Gypsum – Plaster & Dental Stone

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Many dental restorations and appliances are constructed outside the patient’s mouth using models and dies which should be accurate replicas of the patient’s hard and soft tissues.

The morphology of the hard and soft tissues is recorded in an impression and models and dies are prepared using materials which are initially fluid and can be poured into the impression, then harden to form a rigid replica.

WHAT IS A MODEL? WHAT IS A DIE?

Model: is a replica of several teeth and their associated soft tissues or, alternatively, to an edentulous arch.

Die: is a replica of a single tooth.

Many materials have been used for producing models and dies but the most popular are the materials based on gypsum products.

Requirements of Model and Die Materials

The model and die materials ideally, should:

  • Dimensional accuracy: the dimensional changes which occur during and after the setting of these model materials should be minimal in order to produce an accurate model or die.
  • Fluid at the time it is poured into the impression so that fine detail can be recorded.
  • Minimize the presence of surface voids on the set model by encouraging surface wetting.
  • Strong to resist accidental fracture.
  • Hard enough to resist abrasion during the carving of a wax pattern.
  • Compatible with all the other materials with which it comes into contact.

Gypsum Products

Gypsum is a naturally occurring, white powdery mineral with the chemical name calcium sulphate dihydrate (CaSO4·2H2O).

Gypsum products used in dentistry are based on calcium sulphate hemihydrate (CaSO4)2·H2O. Their main uses are for casts or models, dies and investments.

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Types of Gypsum Products

The current ISO Standard for Dental Gypsum Products identifies 5 types of material as follows:

Type 1 Dental plaster, impression

Type 2 Dental plaster, model

Type 3 Dental stone, die, model

Type 4 Dental stone, die, high strength, low expansion

Type 5 Dental stone, die, high strength, high expansion

Chemical Composition

Gypsum products used in dentistry are formed by driving off part of the water of crystallization from gypsum to form calcium sulphate hemihydrate.

Gypsum                 →   Gypsum product (plaster or stone)   + water

2CaSO4·2H2O       →   (CaSO4)2·H2O         +  3H2O

Calcium sulphate dihydrate  →   Calcium sulphate hemihydrate  +  water

MANUFACTURE OF PLASTER aka PLASTER OF PARIS  “POP”

Plaster is produced by a process known as calcination. Gypsum is heated to a temperature of about 120̊ C in order to drive off part of the water of crystallization. This produces irregular, porous particles referred to as β-hemihydrate particles.

Overheating the gypsum may cause further loss of water to form calcium sulphate anhydrite (CaSO4), whilst underheating produces a significant concentration of residual dihydrate. The presence of both components has a marked influence upon the setting characteristics of the resultant plaster.

MANUFACTURE OF DENTAL STONE

Dental stones may be produced by one of two methods:

  • Gypsum is heated to about 125̊ C under steam pressure in an autoclave to form α-hemihydrate (more regular and less porous than β-hemihydrate).
  • Gypsum is boiled in a solution of a salt such as CaCl2. This gives a material similar to that produced by autoclaving but with even less porosity. Manufacturers normally add small quantities of a dye to dental stones to differentiate it from dental plaster.

Advantages

Gypsum model and die materials have the advantages of

  • Inexpensive and easy to use.
  • The accuracy and dimensional stability are good
  • They are able to reproduce fine detail from the impression, providing precautions are taken to prevent blow holes.

Disadvantages

  • The mechanical properties are not ideal and the brittle nature of gypsum occasionally leads to fracture , particularly through the teeth, which form the weakest part of any model.
  • Problems occasionally arise when gypsum model and die materials are used in conjunction with alginate impression. The surface of the model may remain relatively soft due to an apparent retarding effect which hydrocolloids have on the setting of gypsum products.

Applications

Stones are normally used when strength, hardness and accuracy are required. These materials are used when any work is to be carried out on the model or die as would be the case when constructing a denture on a model or a cast alloy crown on a die.

