Wax elimination (burnout):

Wax elimination or burnout consists of heating the investment in a thermostatically controlled furnace until all traces of the wax are vaporized in order to obtain an empty mold ready to receive the molten alloy during procedure.

• The ring is placed in the furnace with the sprue hole facing down to allow for the escape of the molten wax out freely by the effect of gravity .
• The temperature reached by the investment determines thethermal expansion. The burnout temperature is slowly increased in order to eliminate the wax and water without cracking the investment.
•For gypsum bonded investment, the mold is heated to650 -6870 c )to cast precious and semiprecious
precious alloys.
• Whereas for phosphate-bonded investment, the mold is heated up to 8340 c to cast nonprecious alloys at high fusing temperature.
The ring should be maintained long enough at the maximum temperature (“heat soak”) to minimize a sudden drop in temperature upon removal from the oven. Such a drop could result in an incomplete casting because of excessively rapid solidification of thealloy as it enters the mold.
• When transferring the casting ring to casting, a quick visual check of the sprue in shaded light is helpful to see whether it is properly heated. It should be a cherry-red color .

WAX BURNOUT AND HEATING THE RING

After the investment has set hard, the crucible former and the metal sprue former is removed carefully, and any loose particles at the opening of the sprue hole are removed with small brush.
The purpose of the wax burnout is to make room for the liquid metal. The ring is placed in the oven at 250C with the sprue end down, thus allowing the melted wax to flow, out for 30min or even up to 60min may be a good procedure to ensure complete elimination of the wax and the carbon.

Heating the ring: The object is to create a mold of such dimension, condition and temperature so that it is best suited to receive the metal.

Hygroscopic Low-Heat Technique. 

After the wax elimination the temperature of the same furnace can be set to a higher temperature for heating or else, the ring can be transferred to another furnace, which has already set to the higher temperature. In any case accurate temperature control is essential and therefore these furnaces have pyrometer and thermocouple arrangement. The ring is placed in the furnace with the sprue hole down and heated to 500C and kept at this temperature for 1 hour. In this low heat technique the thermal expansion obtained is less but together with the previously obtained hygroscopic expansion the total expansion amounts to 2.2 percent, which is slightly higher than what is required for gold alloys.

So this technique obtains its compensation expansion from three sources:
(1)   The 37º C water bath expands the wax pattern
(2)   The warm water entering the investment mold from the top adds some hygroscopic expansion
(3)   The thermal expansion at 500' C provides the needed thermal expansion.

High-Heat Thermal Expansion Technique. 

After the wax elimination, the ring should be placed in the furnace which is at room temperature and then the temperature is gradually raised, until it comes to 700C in 1 hour. Then the ring is heat soaked at this temperature for ½ hour. This slow rise in temperature is necessary to prevent 
This approach depends almost entirely on high-heat burnout to obtain the required expansion, while at the same time eliminating the wax pattern.  Additional expansion results from the slight heating of gypsum investments on setting, thus expanding the wax pattern, and the water entering the investment from the wet liner, which adds a small amount of hygroscopic expansion to the normal setting expansion.

Suspension liners

Applications

o    Dentin lining under amalgam restorations
o    Stimulation of reparative dentin formation

Components

-Calcium hydroxide powder
-Water
-Modifiers

Manipulation

Used as W/P or pastes Paint thin film on dentin → Use forced air for 15 to 30 seconds to dry → Film is thicker (15 µm) than varnishes → Do not use on enamel or cavosurface margins

Properties

Physical

-Electrically insulating barrier
-Too thin to be thermally insulating

Chemical

-High basicity for calcium hydroxide (pH is II)
-Dissolves readily in water and should not be used at exposed cavosurface margins or gaps may form

Mechanical - weak film

Biologic - calcium hydroxide dissolves, diffuses, and stimulates odontoblasts to occlude dentin tubules below cavity preparation
 

Properties of Amalgam.

