Casting of glass or ceramic

A castable ceramic is prepared in a similar manner as metal cast preparation .
Glass is heated to 1360 degrees & then cast.
Phosphate bonded investments are used for this purpose .

CRUCIBLE FORMER

It serves as a base for the casting ring during investing .Usually convex in shape.
May be metal , plastic or rubber .
Shape depends on casting machine used .
Modern machines use tall crucible to enable the pattern to be positioned near the end of the casting machine .

SELECTION OF SPRUE 

1 . DIAMETER :
It should be approximately the same size of the thickest portion of the wax pattern .
Too small sprue diameter suck back porosity results .

2 . SPRUE FORMER ATTACHMENT :
Sprue should be attached to the thickest portion of the wax pattern .
It should be Flared for high density alloys & Restricted for low density alloys .

3 . SPRUE FORMER POSITION

Based on the
1. Individual judgement .
2. Shape & form of the wax pattern .

Patterns may be sprued directly or indirectly .
Indirect method is commonly used

Cement Bases

Applications

•    Thermal insulation below a restoration
•    Mechanical protection where there is inadequate dentin to support amalgam condensation pressures

Types

•    Zinc phosphate cement bases
•    Polycarboxylate cement bases
•    Glass ionomer cement bases (self-curing and light-curing)

Components

o    Reactive powder (chemically basic)
o    Reactive liquid (chemically acidic)

Reaction

o    Acid-base reaction that forms salts or cross linked matrix
o    Reaction may be exothermic

Manipulation-consistency for basing includes more powders, which improves all of the cement properties

Properties

Physical-excellent thermal and electrical insulation

Chemical-much more resistant to dissolution than cement liners

    Polycarboxylate and glass ionomer cements are mechanically and chemically adhesive to tooth structure

    Solubility of all cement bases is lower than cement liners if they are mixed at higher powder- to-liquid ratios

Mechanical- much higher compressive strengths (12,000 to 30,000 psi)
    Light-cured hybrid glass ionomer cements are the strongest
    Zinc oxide-eugenol cements are the weakest

Biologic (see section on luting cements for details)

    Zinc oxide-eugenol cements are obtundent to the pulp
    Polycarboxylate and glass ionomer cements are kind to the pulp
 

Mercury hygiene

• Do not contact mercury with skin
• Clean up spills to minimize mercury vaporization
• Store mercury or precapsulated products in tight containers
• Only triturate amalgam components-in tightly- sealed capsules
• Use amalgam with covers
• Store spent amalgam under water or fixer in a tightly sealed jar
• Use high vacuum suction during amalgam alloy placement, setting, or removal when mercury may be vaporized
• Polishing amalgams generally causes localized melting of silver-mercury phase with release of mercury vapor, so water cooling and evacuation must be used

POLYCARBOXYLATE CEMENT 

Use:. The primary use of polycarboxylate cement is as a cementing medium of cast alloy and porcelain restorations. In addition, it can be used as a cavity liner, as a base under metallic restorations, or as a temporary restorative material. 

Clinical Uses

Polycarboxylate cement is used in the same way as zinc phosphate cement, both as an intermediate base and as a cementing medium. 

c. Chemical Composition. 

(1) Powder:. It generally contains zinc oxide, 1 to 5 percent magnesium oxide, and 10 to 40 percent aluminum oxide or other reinforcing fillers. A small percentage of fluoride may be included. 
(2) Liquid. Polycarboxylate cement liquid is approximately a 40 percent aqueous solution of polyacrylic acid copolymer with other organic acids such as itaconic acid. Due to its high molecular weight, the solution is rather thick (viscous). 

d. Properties. 

The properties of polycarboxylate cement are identical to those of zinc phosphate cement with one exception. Polycarboxylate cement has lower compressive strength. 

e. Setting Reactions: 

The setting reaction of polycarboxylate cement produces little heat. This has made it a material of choice. Manipulation is simpler, and trauma due to thermal shock to the pulp is reduced. The rate of setting is affected by the powder-liquid ratio, the reactivity of the zinc oxide, the particle size, the presence of additives, and the molecular weight and concentration of the polyacrylic acid. The strength can be increased by additives such as alumina and fluoride. The zinc oxide reacts with the polyacrylic acid forming a cross-linked structure of zinc polyacrylate. The set cement consists of residual zinc oxide bonded together by a gel-like matrix. 

Precautions. 
The following precautions should be observed. 
o    The interior of restorations and tooth surfaces must be free of saliva. 
o    The mix should be used while it is still glossy, before the onset of cobwebbing. 
o    The powder and liquid should be stored in stoppered containers under cool conditions. Loss of moisture from the liquid will lead to thickening.