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 .

Applications/Use

• Load -bearing restorations for posterior  teeth  (class I, II)
• Pinned restorations
• Buildups or cores for cast restorations
• Retrograde canal filling material

 (1) Alloy. An alloy is a solid mixture of two or more metals. It is possible to produce a material in which the desirable properties of each constituent are retained or even enhanced, while the less desirable properties are reduced or eliminated.

(2) Amalgam. When one of the metals in an alloy mixture is mercury, an amalgam is formed. A dental amalgam is a combination of mercury with a specially prepared silver alloy, which is used as a restorative material.

(3) Mercury. Mercury is a silver-white, poisonous, metallic element that is liquid at room temperature

Waxes

Many different waxes are used in dentistry. The composition, form, and color of each wax are designed to facilitate its use and to produce the best possible results.

Applications

o    Making impressions
o    Registering of tooth or soft tissue positions
o    Creating restorative patterns for lab fabrication
o    Aiding in laboratory procedures

Classification

a. Pattern waxes-inlay, casting, and baseplate waxes
b. Impression waxes-corrective and biteplate waxes
c. Processing waxes-boxing, utility, and sticky waxes

Types

1) Inlay wax-used to create a pattern for inlay, onlay or crown for subsequent investing and casting in a metal alloy.
2) Casting wax-used to create a pattern for metallic framework for a removable partial denture
3) Baseplate wax-used to establish the vertical dimension. plane of occlusion. and  initial arch form of a complete denture
4) Corrective impression wax-used to form a registry pattern of soft tissues on an impression
5) Bite registration wax-used to form a registry pattern for the occlusion of opposing models or casts
6) Boxing wax-used to form a box around an impression before pouring a  model or cast
7) Utility wax -soft pliable adhesive wax for modifying appliances, such as alginate impression trays
8) Sticky  wax-sticky when melted and used to temporarily adhere pieces of metal or resin in laboratory procedures

Components

a. Base waxes-hydrocarbon (paraffin) ester waxes    
b. Modifier waxes-carnauba, ceresin, bees wax, rosin, gum dammar, or microcrystalline waxes
c. Additives-colorants

Reaction-waxes are thermoplastic

Properties

Physical

a. High coefficients of thermal expansion and contraction
b. Insulators and so, cool unevenly; should be waxed in increments to allow heat dissipation

Chemical

a. Degrade prematurely if overheated
b. Designed to degrade into CO2and H2Oduring burnout

Mechanical-stiffness, hardness, and strength depend on modifier waxes used
 

COMPOSITE RESINS

Applications / Use

• Anterior restorations for aesthetics (class III, IV, V, cervical erosion abrasion lesions)
• Low-stress posterior restorations (small class I, II)
• Veneers
• Cores for cast restorations
• Cements for porcelain restorations
• Cements for acid-etched Maryland bridges
• Repair systems for composites or porcelains

Polymerization--reaction of small molecules (monomers) into very large molecules (polymers)

Cross-linking-tying together of polymer molecules by chemical reaction between the molecules to produce a continuous three-dimensional network

Mercury bioactivity

• Metallic mercury is the least toxic from and is absorbed primarily through the lungs rather than the GI tract or skin
• Mercury in the body may come from air, water, food. dental (a low amount). Or medical sources
•  Half life for mercury elimination from body is 55 days .-
• mercury toxicity is <50 µm / m³ on average per 40-hour work week.
• Mercury hypersensitivity is estimated as less than 1 per 100,000,000 persons
• Indium-containing amalgams can have lower Hg vapor pressures than conventional dental amalgam

Zinc Phoshate Cement

Uses. Zinc phosphate cement is used both as an intermediate base and as a cementing medium. 

(1) Intermediate base. A thick mix  is used under permanent metallic restoration. This layer of cement protects the pulp from sudden temperature changes that may be transmitted by the metallic restoration. 

(2) Cementing medium. Zinc phosphate cement is used to permanently cement crowns, inlays, and fixed partial dentures upon the remaining tooth structure. A creamy mix of cement is used to seat the restoration or appliance completely into place. The cementing medium does not cement two objects together. Instead, the cement holds the objects together by mechanical interlocking, filling the space between the irregularities of the tooth preparation and the cemented restoration

c. Chemical Composition. 

(1) Powder. primary ingredients - zinc oxide and magnesium oxide. 
(2) Liquid. Phosphoric acid and water in the ratio of two parts acid to one part water. The solution may also contain aluminum phosphate and zinc phosphate Liquids exposed in open bottles will absorb moisture from the air in high humidity. The liquids will lose moisture if humidity is low. Water gain hastens setting; water loss lengthens setting time.
 
PROPERTIES OF ZINC PHOSPHATE CEMENT

a. Advantages. Some advantages of zinc phosphate cement as a cementing medium are:

o    Inconspicuous appearance. 
o    Speed and ease of usage. 
o    Sufficient flow to form a thin layer for the cementing of closely adapted crowns, fixed partial dentures, and inlays. 
o    Low thermal conductivity beneath a metallic restoration.

b. Disadvantages. Some disadvantages of zinc phosphate cement as a cementing medium are:

o    Low crushing strength that varies between 12,000 and 19,000 psi. 
o    Slight solubility in mouth fluids. 
o    Opaque material not suitable for visible surfaces. 

c. Strength. The ratio of powder to liquid increases the strength of phosphate cements to a certain point. For this reason, the dental specialist must use as thick a mix as practical for the work being performed. 

SETTING REACTIONS OF ZINC PHOSPHATE CEMENT 

a. Chemical Reaction. The chemical reaction that takes place between the powder and liquid of setting phosphate cement produces heat. The amount of heat produced depends upon the rate of reaction, the size of the mix, and the amount of heat extracted by the mixing slab. 

b. Powder to Liquid Ratio. The less powder used in ratio to the liquid, the longer the cement will take to harden. Good technique minimizes the rise in temperature and acidity of the setting cement that can injure the pulp. Generally, for increased strength, decreased shrinkage, and resistance to solubility, it is advisable to blend as much powder as possible to reach the desired consistencies. 

c. Setting Time. The setting time of zinc phosphate cement is normally between 5 and 9 minutes. 
 Lower the temperature of the glass mixing slab to between 65° and 75° F (18° to 24° C), if the glass mixing slab is not already cooled below the temperature at which moisture will condense on it. → Blend the powder slowly. →  Mix the powder over a large area of the cool slab. →  Use a longer mixing time, within optimum limits. 
 
Precautions. The following precautions should be observed. 

o    Prevent loss or gain of moisture in liquid cement by keeping bottles tightly stoppered. 
o    Dispense drops only when ready to mix. 
o    Use a cool, dry glass slab (65° to 75° F). 
o    Use the same brand of powder and liquid. 
o    Add increments of powder slowly. 
o    Use the maximum amount of powder to obtain the desired consistency. 

(To incorporate the most powder, the material should be mixed with a moderate circular motion over a large area of the slab, turning the spatula often.)