CONTENTS:
INTRODUCTION
GYPSUM
CLASSIFICATION
MANUFACTURE
SETTING PROCESS
PROPERTIES
MANIPULATION
COMPATIBILITY
INFECTION CONTROL
RECENT DEVELOPMENTS
CONCLUSION
REFERENCES
INTRODUCTION
Auxiliary dental material.
Glossary of
Prosthodontic terms:
Model: Facsimile used for display purposes, a miniature representation of something. Dental Cast: A positive life size reproduction of a part
or parts of the oral
cavity. Die: The positive reproduction of the form of a prepared tooth in any suitable substance.
Direct link between the clinical phase of
treatment and the technical
laboratory procedures.
Contributes significantly to the ultimate
success of the prosthesis.
GYPSUM:
Originates from the Greek word
‘Gypos’ which means chalk.
Dihydrate of calcium sulfate.
Chemical formula CaSO4.2H2O.
Sulfate mineral most commonly
found.
Usually white to yellowish
white in color.
Large beds of gypsum were
formed when seawater evaporated, leaving dissolved Calcium and Sulfate ions to
form deposits of gypsum.
United States is the largest producer as well as the biggest consumer of gypsum.
Others are Canada, France, Japan
and Iran.
Gypsum mineral can be found in various
forms:
- ROCK–GYPSUM: widely occurring massive dull coloured rocks.
2. ALABASTER: Large fine-grained white stones. Often
used for carving into vases and ornaments. Also used in building of King
Solomon’s Temple
3. SELENITE: as transparent crystals
4. SATIN SPAR: as fibrous crystals.
SYNTHETIC GYPSUM:
•
Also produced as a by-product of manufacture
of phosphoric acid.
•
CHEMICAL GYPSUM.
3 H2SO4(l) + Ca3(PO4)2(s) + 6
H2O(l) 2 H3PO4(s) + 3
CaSO4·2H2O(s)
PROPERTIES:
ü Found as prismatic, curved or twisted monoclinic crystals of vitreous luster. 
ü Specific gravity: 2.3
ü It cleaves perfectly in one direction.
ü Moh hardness no: 2, which means that it can be scratched by a
fingernail.
USES:
- As a raw material for making Plaster of Paris. Plaster of Paris is called so, because the gypsum that was used to manufacture it came from a village called Montamarte, near Paris. It is used extensively in construction purposes.
- Grounded gypsum (land plaster) is sometimes used as a fertilizer for soil that needs calcium.
- Raw gypsum is also used to keep Portland cement from hardening too quickly.
- It is also used to make paint (as a filler), filters, insulation and wall plaster.
- Alabaster is used for carving ornaments and vases.
- Selenite is sometimes used as an optical material.
Gypsum produced for dental application is
nearly pure calcium sulfate dihydrate.
DENTAL USES OF GYPSUM PRODUCTS:
•
Impression plaster is used to
make the impression of the edentulous
mouth.
•
For preparation of study models
of oral and maxillofacial structures.
•
To form cast and dies on which
dental prosthesis are constructed.
•
As a mold material for
processing complete dentures.
•
For mounting of casts on the
articulator.
•
Also used as a binder for
silica, gold alloy casting investment, soldering investment and investment for
low melting point nickel-chromium alloys.
GYPSUM PRODUCTS:
Refers to the various forms of
calcium sulfate, hydrous and anhydrous.
Manufactured by the calcination
of calcium sulfate dihydrate.
Calcination can be controlled
to produce partial or complete dehydration.
ADA No: 25. ISO No: 6873.
Gypsum products can be classified into five
types:
GYPSUM
PRODUCTS
IMPRESSION PLASTER PLASTER HIGH STRENGTH PLASTER
(
ISO TYPE I ) ( ISO
TYPE II )
STONE ( ISO TYPE III )
STONE, HIGH STRENGTH, LOW EXPANSION ( ISO TYPE IV )
STONE, HIGH STRENGTH, HIGH EXPANSION (
ISO TYPE V )
CHEMISTRY OF GYPSUM:
In the temperature range of 20º to 700ºC
that is important in dental manipulation of gypsum products, three phase
changes occur in the CaSO4-H2O system.
40º – 50ºC
1)
CaSO4.2H2O
CaSO4.1/2H2O + Water
(Calcium Sulfate
Hemihydrate)
90º – 100ºC
2) CaSO4.1/2H2O γ-CaSO4 +
Water
(Hexagonal Form)
(Soluble
anhydrite)
300º – 400ºC
3) γ-CaSO4 CaSO4
(Orthorhombic CaSO4)
(Insoluble anhydrite)
MONOCLINIC HEXAGONAL ORTHORHOMBIC
Calcium Sulfate Hemihydrate Soluble anhydrite Insoluble anhydrite
Calcium Sulfate Dihydrate
MANUFACTURE:
From the conversion temperature given
before, it can be seen that calcium sulfate hemihydrate would be produced by
heating gypsum to temperatures in the range of 50o to 90 oC. However, at these
temperatures the reaction is slow; even at 90oC substantially complete
conversion takes about 12 hours (Khalil et. al., 1971). Therefore, in
commercial processes temperatures higher than this are used, for shorter times.
The stable phase at these higher
temperatures is hexagonal calcium sulfate, so the initial product of
calcination is partly of very largely this anhydrous form. However, on cooling
to temperatures below 85 oC and exposure to atmospheric moisture, the hexagonal
calcium sulfate rehydrates to form the hemihydrate.
In the production of plaster, the gypsum is
ground to a fine powder, impurities such as sulphur (S) and quartz (SiO2) are
removed, and then it is subjected to calcination.
dehydration
by heat
or
other means
Mineral Gypsum
formulation
Plasters Dental plaster
Synthetic Gypsum Hydrocal Dental Stone
Densite High strength
dental stone
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