Wednesday, August 28, 2013
• Desirable Properties
• Chemical & Physical nature of gypsum products
• Manufacture of dental plaster & stone
• Types of gypsum products
- Impression Plaster
- Model Plaster
- Dental Stone
- Dental Stone, High Strength
- Dental Stone, High Strength, High Expansion
- Synthetic Gypsum
• Setting Reaction
• Theories of Setting Reaction
• Setting Time
• Control of Setting Time
• Setting Expansion
• Control of Setting Expansion
• Hygroscopic Setting Expansion
• Surface Hardness & Abrasion Resistance
• Reproduction of Details
• Proportioning, Mixing & Caring for Gypsum Products
- Gypsum is a naturally occurring white powdery mineral mined in various parts of the world, with chemical name calcium sulfate dihydrate ( CaSO4.2H2O ).
- Gypsum is derived from a greek word “Gypsas” (chalk).
1. ROCK: - Dull colored rock.
2. ALABLASTER: - Fine grained variety.
3. GYPCRETE (gypcrust): - Hard layer formed on soil.
5. SATIN SPUR: - Fibrous with silky luster.
1. For construction purposes.
2. Used in industry for making pottery, moulds etc.
3. Used in orthopedics to make plaster casts.
APPLICATION IN DENTISTRY
1. For cast preparation.
2. Models and dies.
3. Impression Material.
4. Investment Material.
5. Mounting of Casts.
6. As a mold material for processing of complete dentures.
2. Dimensional Stability
3. Ability to reproduce fine detail.
4. Strength & resistance to abrasion.
5. Compatibility with the impression materials.
7. Biological safety
8. Ease of use
- As gypsum is dihydrate form of calcium sulphate (CaSO4.2H2O), on heating, it
loses 1.5gm mol of its 2gm mol of water & is converted to calcium sulphate
CaSO4. 2H2O on heating CaSO4. ½H2O + 1½H2O (1)
(gypsum) (calcium sulphate (water)
- When calcium sulphate hemihydrate is mixed with water, the reverse reaction
takes place & calcium sulphate hemihydrate is converted back to calcium
CaSO4.½H2O + 1½H2O CaSO4. 2H2O + 3900 (2)
(plaster of paris) (water) (gypsum) (cal)
- Reaction is exothermic.
MANUFACTURE OF DENTAL PLASTER & STONE
- Formed by calcining of gypsum.
- Gypsum is ground & subject to heat 110°C - 130 °C to drive off a part of water
- As the temperature is raised further the remaining water of crystallization is also
removed & products are formed.
CaSO4 . 2H2O 110-130ºC(CaSO4)2 H2O130-200ºCCaSO4200-1000ºCCaSO4
gypsum plaster/stone hexagonal ortho-
(calcium sulphate (calcium sulphate anhydrite rhombic
dihydrate) hemihydrate) anhydrite
- Gypsum is heated in a kettle, vat or rotatory kiln open to air.
- Crystals – spongy & irregular.
- Gypsum is heated to 125ºc under steam pressure in an autoclave or boiled in a
solution of CaCl2 .
- Crystals- more dense & prismatic.
HYDRATES OF CALCIUM SULPHATE
Mineral source By product of other industries
Calcium Sulfate Dihydrate( gypsum)
Heat in an open Heat in autoclave Heat ground gypsum Heat in boiling30%
vessel, 120ºc under steam in H2O with small aqueous solution of
pressure, 120-130ºc quantity, organic CaCl2/ MgCl2
acid or salt, in an
Calcined CaSO4 Autoclaved CaSO4 Autoclaved CaSO4 CaSO4 hemihydrate
hemihydrate hemihydrate hemihydrate ( DENSITE)
(β- hemihydrate) (HYDROCAL or (α- hemihydrate)
1. Depending on the method of calcination:-
- Dental plaster or β- hemihydrate
- Dental stone or α- hemihydrate or hydrocal
- Dental stone, high strength or densite
2. Other Gypsum Products:-
- Impression plaster
- Dental Investments:-
a) Gypsum bonded investments
b) Phosphate bonded investments
c) Silica bonded investments
1. IMPRESSION PLASTER(Type 1)-
- They are composed of Plaster of Paris to which modifiers have been added to
regulate setting time & setting expansion.
