer leaves anthe needle is lowered onto the surface. The time at which it no longer leaves an
impression is called the “Initial Set.”impression is called the “Initial Set.”
This is marked by a definite increase in strength.This is marked by a definite increase in strength.
- VICAT TEST FOR SETTING TIME: -- VICAT TEST FOR SETTING TIME: - In thisIn this Vicat Penetrometer is used.Vicat Penetrometer is used.
The needle with a weighed plunger rod is supported & held just in contact with theThe needle with a weighed plunger rod is supported & held just in contact with the
mix. After the gloss is lost, the plunger is released.mix. After the gloss is lost, the plunger is released.
The time elapsed until the needle no longer penetrates to the bottom of the mix isThe time elapsed until the needle no longer penetrates to the bottom of the mix is
known as the “Setting Time.”known as the “Setting Time.”
- GILLMORE TEST FOR FINAL SETTING TIME:-- GILLMORE TEST FOR FINAL SETTING TIME:-
Heavier Gillmore Needle is used.Heavier Gillmore Needle is used.
The time elapsed at which this needle leaves only a barely perceptible mark on theThe time elapsed at which this needle leaves only a barely perceptible mark on the
surface is called the “surface is called the “Final Setting TimeFinal Setting Time.”.”
- READY- FOR- USE CRITERION:-- READY- FOR- USE CRITERION:- The subjective measure of the time atThe subjective measure of the time at
which the set material may be safely handled in the usual manner.which the set material may be safely handled in the usual manner.
Ready for use state is reached in approx. 30 min.Ready for use state is reached in approx. 30 min.

CONTROL OF SETTING TIMECONTROL OF SETTING TIME
TemperatureTemperature
W:P ratioW:P ratio
phph
FinenessFineness
Modifiers, Accelerators and RetardersModifiers, Accelerators and Retarders
Mixing and spatulationMixing and spatulation
Manufacturing processManufacturing process

SETTING EXPANSIONSETTING EXPANSION
-- Expansion may vary from 0.06% to 0.5%Expansion may vary from 0.06% to 0.5%
- Volume of dihydrate formed is less than equal to- Volume of dihydrate formed is less than equal to
the volume of hemi hydrate & water i.e. actually athe volume of hemi hydrate & water i.e. actually a
volumetric contraction should occur during settingvolumetric contraction should occur during setting
reaction, but instead a setting expansion is observed.reaction, but instead a setting expansion is observed.
PHENOMENON:-PHENOMENON:- Based on crystallizationBased on crystallization
mechanism.mechanism.
RESULT:-RESULT:-
The set material must be porous.The set material must be porous.

CONTROL OF SETTING EXPANSIONCONTROL OF SETTING EXPANSION
1. Increased spatulation increases setting expansion1. Increased spatulation increases setting expansion
2. Increase in W: P ratio reduces the setting expansion2. Increase in W: P ratio reduces the setting expansion
3. Accelerators & Retarders:-3. Accelerators & Retarders:-
Accelerators:-Accelerators:-
- Sodium Chloride (up to 2% of hemi hydrate)- Sodium Chloride (up to 2% of hemi hydrate)
- Sodium Sulfate (max. effect at 3.4%)- Sodium Sulfate (max. effect at 3.4%)
- Potassium Sulfate (>2%)- Potassium Sulfate (>2%)
- Potassium Tartrate- Potassium Tartrate
Retarders: -Retarders: - Citrates, acetates & boratesCitrates, acetates & borates

HYGROSCOPIC SETTINGHYGROSCOPIC SETTING
EXPANSIONEXPANSION
Setting expansion that occurs under water isSetting expansion that occurs under water is
called as “Hygroscopic Setting Expansion.”called as “Hygroscopic Setting Expansion.”
Setting expansion without water immersion isSetting expansion without water immersion is
called “Normal Setting Expansion.”called “Normal Setting Expansion.”

Stages of Hygroscopic settingStages of Hygroscopic setting
expansionexpansion:-:-
STRENGTH:STRENGTH: The greater the amount of free water, the lessThe greater the amount of free water, the less
the strength.the strength.
There are 2 Strength properties of gypsumThere are 2 Strength properties of gypsum
--
1.1. WET STRENGTH (Green Strength):-WET STRENGTH (Green Strength):-Strength obtainedStrength obtained
when the water in excess of that required for hydration ofwhen the water in excess of that required for hydration of
the hemi hydrate is left in the test specimen.the hemi hydrate is left in the test specimen.
2.2. DRY STRENGTH: -DRY STRENGTH: - Strength obtained when the excessStrength obtained when the excess
water in the specimen has been driven off by drying.water in the specimen has been driven off by drying.
Strength depends upon:-Strength depends upon:-www.indiandentalacademy.comwww.indiandentalacademy.com