The most important physical properties of amalgam are

• Coefficient of thermal expansion = 25-1 >ppm/ C (thus amalgams allow percolation during temperature changes)

• Thermal conductivity-high (therefore, amalgams need insulating liner or base in deep restorations)
• Flow and creep. Flow and creep are characteristics that deal with an amalgam undergoing deformation when stressed. The lower the creep value of an amalgam, the better the marginal integrity of the restoration. Alloys with high copper content usually have lower creep values than the conventional silver-tin alloys.

 Dimensional change. An amalgam can expand or contract depending upon its usage. Dimensional change can be minimized by proper usage of alloy and mercury. Dimensional change on setting, less than ± 20 (excessive expansion can produce post operative pain)

•  Compression strength. Sufficient strength to resist fracture is an important requirement for any restorative material. At a 50 percent mercury content, the compression strength is approximately 52,000 psi. In comparison, the compressive strength of dentin and enamel is 30,000 psi and 100,000 psi, respectively. The strength of an amalgam is determined primarily by the composition of the alloy, the amount of residual mercury remaining after condensation, and the degree of porosity in the amalgam restoration.
• Electrochemical corrosion produces penetrating corrosion of low-copper amalgams but only produces superficial corrosion of high copper amalgams, so they last longer

• Because of low tensile strength, enamel support is needed at margins

• Spherical high-copper alloys develop high tensile strength faster and can be polished sooner
• Excessive creep is associated with silver mercury phase of low-copper amalgams and contributes to early marginal fracture
• Marginal fracture correlated with creep and electrochemical corrosion in low-copper amalgams
• Bulk fracture (isthmus fracture) occurs across thinnest portions of amalgam restorations because  of high stresses during traumatic occlusion and/or the accumulated effects of fatigue
• Dental amalgam is very resistant to abrasion

Lost Wax Process

The lost wax casting process is widely used as it offers asymmetrical casting withnvery fine details to be manufactured relatively inexpensively. The process involves producing a metal casting using a refractory mould made from a wax replica pattern.
The steps involved in the process or the lost wax casting are:

1 . Create a wax pattern of the missing tooth / rim
2 . Sprue the wax pattern
3 . Invest the wax pattern
4 . Eliminate the wax pattern by burning it (inside the furnace or in hot water). This will create a mould.
5 . Force molten metal into the mould - casting.
6 . Clean the cast.
7 . Remove sprue from the cast
8 . Finish and polish the casting on the die .

The lost-wax technique is so named because a wax pattern of a restoration is invested in a ceramic material, then the pattern is burned out ("lost") to create a space into which molten metal is placed or cast. The entire lost-wax casting process . 

Wax pattern removal:

Sprue former can be used to remove the pattern. If not the pattern is removed with a sharp probe. Then the sprue former is attached to it. The pattern should be removed directly in line with the principle axis of the tooth or the prepared cavity. Any rotation of the pattern will distort it. Hollow sprue pin is advisable because of its greater retention to the pattern.

POLISHING MATERIALS

1 Tin Oxide. Tin oxide is used in polishing teeth and metal restorations. Tin oxide is a fine, white powder that is made into a paste by adding water or glycerin.

2. Pumice. Pumice is used as an abrasive and polishing agent for acrylic resins, amalgams, and gold. It consists mainly of complex silicates of aluminum, potassium, and sodium. Two grades--flour of pumice and coarse pumice--are listed in the Federal Supply Catalog.

3. Chalk (Whiting). Chalk is used for polishing acrylic resins and metals. It is composed primarily of calcium carbonate.

4.Tripoli. Tripoli is usually used for polishing gold and other metals. It is made from certain porous rocks.

5. Rouge (Jeweler's). Rouge is used for polishing gold and is composed of iron oxide. It is usually in cake or stick form.

6. Zirconium Silicate. Zirconium silicate is used for cleaning and polishing teeth. It may be mixed with water or with fluoride solution for caries prevention treatment. For full effectiveness, instructions must be followed exactly to obtain the proper proportions of powder to liquid.

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