- Impression plaster is rarely used any more for dental impression because it has
been replaced by less rigid materials, such as hydrocolloids & elastomers.
- Modifiers such as:-
- Potassium sulphate – decreases setting expansion so as to prevent warpage
of impression & also decreases setting time drastically.
- Advantage:- Records excellent fine details.
- Disadvantages:- Small dimensional changes.
Fracture on removal from undercuts
Separating media is required
Non toxic but causes dryness
2. MODEL PLASTER ( Type 2):-
- β – hemihydrate
- Powder particles are porous & irregular.
- It is usually white in colour.
- Use:- For primary cast for complete dentures.
For articulation purposes.
For flasking in denture construction.
- Advantage:- Inexpensive
- Disadvantage:- Low strength
3. DENTAL STONE( Type 3):-
- Discovered in 1930
- α – hemihydrate or Hydrocal
- Powder particles are more dense & regular in shape.
- Comes in different colours, like yellow, green.
- Use:- - Making casts for diagnostic purposes & for complete or partial denture
- Advantages:- Greater strength & surface hardness.
- Disadvantage:- More expensive than plaster.
4. DENTAL STONE, HIGH STRENGTH(Type 4):-
- Modified α – hemihydrate, Densite or Die stone.
- Powder particles are very dense, cuboidal in shape &has reduced surface area.
- Use:- For making casts or dies for crown, bridge & inlay fabrication.
- Advantages:- High strength
Minimum setting expansion
5. DENTAL STONE, HIGH STRENGTH, HIGH EXPANSION(Type 5):-
- Most recent gypsum product.
- Use:- When inadequate expansion has been achieved during the fabrication
of cast crowns.
- Advantages:- Higher compressive strength
Higher setting expansion(0.10-0.30%)
- α & β- hemihydrates can also be made from the by products/ waste products
of the manufacture of phosphoric acid.
- Synthetic product is more expensive than that made from natural gypsum.
- It follows reversal in reaction of calcium sulfate hemihydrate powder with
water to produce gypsum.
(CaSO4)2.H2O + 3H2O 2CaSO4.2H2O
unreacted (CaSO4)2.½H2O + heat
- The product of the reaction is gypsum & the heat evolved in the exothermic
reaction is equivalent to the heat used originally in calcinations.
- The products formed during calcination react with water to form gypsum,
but at different rates.
- Eg – Hexagonal anhydrite reacts very rapidly, whereas when orthorhombic
anhydrite is mixed with water the reaction may require hours since, the
orthorhombic anhydrite has a more stable & closely packed crystal lattice.
- Gypsum is a unique material.
- Various hydrates have a relatively low solubility, with a distinct difference
between the greater solubility of hemihydrate & dihydrate.
- Dihydrate is too soluble for use in structures exposed to atmosphere.
1. Dissolution of calcium sulphate hemihydrate.
2. Formation of saturated solution of calcium sulphate.
3. Subsequent aggregation of less soluble calcium sulphate dihydrate.
4. Precipitation of the dihydrate crystals.
1. Colloidal/ Gel Theory.
2. Hydration Theory.
3. Crystalline Theory.
4. Dissolution- precipitation Theory.
1. Colloidal/ Gel Theory:-
- Originated in 1893 by M. Michaelis.
- When plaster is mixed with water, plaster enters into the colloidal state through
a sol- gel mechanism.
- In the sol state, hemihydrate particles are hydrated to form dihydrate, thereby
entering into an active state.
- As the measured amount of water is consumed, the mass converts to a solid gel.
2. Hydration Theory:-
- The rehydrated plaster particles join together through hydrogen bonding to the
sulfate groups to form the set material.
3. Crystalline Theory:-
- Originated in 1887 by Henry Louis Le Chatelier
- In 1907, supported by Jacobus Hendricus van’t Hoff
- The difference in the solubilities of calcium sulphate dihydrate & hemihydrate
causes setting differences.
- Dissolved CaSO4 precipitates as calcium sulphate dihydrate, since it is less
soluble than hemihydrate.
- X-ray diffraction studies – not all hemihydrate is converts to dihydrate.
