A number of gypsum products are used in dentistry as adjuncts to dental

operation.

1. Type I: Impression plaster.

2. Type II: Dental plaster.

3. Type III: Dental stone (medium strength stone).

4. Type IV: Improved stone (high strength stone) (die stone).

5. Type V: high strength/high expansion stone.

1- Impression plaster.

2- Mounting the casts to the articulation.

3- Form casts and dies.

4- Used as a binder for silica.

5- Used as a mold for processing dental polymers.

6- Used for bite registration (record centric jaw relation).

Properties of ideal model material (gypsum products):

Dimensional stability, no expansion or contraction during or after setting.

High compressive strength to withstand the force applied on it.

Hardness, soft material can be easily scratched.

Reproduce the fine details.

Produce smooth surface.

Reasonable setting time.

Compatible with the impression material.

Can be disinfected without damaging the surface.

Most gypsum products are obtained from natural gypsum rock. Because

gypsum is the dihydrate form of calcium sulfate (CaSO4. 2H2O), on

heating, it loses 1.5 g mol of its 2 g mol of H2O and is converted to

calcium sulfate hemihydrate (CaSO4. 0.5H2O). When calcium sulfate

hemihydrate is mixed with water, the reverse reaction takes place, and the

calcium sulfate hemihydrate is converted back to calcium sulfate

dihydrate.

1- Plasters are produced when the gypsum mineral is heated in an open

kettle at a temperature of about 110° to 120°C (dry calcination). The

hemihydrate produced is called β-calcium sulfate hemihydrate. Such

a powder is known to have a somewhat irregular shape and is porous

in nature. These plasters are used in formulating model and lab

plasters.

2- Stones are produced when the gypsum is dehydrated under pressure

and in the presence of water vapor at about 125°C (wet calcination),

the product is called hydrocal. The powder particles of this product

are more uniform in shape and denser than the particles of plaster.

Calcium sulfate hemihydrate produced in this manner is designated as

α-calcium sulfate hemihydrate. Hydrocal is used in making low- to

moderate-strength dental stones.

3- High-strength stones are produced when the gypsum rock is boiling

in a 30% calcium chloride solution, after which the chloride is

washed away with hot water (100°C), the product is called densite,

and the material is ground to the desired fineness. This variety is

made by gypsum The calcium sulfate hemihydrate in the presence of

100°C water does not react to form calcium sulfate dihydrate because

at this temperature their solubilities are the same. The powder

obtained by this process is the densest of the types.

Potassium sulfate, and terra alba (set calcium sulfate dihydrate) are 1-

effective accelerators.

Sodium chloride in small amounts shortens the setting reaction but 2-

increases the setting expansion of the gypsum mass.

Sodium citrate is a dependable retarder. 3-

A mixture of calcium oxide (0.1%) and gum arabic (1%) reduces the 4-

amount of water necessary to mix gypsum products, resulting in

improved properties.

The setting reaction is explained on the basis of difference in the

solubilities of calcium sulfate dihydrate and hemihydrate. Hemihydrate is

four times more soluble than dihydrate.

When hemihydrate is mixed in water a suspension is formed which is

fluid and workable.

Hemihydrate dissolves until it forms a saturated solution. Some dihydrate

is formed due to the reaction.

Since solubility of dihydrate is much less than hemihydrate, the saturated

hemihydrate is supersaturated with respect to the dihydrate.

All supersaturated solutions are unstable. So the dihydrate crystals

precipitate out.

As the dihydrate precipitates out, the solution is no longer saturated with

hemihydrate and so it continues to dissolve. The process continues until

all hemihydrate converts to dihydrate.

Other theories include .

The mixing process, called spatulation, has a definite effect on the setting

time and setting expansion of the material. Within practical limits an

increase in the amount of spatulation (either speed of spatulation or time

or both) shortens the setting time. Obviously when the powder is placed

in water, the chemical reaction starts, and some calcium sulfate dihydrate

is formed. During spatulation the newly formed calcium sulfate dihydrate

breaks down to smaller crystals and starts new centers of nucleation,

around which the calcium sulfate dihydrate can be precipitated. Because

an increased amount of spatulation causes more nuclei centers to be

formed, the conversion of calcium sulfate hemihydrate to dihydrate

requires somewhat less time.

The first effect of increasing temperature is a change in the relative

solubilities of calcium sulfate hemihydrate and calcium sulfate dihydrate,

which alters the rate of the reaction. As the temperature increases, the

solubility ratios decrease, until 100°C is reached and the ratio becomes

one. As the ratio of the solubilities becomes lower, the reaction is slowed,

and the setting time is increased.

The second effect is the change in ion mobility with temperature. In

general, as the temperature increases, the mobility of the calcium and

sulfate ions increases, which tends to increase the rate of the reaction and

shorten the setting time.

Practically, the effects of these two phenomena are superimposed, and the

total effect is observed.

Plaster can easily absorb water vapor from a humid atmosphere to form

calcium sulfate dihydrate. The presence of small amounts of calcium

sulfate dihydrate on the surface of the hemihydrate powder provides

additional nuclei for crystallization. Increased contamination by moisture

produces sufficient dihydrate on the hemihydrate powder to retard the

solution of the hemihydrate. Experience has shown that the common

overall effect of contamination of gypsum products with moisture from

the air during storage is a lengthening of the setting time.

Colloidal systems such as agar and alginate retard the setting of gypsum

products. Accelerators such as potassium sulfate are added to improve the

surface quality of the set CaSO4 .2H20 against agar or alginate.

Liquids with low pH, such as saliva, retard the setting reaction. Liquids

with high pH accelerate setting.

The operator also can change the setting time of model plaster to a certain

extent by changing the water/powder (W/P) ratio. The W/P ratio has a

pronounced effect on the setting time. The more water in the mix of

model; (plaster, dental stone, or high-strength dental stone); the longer the

setting time.

When set, gypsum products show relatively high values of compressive

strength. The compressive strength is inversely related to the W/P ratio of

the mix. The more water used to make the mix, the lower the compressive

strength. Model plaster has the greatest quantity of excess water, whereas

high-strength dental stone contains the least excess water. The set model

plaster is more porous than set dental stone, causing the apparent density

of model plaster to be lower.

After most excess water is evaporated from the surface, the hardness will

increase. Attempts have been made to increase the hardness of gypsum

products by impregnating the set gypsum with epoxy or methyl

methacrylate monomer that is allowed to polymerize.

The tensile strength of model plaster and dental stone is important in

structures in which bending tends to occur because of lateral force

applications, such as the removal of casts from flexible impressions.

Because of the brittle nature of gypsum materials, the teeth on the cast

may fracture rather than bend.

ANSI/ADA Specification No. 25 requires that types I and II reproduce a

groove 75 μm in width, whereas types III, IV, and V reproduce a groove

50 μm in width. Air bubbles are often formed at the interface of the

impression and gypsum cast because freshly mixed gypsum does not wet

some rubber impression materials (e.g., some silicone types). The use of

vibration during the pouring of a cast reduces the presence of air bubbles.

Contamination of the impression with saliva or blood can also affect the

detail reproduction.

When set, all gypsum products show a measurable linear expansion.

Under ordinary conditions, plasters have (0.2-0.3 %) setting expansion,

low to moderate strength dental stone about (0.15-0.25 %), and high-

strength dental stone only (0.08-0.10 %). Typically, (over 75 %) of the

expansion observed at 24 hours occurs during the first hour of setting.

Increasing the W/P ratio; reducing the setting expansion. If during the

setting process, the gypsum materials are immersed in water, the setting

expansion increases slightly. This is called hygroscopic expansion.