The cheaper dental plaster is used when mechanical properties and accuracy are not of primary importance. Thus, plaster is often used for mounting stone models onto articulators and sometimes for preparing study models.


Manipulation and Setting Characteristics

Chemical Reaction

Application of gypsum products in dentistry involves hydration of the Calcium sulphate hemihydrate with water to produce Calcium sulphate dihydrate.

Gypsum product (plaster or stone) + water   →   Gypsum

(CaSO4)⋅H2O        + 3H2O   →   2CaSO4⋅2H2O

Calcium sulphate hemihydrate + water   →  Calcium sulphate dihydrate

Water/Powder Ratio

Plaster and stone powders are mixed with water to produce a workable mix. The table illustrates Water/Powder ratios for gypsum model and die materials.

  Water (ml) Powder (g) W/P ratio (ml/g)
Plaster 50–60 100 0.55
Stone 20–35 100 0.30
Theoretical ratio 18.6 100 0.186

Excess water is absorbed by the porosities of gypsum particles.

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Armamentarium for hand mixing:

  • Clean, scratch free rubber or plastic bowl having a top diameter of about 130 mm.
  • Stiff spatula with a round-edged blade of around 20–25 mm width and 100 mm length.

The presence of gypsum residues in the mixing bowl can noticeably alter the working and setting characteristics of a fresh mix and so the need for cleanliness is emphasized.

Mixing Steps

  1. The requisite amount of water is added to a moist bowl and the powder added slowly to the water over about 10 seconds.
  2. The mix is allowed to soak for about another 20 seconds.
  3. Then mixing/spatulation carried out for around 60 seconds using a circular stirring motion.
  4. The material should be used as soon as possible after mixing since its viscosity increases to the stage where the material is unworkable within a few minutes.
  5. After the material has been mixed and used, the mixing bowl should be thoroughly cleaned before the next mix is performed.

Air Porosity

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Considerable quantities of air may be incorporated during mixing and this may lead to porosity within the set material.

Air porosity may be reduced either by:

  • Vibrating the mix of plaster or stone in order to bring air bubbles to the surface
  • Mixing the material mechanically under vacuum
  • Both

Setting Process

The setting process begins rapidly after mixing.

  1. The water becomes saturated with hemihydrate, which has a solubility of around 0.8% at room temperature.
  2. The dissolved hemihydrate is then rapidly converted to dihydrate which has a much lower solubility of around 0.2%, since the solubility limit of the dihydrate is immediately exceeded it begins to crystallize out of solution.
  3. The process continues until most of the hemihydrate is converted to dihydrate.
  4. The crystals of dihydrate grow from specific sites called nuclei of crystallization. These may be small particles of impurity, such as unconverted gypsum crystals, within the hemihydrate powder.

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Setting Time

Two stages can be identified during setting.

A) Initial setting time

The time at which the material develops the properties of a weak solid and will not flow readily. At this time, it is possible to carve away excess material with a knife.

B) Final setting time

The time taken to reach a stage when the models or dies are strong and hard enough to be worked upon. The term is misleading since it implies that the material has reached its ultimate strength, which is reached several hours later.

Type 1 Type 2 Type 3 Type 4 Type 5
Initial setting time (min) 5 – 10 5 – 20 5 – 20 5 – 20
Final setting time (min) 4 20 20 20 20

Factors Controlling Setting Time

A) Factors controlled by manufacturers

  • The concentration of nucleating agents in the hemihydrate powder: a higher concentration of nucleating agent, produced by ageing or from unconverted calcium sulphate dihydrate, results in more rapid crystallization.
  • Addition of chemical accelerators or retarders to dental stones: Potassium sulphate is an accelerator which act by increasing the solubility of the hemihydrate. Borax is a widely used retarder, the mechanism by which it works is not clear.