SURFACE HARDNESS & ABRASIONSURFACE HARDNESS & ABRASION
RESISTANCERESISTANCE
Surface hardness of gypsum materials is related toSurface hardness of gypsum materials is related to
their compressive strength.their compressive strength.
Surface hardness increases at a faster rate than theSurface hardness increases at a faster rate than the
compressive strength.compressive strength.
Abrasive Resistance of gypsum product is increasedAbrasive Resistance of gypsum product is increased
by impregnating the set gypsum with epoxy resins.by impregnating the set gypsum with epoxy resins.
Surface hardness of set gypsum is improved bySurface hardness of set gypsum is improved by
mixing stone with a hardening solution containingmixing stone with a hardening solution containing
colloidal silica (about 30%)colloidal silica (about 30%)

VISCOSITY-VISCOSITY-It is the resistance of a fluid to flow.It is the resistance of a fluid to flow.
REPRODUCTION OF DETAILREPRODUCTION OF DETAIL
Gypsum dies do not reproduce surface detail as wellGypsum dies do not reproduce surface detail as well
as electroformed or epoxy dies.as electroformed or epoxy dies.
Air bubbles are formed at the interface of impressionAir bubbles are formed at the interface of impression
& gypsum cast.& gypsum cast.
Contamination of the impression (by saliva orContamination of the impression (by saliva or
blood) in which the gypsum die is poured can alsoblood) in which the gypsum die is poured can also
affect the detail reproduction.affect the detail reproduction.

PROPORTIONING, MIXING & CARINGPROPORTIONING, MIXING & CARING
FOR GYPSUM PRODUCTSFOR GYPSUM PRODUCTS
--PROPORTIONING:-PROPORTIONING:- Strength of a stone isStrength of a stone is
inversely proportional to the W:P ratio.inversely proportional to the W:P ratio.
--MIXING:-MIXING:-
Trapping of air should be avoided while mixing toTrapping of air should be avoided while mixing to
avoid porosity weak spots & surface inaccuracies.avoid porosity weak spots & surface inaccuracies.
Longer spatulation decreases the working timeLonger spatulation decreases the working time
-- Method of mixing:-Method of mixing:-
Add measured amount of water.Add measured amount of water.
GradualGradualwww.indiandentalacademy.comwww.indiandentalacademy.com

CARING FOR THE CAST:-CARING FOR THE CAST:-
Once the setting reaction in the cast is completed,Once the setting reaction in the cast is completed,
its dimensions remain constant under roomits dimensions remain constant under room
temperature & humidity.temperature & humidity.
If stone cast is immersed in running water, its linearIf stone cast is immersed in running water, its linear
dimensions may vary 0.1% for every 20min. ofdimensions may vary 0.1% for every 20min. of
immersion.immersion.
If storage temperature is raised from 90ºC to110ºCIf storage temperature is raised from 90ºC to110ºC
– shrinkage occurs– shrinkage occurs

CARING FOR GYPSUMCARING FOR GYPSUM
PRODUCT:-PRODUCT:-
- All types of gypsum products should be stored in aAll types of gypsum products should be stored in a
dry atmosphere.dry atmosphere.
- Products should be sealed in a moisture proof metal- Products should be sealed in a moisture proof metal
container.container.

INFECTION CONTROL:-INFECTION CONTROL:-
If an impression has not been disinfected, it isIf an impression has not been disinfected, it is
necessary to disinfect the stone cast.necessary to disinfect the stone cast.
Disinfection solutions that do not adversely affectDisinfection solutions that do not adversely affect
the quality of the gypsum product can be used.the quality of the gypsum product can be used.
Dental stone containing a disinfectant may also beDental stone containing a disinfectant may also be
used.used.
Useful disinfectants for stone casts include sprayUseful disinfectants for stone casts include spray
disinfectants, hypochlorites, & iodophores.disinfectants, hypochlorites, & iodophores.

Difference between dental plaster andDifference between dental plaster and
dental stone.dental stone.
ManufactureManufacture –– dental stone by wet calcinations and dental plaster bydental stone by wet calcinations and dental plaster by
dry calcinations.dry calcinations.
Particle shape and size-Particle shape and size- dental plaster- particles are larger, irregulardental plaster- particles are larger, irregular
and porous while dental stone – particles are smaller, regular and non-and porous while dental stone – particles are smaller, regular and non-
porous.porous.
W/P ratioW/P ratio—— Dental plaster- 0.5 (50ml/100gm)Dental plaster- 0.5 (50ml/100gm)
Dental stone – 0.3 (30ml/100gm)Dental stone – 0.3 (30ml/100gm)
Mechanical properties of set material-Mechanical properties of set material- the strength and hardnessthe strength and hardness
depends on the density of the material, thus stone is harder anddepends on the density of the material, thus stone is harder and
stronger than plaster.stronger than plaster.
Application-Application- when strength and hardness are required, stone iswhen strength and hardness are required, stone is
material of choice e.g. denture construction, dies for crowns, bridgesmaterial of choice e.g. denture construction, dies for crowns, bridges
and inlay work. When mechanical properties is not of primaryand inlay work. When mechanical properties is not of primary
importance plaster can be used e.g. mounting models, orthodonticimportance plaster can be used e.g. mounting models, orthodontic
study models.study models. www.indiandentalacademy.comwww.indiandentalacademy.com

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Gypsum / dental lab technology courses

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