Wednesday, August 28, 2013

gypsum products


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.

4.      SELENITE.

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.

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.


1.      Accuracy

  2. Dimensional Stability

  3. Ability to reproduce fine detail.

  4. Strength & resistance to abrasion.

  5. Compatibility with the impression materials.

  6. Colour

  7. Biological safety

  8. Ease of use

  9. Cost.


-  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

    hemihydrate (CaSO4.1/2H2O).

                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

    sulphate dihydrate.

              CaSO4.½H2O + 1½H2O               CaSO4. 2H2O + 3900 (2)
               (plaster of paris)    (water)                  (gypsum)          (cal)

-  Reaction is exothermic.


-  Formed by calcining of gypsum.                                            

-  Gypsum is ground & subject to heat 110°C - 130 °C to drive off a part of water

    of crystallization

-  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.


     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)
                                 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

Five types of gypsum products & their properties



  -  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.

          -  Borax

  -  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.


    - 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

                               Surface hardness

                               Abrasion resistant

                               Minimum setting expansion


      - 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.

The setting reaction of gypsum occurs by:-

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

      out dihydrate.

 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.

W: P ratio:-

  -  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 Strength

                                                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

   setting reaction.
 - 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.”


    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.


- The setting time depends on :-

1. Temperature

   -  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

       retardation occurs.

   -  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.

   3. Fineness

       -  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.

   4. Humidity

       -  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

           setting time.

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.


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

      setting expansion.


     - 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

    setting time.
- 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.


        Strength obtained when the excess water in the specimen has been driven off

        by drying.

        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

      gypsum products.

2.      Increase in W:P ratio increases porosity, which decreases dry strength.

Effect of W:P ratio & Mixing time on the Compressive Strength of plaster of paris



  - 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

     maximum strength.  


 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%).


-  It is the resistance of a fluid to flow.    


  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 &

   surface inaccuracies.

- 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