Properties

I. Physical

a. Excellent thermal and electrical insulator
b. Very dense
c. Excellent dimensional stability
d. Good reproduction of fine detail of hard and soft tissues

2. Chemical

a. Heating will reverse the reaction (decompose the material into calcium sulfate hemihydrate, the original dry component)
b. Models, casts, and dies should be wet during grinding or cutting operations to prevent heating

3. Mechanical

a. Better powder packing and lower water contents at mixing lead to higher compressive strengths (plaster < stone < diestone)
b. Poor resistance to abrasion

4. Biologic

a. Materials are safe for contact with external - epithelial tissues
b. Masks should be worn during grinding or polishing operations that are likely to produce gypsum dust

Finishing and Polishing

Remove oxygen-inhibited layer .Use stones or carbide burs for gross reduction.Use highly fluted carbide burs or special diamonds for fine reduction.Use aluminum oxide strips or disks for finishing. Use fine aluminum oxide finishing pastes. Microfills develop smoothest finish because of small size of filler particles
 

Gypsum Products

Classification of Dental amalgam

1. By powder particle shape .

• Irregular (comminuted, filing, or lathecut)
• Spherical (spherodized)
• Blends (e.g., irregular-irregular, irregularspherical, or spherical-spherical)

2. By total amount of copper

• Low-copper alloys (e.g., conventional, traditional); <5% copper
• High-copper alloys (e,g.  corrosion resistant); 12% to 28% copper

3.By presence of zinc

Examples

• Low-copper, irregular-particle alloy-silver (70%)-tin (26%)-copper (4%)
• High-copper, blended-particles alloy-irregular particles, silver (70%) –tin (26%) -Copper (4%); spherical particles, silver (72%)-copper (28%)
• High-copper, spherical-particles alloy-silver (60%) - tin (27%)-copper (13%)

The Sprue :

Its a channel through which molten alloy can reach the mold in an invested ring after the wax has been eliminated. Role of a Sprue: Create a channel to allow the molten wax to escape from the mold. Enable the molten alloy to flow into the mold which was previously occupied by the wax pattern.

FUNCTIONS OF SPRUE

1 . Forms a mount for the wax pattern .
2 . Creates a channel for elimination of wax .
3 .Forms a channel for entry of molten metal
4 . Provides a reservoir of molten metal to compensate for the alloy shrinkage .

SELECTION OF SPRUE

Sprue former gauge selection is often empirical, is yet based on the following five general principles:
1.   Select the gauge sprue former with a diameter that is approximately the same size as the thickest area of the wax pattern. If the pattern is small, the sprue former must also be small because a large sprue former attached to a thin delicate pattern could cause distortion. However if the sprue former diameter is too small this area will solidify before the casting itself and localized shrinkage porosity may result.
2.   If possible the sprue former should be attached to the portion of the pattern with the largest cross-sectional area. It is best for the molten alloy to flow from the thick section to the surrounding thin areas. This design minimizes the risk of turbulence.
3.   The length of the sprue former should be long enough to properly position the pattern in the casting ring within 6mm of the trailing end and yet short enough so the molten alloy does not solidify before it fills the mold.
4.   The type of sprue former selected influences the burnout technique used. It is advisable to use a two-stage burnout technique whenever plastic sprue formers or patterns are involved to ensure complete carbon elimination, because plastic sprues soften at temperatures above the melting point of the inlay waxes.
5.   Patterns may be sprued directly or indirectly. For direct sprueing the sprue former provides the direct connection between the pattern area and the sprue base or crucible former area. With indirect spruing a connector or reservoir bar is positioned between the pattern and the crucible former. It is common to use indirect spruing for multiple stage units and fixed partial dentures.

WETTABILITY
To minimise the irregularities on the investment & the casting a wetting agent can be used .

FUNCTIONS OF A WETTING AGENT

1 . Reduce contact angle between liquid & wax surface .
2 .Remove any oily film left on wax pattern .