Effects of Amalgam.

(1) The combined effects of the properties of its ingredients should provide the most satisfactory restorative material.

(2) Quantity of mercury. Too little mercury in the mix results in a grainy, weak, readily tarnished, and corroded amalgam. Too much mercury will cause excessive expansion and weakened amalgam.

 (3) Composition of the alloy. Composition of the alloy must include a minimum of 65 percent silver, a maximum of 29 percent tin, a maximum of 6 to 13 percent copper, and a maximum of two percent zinc by weight

 (4) Correct proportion important. Before use, the silver alloy is mixed with pure and uncontaminated mercury. There are some alloys that are completely zinc free. They can therefore be used more successfully in a moisture-contaminated environment.

 (5) Properties of the finished product.

Silver imparts strength, durability, and color, gives the alloy desirable setting expansion, decreases flow, and accelerates (decreases) the setting time.

Tin makes the amalgam easier to work, controls excessive setting expansion, and increases both flow and setting time.

Copper increases hardness, contributes to setting expansion, reduces flow, and decreases setting time.

Zinc increases workability, and unites with oxygen and other "impurities" to produce a clean amalgam.

RINGLESS INVESTMENT TECHNIQUE
Used for phosphate bonded investments .
This method uses paper or plastic casting ring .
It is designed to allow urestricted expansion .
Useful for high melting alloys .

Manipulation

Mixture of powder and liquid is painted onto working cast to create shape for acrylic appliance à  After curing of mixture, the shape and fit are adjusted by grinding with burrs and stones with a slow-speed handpiece .Acrylic dust is irritating to epithelial tissues of nasopharynx and skin and may produce allergic dermatitis or other reactions.  Grinding may heat polymer to temperatures that depolymerize and release monomer vapor. which may be an irritant

Dental Porcelain and PFM Porcelains

Applications/Use

a. Porcelain inlays and jacket crowns
b. PFM crowns and bridges
c. Denture teeth

Terms

PFM-porcelain fused to metal
Fusing-adherence of porcelain particles into a single porcelain mass

Classification

 Dental porcelain is manufactured as a powder. When it is heated to a very high temperature in a special oven, it fuses into a homogeneous mass. The heating process is called baking. Upon cooling, the mass is hard and dense. The material is made in a variety of shades to closely match most tooth colors. Baked porcelain has a translucency similar to that of dental enamel, so that porcelain crowns, pontics, and inlays of highly pleasing appearance can be made. Ingredients of porcelain include feldspar, kaolin, silica in the form of quartz, materials which act as fluxes to lower the fusion point, metallic oxide, and binders. Porcelains are classified into high-, medium-, and low-fusing groups, depending upon the temperature at which fusion takes place. 
 
High-Fusing Porcelains. High-fusing porcelains fuse at 2,400o Fahrenheit or over. They are used for the fabrication of full porcelain crowns (jacket crowns). 

Medium-Fusing Porcelains. Medium-fusing porcelains fuse between 2,000o and 2,400o Fahrenheit. They are used in the fabrication of inlays, crowns, facings, and pontics. A pontic is the portion of a fixed partial denture, which replaces a missing tooth. 

Low-Fusing Porcelains. Low-fusing porcelains fuse between 1,600o and 2,000o Fahrenheit. They are used primarily to correct or modify the contours of previously baked high- or medium-fusing porcelain restorations. Eg  for PFM restorations

Structure

Components

a. Large number of oxides but principally silicon oxide, aluminum oxide. and potassium oxide    
b. Oxides are supplied by mixing clay, feldspar, and quartz.

Manipulation

Porcelain powders mixed with water and compacted into position for firing
Shrinkage is 30% on firing because of fusing and so must be made oversized and built up by several firing steps

Properties

1. Physical

a. Excellent electrical and thermal insulation
b. Low coefficient of thermal expansion and contraction
c. Good color and translucency; excellent aesthetics

2. Chemical

a. Not resistant to acids (and can be dissolved by  contact with APF topical fluoride treatments)
b. Can be acid-etched with phosphoric acid or  hydrofluoric acid for providing microll1echanical retention for cements

3. Mechanical

a. Harder than tooth structure and ,will cause opponent wear
b. Can be polished with aluminum oxide pastes

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

ZINC OXIDE AND EUGENOL 

This material is used for many dental purposes ranging from temporary restorative material to pulp capping. The material is composed of a powder that is basically zinc oxide and a liquid that is called eugenol.

Chemical Composition.

The powder must contain between 70 and 100 percent zinc oxide. The manufacturer may add hydrogenated resins to increase strength and zinc acetate to hasten the set. 

Eugenol is usually derived from oil of cloves. The oil of cloves contains more eugenol (82 percent) Eugenol is an obtundent (pain-relieving agent). It is a clear liquid that gradually changes to amber when exposed to light. 

Physical Properties. 
This material relieves pain, makes tissue less sensitive to pain, is slightly antiseptic, and is low in thermal conductivity. It provides a good marginal seal when placed in tooth cavities. The crushing strength (compression strength) of pure zinc oxide and eugenol is about 2,000 psi, which is low in comparison to other cements. The addition of hydrogenated resin increases the crushing strength to 5,000 psi. 

CLINICAL USES OF ZINC OXIDE AND EUGENOL 

Treatment Restoration. It helps prevent pulpal irritation in carious teeth, lost restorations, advanced caries, or pulpitis. This dental material also exerts a palliative effect on the pulp. 

Temporary Cementing Medium. Zinc oxide and eugenol is used as a temporary cementing medium for crowns, inlays, and fixed partial dentures. 

Intermediate Base. Zinc oxide and eugenol is used as an intermediate base. This material provides insulation between metallic restorations and vital tooth structure. Because of the low crushing strength, its use is sometimes contraindicated. 

Surgical Packing or Dressing. The surgical dressing applied and adapted over the gingival area after a gingivectomy. This dressing protects the area and makes the tissue less sensitive.