- In a setting mass of plaster 2 types of centers are there:-
a) Dissolution center- around CaSO4 hemihydrate
b) Precipitation center- around CaSO4 dihydrate
4. Dissolution- Precipitation Theory:-
- Based on dissolution of plaster & instant recrystallization of gypsum to
interlocking of crystals.
Solubility of gypsum & gypsum products
The Setting Reaction is as follows:-
1. When the hemihydrate is mixed with water, a suspension is formed that is
fluid & workable.
2. The hemihydrate dissolves until it forms a saturated solution.
3. This saturated solution of hemihydrate, supersaturated in dihydrate, precipitates
4. As the dihydrate precipitates, the solution is no longer saturated with the
hemihydrate, so it continues to dissolve. Dissolution of hemihydrate &
precipitation of dihydrate as either new crystals or further growth on the
already present. The reaction continues until no further dihydrate precipitates
out of solution.
The reaction rate is followed by the exothermic heat evolved as shown in fig.
Temperature increases during the setting of plaster of paris
- As the gypsum forming increases, mass hardens into needle- like clusters called
- The intermeshing & entangling of crystals lead to a strong, solid structure.
- The amount of water & hemihydrate should be gauged accurately by weight.
- W: P ratio is an important factor in determining the physical & chemical
properties of the final gypsum product.
ed W: P ratio ed Setting Time
ed Setting Expansion
- Typical recommended ranges are:-
W: P ratio
Type 2 plaster 0.45-0.50
Type 3 stone 0.28-0.30
Type 4 stone 0.22-0.24
2 main effects on setting reaction:-
1. Change in temperature causes change in the relative solubilities of
hemihydrate & dihydrate, which alter the rate of the reaction.
Solubility of hemihydrate & dihydrate at different temperatures
- Temperature es Solubility ratio es
- Solubility ratio es Setting Reaction es& Setting Time es
- Solubility ratio es Setting Reaction es & Setting Time es
2. There is change in ion mobility with temperature.
in temperature in the mobility of Ca & SO4 ions
ed setting time in rate of reaction
- Practically the effects of these 2 phenomena are superimposed, & the total
effect is observed.
- Therefore, by increasing the temperature from 20ºC to 30ºC, the solubility ratio
decreases from 4.5 – 3.44, which should retard the reaction.
- At the same time as the mobility of ions increases, it should accelerate the
- Experimentation has shown that, by increasing the temperature from room
temperature of 20ºC to body temperature of 37ºC increases the rate of reaction
& decreases the setting time.
- Liquids with low pH(saliva) in setting reaction
- Liquids with high pH in setting reaction
- Measured by “Penetration Test”( time taken from the start of mix until the needle
no longer penetrates to the bottom) with the help of Vicat & Gillmore needles.
There are number of stages in the setting of a gypsum product:-
- MIXING TIME:- The time from the addition of powder to the water until the
mixing is completed.
Mechanical mixing – 20-30 secs.
Hand spatulation - 1 min.
- WORKING TIME:- The time available to use a workable mix.
Working time – 3min.
- LOSS OF GLOSS TEST FOR INITIAL SET:- Some of the excess water is
taken up in forming the dihydrate so that the mix loses its gloss.
- INITIAL GILLMORE TEST FOR INITIAL SET:- The mixture is spreadout, &
the needle is lowered onto the surface. The time at which it no longer leaves an
impression is called the “Initial Set.”
This is marked by a definite increase in strength.
- VICAT TEST FOR SETTING TIME:- Vicat Penetrometer is used.
The needle with a weighed plunger rod is supported & held just in contact with
the 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
is known as the “Setting Time.”
- GILLMORE TEST FOR FINAL SETTING TIME:-
Heavier Gillmore Needle is used.
The time elapsed at which this needle leaves only a barely perceptible mark on
the surface is called the “Final Setting Time.”
- READY- FOR- USE CRITERION:- The subjective measure of the time at
which the set material may be safely handled in the usual manner.
Ready for use state is reached in approx. 30 min.
CONTROL OF SETTING TIME
- The setting time depends on :-
- Effect of temperature on setting time may vary from one plaster or stone to
another, little change occurs between 0ºC & 50ºC.
- If the temperature of plaster water mixture exceeds 50ºC, a gradual
- As the temperature approaches 100ºC, no reaction takes place.
