Investment is a material that comes in a powder form which is mixed with water and poured around the wax models that are supported inside the stainless steel flask. It has the basic appearance of Plaster of Paris, but it is not the same formulation. It has been developed specifically for the jewelry and dental industries. It can be mixed in a number of ways. I use an electric egg beater at low speeds to mix the investment and water in a flexible rubber bowl.
Once thoroughly mixed, the container of investment is placed under vacuum to draw out gases and air bubbles that have been introduced while mixing. The investment is then carefully poured around the wax models making certain that all areas are filled in the process. If investment containing air bubbles is poured around the waxes, many of these bubbles will cling to the wax and will translate to small metal bubbles on the piece once it is cast. As a further means of trying to prevent the formation of very tiny metal bubbles that result from minute air pockets that cling to the wax, a de-bubblizing solution can be applied to the wax surface prior to investing.
This next step is optional but can be quite useful depending on the types of waxes that are being dealt with. Most waxes tend to be fairly soft and melt at lower temperatures, making them good candidates for undergoing wax elimination prior to the burnout in the kiln. (The most notable exception here is carving wax which contains a sizable quantity of plastic causing it to melt at too high a temperature for this process to be effectively employed). In this procedure the sprue base is first removed from the flask. The flask is placed sprue side down over a body of boiling water for one to two hours. Waxes that melt at at lower temperatures drip out into the water below. This effectively eliminates most of the fumes and smoke that would develop if it were burnt out in the kiln.
Whether or not the the bulk of the wax has been eliminated using the above described process, the flask must undergo a heat treatment process to eliminate any remaining wax and residues left by it. When molten metal comes into contact with the the empty space where the wax was, it reacts with whatever sorts of materials are in its path. If it encounters wax or its residues, unwanted gases are formed inside the metal which adversely effects the final casting.
The flask or flasks with the sprue bases removed, are placed inside of a kiln, either electric or gas. Both varieties work effectively. This process is called the burn out. The temperature of the kiln is slowly brought up to well in excess of one thousand degrees Fahrenheit over a period of several hours. A well burnt out flask will have investment that is very white in the area where the sprue base was. If it is at all gray, it needs to be burnt out further. Once all residues have been eliminated and the investment is thoroughly cured, the temperature is brought back down to specific levels and allowed to settle. The particular temperature depends on the type of metal that is to be cast as well as the fineness of the detail. As a general rule, the greater the detail and the finer the areas the metal must fill, the hotter the flask needs to be in order to allow the metal to completely fill the voids where the wax was before the metal freezes. Most generally, these temperatures range from 700-900 degrees F.
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