B) Factors under the control of the operator

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  • Temperature variation has little effect on the setting time. Increasing the temperature accelerates the dissolution of hemihydrate but retards the crystallization of dihydrate.
  • Increasing the W/P ratio retards setting by decreasing the concentration of crystallization nuclei.
  • Increasing mixing time accelerates setting by breaking up dihydrate crystals during the early stages of setting, producing more nuclei on which crystallization can be initiated.

Exothermic Setting Reaction

The setting reaction is exothermic, the maximum temperature is reached during the stage when final hardening occurs. Temperature rise is negligible at the time of the initial set.

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Temperature–time profile for a gypsum material during setting. Points I and F correspond to the initial set and final set points indicated by indentors

The magnitude of temperature rise depends on the bulk of material used and can reach 30̊ C at the centre of a mass of setting material. This may be maintained for several minutes due to the thermal insulating characteristics of the materials.

This marked rise in temperature can be used to good effect when flasking dentures since it softens the wax of the trial denture and enables it to be easily removed from the mould.

Setting Expansion

A small expansion caused by the outward thrust of growing crystals. The maximum rate of expansion occurs at the time when the temperature is increasing most rapidly. The expansion is only apparent since the set material contains a considerable volume of porosity.

Type 1 Type 2 Type 3 Type 4 Type 5
Setting expansion (%) 0-0.15 0-0.30 0-0.20 0-0.15 0.16-0.30

In order to produce an accurate model or die it is necessary to maintain the setting expansion at as low a value as possible.

Accelerators or retarders which are added by manufacturers to dental stones in order to control the setting time also have the effect of reducing the setting expansion and are sometimes referred to as antiexpansion agents.

Alterations in W/P ratio and mixing time have only a minimal effect on setting expansion.

Hygroscopic Expansion

If the material is placed in water at the initial set stage, considerably more expansion occurs during setting.

This increased expansion is sometimes used to increase the setting expansion of gypsum-bonded investment materials.


Properties of the Set Material

A) Compressive Strength

The strength of gypsum depends on:

  • The porosity of the set material.
  • The time for which the material is allowed to dry out after setting.

The porosity, and hence the strength, is proportional to the W/P ratio. Since stone is always mixed at a lower W/P ratio than plaster it is less porous and consequently much stronger and harder.

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Although a gypsum model or die may appear completely set within a relatively short period its strength increases significantly if it is allowed to stand for a few hours.

The increase in strength is a function of the loss of excess water by evaporation. It is thought that evaporation of water causes a precipitation of any dissolved dihydrate and that this effectively cements together the crystals of gypsum formed during setting.

Type 1 Type 2 Type 3 Type 4 Type 5
Compressive strength 1h (MPA) 6 12 25 40 40
Compressive strength 24h (MPA) 24 70 75 75

B) Flexural Strength

Gypsum is a very brittle material.

Plaster is fragile with very low value of flexural strength. Stone is less fragile but must be treated with care if fracture is to be avoided. It is relatively rigid but has a poor impact strength and is likely to fracture if dropped.

Type 1 Type 2 Type 3 Type 4 Type 5
Flexural strength 24h (MPA) 1 1 15 20 20

C) Dimensional Stability

The dimensional stability of gypsum is good.

Following setting, further changes in dimensions are immeasurable and the materials are sufficiently rigid to resist deformations when work is being carried out upon them.

D) Solubility

Set plaster is slightly soluble in water.

Solubility increases with the temperature of the water and if hot water is poured over the surface of a plaster cast, as happens during the boiling out of a denture mould, a portion of the surface layer becomes dissolved leaving the surface roughened.

Frequent washing of the surface with hot water should therefore be avoided.

E) Detail Reproduction

The ability of dental gypsum products to reproduce surface details of hard or soft tissues either directly or from impressions is central to their suitability as model and die materials.

Types 3, 4, and 5 stones are capable of recording greater fine detail than type 2 plaster material.

Type 1 Type 2 Type 3 Type 4 Type 5
Detail reproduction (µm) 75 75 50 50 50

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