- At higher temperature range (50-100ºC), there is a tendency for any gypsum
crystals formed to be converted back to the hemihydrate form.
2. W:P ratio
- The more water used for mixing, the fewer nuclei there are per unit volume,
consequently, setting time is prolonged.
- The finer the particle size of the hemihydrate, the faster the mix hardens, the
rate of hemihydrate dissolution increases & the gypsum nuclei are also
more numerous. Therefore, a more rapid rate of crystallization occurs.
- Increased contamination by moisture produces sufficient dihydrate on
hemihydrate powder to retard the solution of hemihydrate.
- Contamination of gypsum with moisture from air during storage increases
Factors that control setting time :-
a) Factors controlled by the operator:-
1. W:P ratio
- More the w/p ratio, fewer the nuclei per unit volume so prolonged setting
2. Mixing time
- Within practical limits, longer & rapid mixing leads to shorter setting time.
- Some gypsum crystals form immediately when the plaster comes in
contact with water & as the mixing begins, formation of crystals increases.
- Some crystals are also broken up by mixing spatula & are distributed
resulting in the formation of more nuclei of crystallization resulting in
decreased setting time.
Effect of W:P ratio & Mixing time on the Setting time of plaster of paris
b) Factors controlled by the manufacturer:-
1. By the addition of Accelerators & Retarders:-
- Gypsum (<20%) - es setting time.
The set gypsum used as an accelerator is called“Terra Alba.”
- Potassium Sulphate(conc. 2-3%) & reduces the setting time of model plaster
from approx. 10min. to 4min.
- Sodium Chloride(<28%)
- Organic materials – glue, gelatin & some gums.
- Potassium citrate, borax, sodium chloride(20%), sodium citrate.
- Expansion may vary from 0.06% to 0.5%
- Volume of dihydrate formed is less than equal volume of hemihydrate & water.
i.e. actually a volumetric contraction should occur during setting reaction, but
instead a setting expansion is observed.
- PHENOMENON:- Based on crystallization mechanism.
- The crystallization process occurs as an outgrowth of crystals from nuclei of
- The dihydrate crystals growing from the nuclei not only intermesh with but
also obstruct the growth of adjacent crystals.
- If this process is repeated by thousands of crystals during growth, an outward
stress or thrust develops that produces an expansion of the entire mass.
- The crystal impingement & movement results in the formation of micropores.
- RESULT:- The gypsum formed is greater in external volume but less in
crystalline volume, therefore, the set material must be porous.
CONTROL OF SETTING EXPANSION
1. W: P ratio:-
- Increase in w/p ratio, decreases the nuclei of crystallization per unit volume,
so there is less growth of dihydrate crystals which leads to less outward thrust.
- Decreased w/p ratio increased mixing time increased setting
Effect of W:P ratio & Mixing time on Setting expansion of plaster of paris
2. Accelerators & Retarders:- Chemicals added by the manufacturer to regulate
- Sodium Chloride ( upto 2% of hemihydrate)
- Sodium Sulfate ( max. effect at 3.4%)
- Potassium Sulfate (>2%)
- Potassium Tartrate
Retarders:- Chemicals that form a coating on the hemihydrate particles &
prevent the hemihydrate from going into the solution in the normal manner.
Citrates, acetates & borates.
- Setting expansion that occurs under water is called as “Hygroscopic Setting
- Setting expansion without water immersion is called “Normal Setting
Stages of Hygroscopic setting expansion:-
Stage I – Initial mix stage.
- Represented by 3 round particles of hemihydrate surrounded by water.
Stage II – Initial crystal growth stage.
- Crystals of dihydrate have started to form.
- In NSE, the water around the particles is reduced by hydration & particles are
drawn close together by surface tension of water.
- In HSE, the setting is taking place under water so that water of hydration is
replaced & the distance between the particles remain the same.
Stage III – Solid phase contact stage.
- As the dihydrate crystals grow, they contact each other & setting expansion
- In NSE, the water around the particles is reduced. The particles with their
attached crystals are drawn together as before, but the contraction is opposed by
the outward thrust of the growing crystals.
- In HSE, the crystals are not inhibited, because the water is again replenished
from the outside. Infact, the original particles are now separated further as the
crystals grow & setting expansion occurs.
