Gypsum in Dentistry: Types, Reactions, and Applications

GYPSUM

CaSO4•2H2O; calcium sulfate dihydrate

Calcination

The process of heating a solid material to drive off volatile chemically combined components such as water and carbon dioxide.

Calcining

Grounding calcium sulfate dihydrate to a powder and heated in an oven to dry.

Cast

A reproduction of the shape and surface of a structure made in an impression of the surface.

Dental Plaster (Plaster of Paris)

The beta form of calcium sulfate hemihydrate (CaSO4• ½ H2O).

Dental stone

The alpha form of calcium sulfate hemihydrate (CaSO4• ½ H2O).

Die

A reproduction of a prepared tooth made from a gypsum product, epoxy resin, a metal, or a refractory material.

Gypsum-based investment

A refractory material consisting of silica and gypsum as a binder used to produce a mold for the metal casting process.

Hygroscopic setting expansion

The expansion that occurs when gypsum or a gypsum-bonded investment sets while immersed in water (usually heated to approximately 38 °C [100 °F]).

Model

A positive likeness of an object.

Normal setting expansion

The expansion that occurs when gypsum or a gypsum-bonded investment sets in ambient air.

Exothermic reaction

Where heat evolves as the gypsum sets.

Production of gypsum products

Commercially, the gypsum is ground and subjected to temperatures of 110 °C to 130 °C (230 °F to 266 °F) in open containers to drive off part of the water of crystallization.

Calcination of gypsum

α-hemihydrate, β-hemihydrate.

Mineral mined on earth

Gypsum is also produced as a by-product of flue gas desulfurization.

Appearance of gypsum

It usually appears white to milky yellowish and is found as compact mass in nature.

Supplied form of gypsum

Supplied as fine hemihydrate powders that are produced by heating ground gypsum particles.

Use of gypsum in ancient Egypt

A mixture of plaster (a gypsum product), lime, and water was used in joining the stone blocks of ancient Egypt's pyramids.

Uses in dentistry

Occasionally used intraorally for completely edentulous patients, preparation of study models, mounting of study models in articulators, study cast in form of die, embed used on flask then for dental prostheses, and as an additive to silica in dental casting investments.

Grounding of calcium sulfate dihydrate

Grounding it to powder and heated in an oven to dry.

Method of calcination

Based upon the method of calcination, different forms of hemihydrate can be obtained.

Calcium sulfate hemihydrate

is the principal constituent of gypsum-based products (CaSO4• ½ H2O)

Colloidal Theory

When mixed with water, hemihydrate enters into the colloidal state through a sol-gel mechanism

Hydration Theory

Rehydrated plaster particles unite through hydrogen bonding with sulfate groups to form the set material

Dissolution-Precipitation Theory

Dissolution of hemihydrate particles in water followed by instant recrystallization to the dihydrate

Wet Calcination

Heating gypsum between 120-130 °C under pressure of 17 lbs/in² for 5-7 hours in a kettle, vat, or rotary kiln

Stronger and harder dihydrate structure

Prismatic and more regular in shape

Mixing Time

Mechanical mixer: 20-30 seconds; Hand spatulation: 1 minute; Time from addition of powder until mixing is completed

Working Time

Time required for the mixture to remain workable after mixing

Dry Calcination

Ground and heated to 110-130 °C in an open kettle/containers

Crystals are spongy

Irregular in shape, more porous in character; Require more water to wet powder particles for stirring and pouring

Anhydrite

As temperature is raised, hemihydrate becomes anhydrite.

Setting Time

Duration until the consistency is no longer acceptable for intended purposes.

Loss of gloss

9 minutes.

Initial Set

Duration: 13 minutes; time elapsed when the mix can resist penetration by a Gillmore needle (2.12mm in diameter, 113.4g).

Final Setting Time

Duration: 30 minutes; elapsed time at which a Gillmore needle (1.06mm tip, 453.6g) leaves only barely perceptible mark on surface.

MODIFIED α- HEMIHYDRATE/ DIE STONE

Also called improved stone; formed by boiling gypsum in 30% aqueous solution of calcium chloride or in the presence of more than 1% of sodium succinate at 100°C.

Crystals

Will be shorter and thicker than those produced in a closed container.

Cube shaped

Reduces surface area that improves flow of mixture.

Primatic

More regular and densely packed.

Wet/ Green Strength

Increases rapidly as the material hardens after the initial set; shortens setting time.

Fineness

The finer the material, the faster the setting time.

Water-Powder Ratio

Weight of the water divided by the weight of hemihydrate powder; ↑ W:P causes prolonged setting time, decrease in strength and setting expansion.

Dry Strength

Strength obtained after excess water is removed by drying.

Mixing

The longer and more rapid the mixing, the shorter the setting time.

Temperature

The higher the temperature, the longer the setting time.

Retarders and Accelerators

Chemical modifiers that increase or decrease the setting time.

Compressive strength

Strength of set gypsum products is generally expressed in terms of this.

Microwave irradiation

Can speed up gypsum cast drying, achieving similar strength in 1 minute as 24 hours of air drying.

Excess water

Presence weakens the set product.

Increased mixing time

Increases the strength; overmixing decreases it.

Incorporation of accelerators and retarders

Decreases both the wet and dry strength.

Chloric Acid/Borax

At 0.2mM or 0.08g/L.

Calcium salts

Includes Potassium Sulfate, NaCl (2%), Sodium Sulfate.

Too much salt

Will cause it not to set.

Type I: Impression Plaster

Used to take impression; rarely used; replaced by hydrocolloids and elastomers; modifiers are added to regulate setting time (4-5 min.) and setting expansion.

Type II: Model Plaster/Plaster of Paris

Used to make study cast/model for record purposes stronger than Type I Plaster; lower water: powder ratio; therefore less porosity, higher expansion; used to make study casts or to mount stone casts in the articulator and to fill a flask in denture construction; natural white.

Type III: Dental Stone (Class 1 Dental Stone)

Stronger than Plaster of Paris/model plaster; used as working cast that can withstand forces; used for production of dentures, for indirect wax patterns of indirect restorations; setting time is 45-60 min. depending on the stone and type of impression material; 82 Rockwell Hardness.

Type IV: Dental Stone, High Strength, or Die Stone (Class II Dental Stone)

Stronger than Type III; used as working die for crown restoration; for accurate measurements; die material refers to single tooth/several teeth; inlay and onlay restoration; harder with minimum setting expansion; 92 Rockwell Hardness.

Type V: Dental Stone, High Strength, High Expansion

Used for the construction of removable partial dentures (RPD) and fixed partial dentures (FPD); higher compressive strength than Type IV with increased setting expansion to compensate for the greater shrinkage of newer, high-melting alloy used for dental castings.

Water: Powder ratio for Type I

0.060 ml.

Water: Powder ratio for Type II

0.050 ml.

Water: Powder ratio for Type III

0.030 ml.

Water: Powder ratio for Type IV

0.24 ml or less.

Water: Powder ratio for Type V

0.18 to 0.22 ml.

Care cast

Store in water bath saturated with calcium sulfate solution.

Care for gypsum

Sealed container in dry atmosphere.

Disinfection of casts

Spray the casts with disinfecting solutions such as iodophor, glutaraldehyde, or phenol and store in sealed plastic bag for 10 minutes; there are stone materials that already contain disinfecting agents.

Mixing of gypsum products

Involves proper proportioning of water and powder to achieve desired consistency.

Manipulation of gypsum products

Includes mixing, proportioning, and ensuring proper setting conditions.

Setting time for Type III

45-60 min. depending on the stone and type of impression material.

Rockwell Hardness for Type III

82

Rockwell Hardness for Type IV

92

Rockwell Hardness for Type V

Higher than Type IV.

Boxing in model plaster

Strips of wax are wrapped around the impressions extending ½ inch beyond the tissue side of the impression.

Mixture preparation for model plaster

Mixture piled on glass slab; the filled impression is inverted over the pile, shaped with spatula before it sets.

Longer spatulation effect

Reduces working time.

Effect of repeated addition of water or powder

Will result in uneven set, low strength, and distortion which will produce an inaccurate gypsum product.