Stage IV & V – Expansion & Termination.
- Effect becomes more marked.
- The crystals that are inhibited in NSE become intermeshed & entangled much
sooner than in HSE in which the crystals grow much more freely during the
early stages before the intermeshing.
- The observed setting expansion that occurs when the gypsum product sets under
water may be greater than that which occurs during setting in air.
- Strength of gypsum product is expressed in terms of compressive strength.
- Strength of plaster & stone increases rapidly as the material hardens after initial
- Free water content of the set product affects its strength.
- 2 Strength properties of gypsum are:-
1. WET STRENGTH ( Green Strength):-
Strength obtained when the water in excess of that required for hydration of
the hemihydrate is left in the test specimen.
2. DRY STRENGTH:-
Strength obtained when the excess water in the specimen has been driven off
Dry strength is two or more times as high as wet strength.
- Strength depends upon:-
1. Addition of Accelerators & Retarders – decrease the wet & dry strength of
2. Increase in W:P ratio increases porosity, which decreases dry strength.
- Compressive strength is inversely related to the W:P ratio of the mix.
Effect of W:P ratio on the Compressive Strength of different materials
- After final setting time the hardened gypsum material appears dry & has
Effect of Drying on the Compressive Strength of plaster of paris
- Effect of drying is reversible, soaking a dry cast in water reduces its strength to
the original level.
- Surface hardness of gypsum materials is related to their compressive strength.
- Surface hardness increases at a faster rate than the compressive strength.
- Abrasive Resistance of gypsum product is ed by impregnating the set gypsum
with epoxy resins.
- Surface hardness of set gypsum is improved by mixing stone with a hardening
solution containing colloidal silica( about 30%).
Viscosity of several High Strength Dental Stones & Impression Plaster
- ADA Specification No. 25 requires that types I & II reproduce a groove 75mm in
width, whereas types III, IV & V reproduce a groove 50mm in width.
- Gypsum dies do not reproduce surface detail as well as electroformed or epoxy
dies because, the surface of set gypsum is porous on a microscopic level.
Air bubbles are formed at the interface of impression & gypsum cast because,
freshly mixed gypsum does not wet some rubber impression materials well.
Contamination of the impression ( by saliva or blood) in which the gypsum die is
poured can also affect the detail reproduction. Rinsing the impression & blowing
away excess water can improve the surface detail recorded by gypsum die
- Strength of a stone is inversely proportional to the W:P ratio.
- Trapping of air should be avoided while mixing to avoid porosity – weak spots &
- Longer spatulation es working time
- Method of mixing:-
Add measured water
Gradual addition of the preweighed powder
- Once the setting reaction in the cast is completed, its dimensions remain constant
under room temperature & humidity.
- If stone cast is immersed in running water, its linear dimensions may 0.1% for
every 20min. of immersion.
- If storage temperature is raised to 90º &110ºC – shrinkage occurs
- White stone or plaster – longer working time
- Gypsum products used for mounting casts are called as “mounting stones or
plasters”- fast setting & ed setting expansion.
- All types of gypsum products should be stored in a dry atmosphere.
- Products should be sealed in a moisture proof metal container.
- If an impression has not been disinfected, it is necessary to disinfect the stone
- Disinfection solutions that do not adversely affect the quality of the gypsum
product can be used.
- Dental stone containing a disinfectant may also be used.
- Useful disinfectants for stone casts include spray disinfectants, hypochlorites, &
- Gypsum products are used for making positive reproductions or replicas of oral
- These replicas are called casts, dies or models.
- The criteria for selection of a GP depends on its use & physical properties.
1. Anusavice K.J.-“Phillips’ Science of Dental materials” 11th edition , 2003
2. Combe E.C. – “Notes on Dental Materials”6th edition , 1992
3. Craig’s R.G., Powers J.M. – “Restorative Dental Materials” 11thedition, 2002
4.Gladwin M, Bagby M – “Clinical Aspects of Dental Materials” 2nd edition,
5. Mc Cabe J.F. – “Applied Dental Materials” 7th edition , 1992
6. Phillips R.W.-“Skinner’s Science of Dental Materials”9th edition , 1992