Alloy Resources: Porosity

There are two mains types of porosity in dental castings: Shrink and Gas

Shrink Porosity
Shrink porosity is irregular in shape, and tends to be located near the sprue or in the thicker parts of a casting. This is a natural phenomenon. As molten metal solidifies, it physically takes up less space. If there is more molten metal available, it will be pulled into the casting to "fill in the gaps." If there is no metal available, the casting will be left with porosity - generally situated along the grain boundaries. Shrink porosity must occur - the challenge to the technician is to keep the porosity out of the casting.
	Keep patterns out of the heat zone (see drawing/discussion in spruing section)
	Use chill vents on thick sections (drawing)
	Use reservoirs or runner bars for bridges and heavier castings
	Keep the connector between the reservoir and the pattern very short - <2mm
	Always wax and sprue with an eye on metal flow - avoid thin areas that may cool rapidly and inhibit metal flow to the rest of the casting

Gas Porosity
Gas Porosity can be found anywhere in the casting and generally appears during polishing as very round pits. Gas porosity occurs when gases are absorbed into the molten metal.
	Avoid overheating the metal
	Be sure that you are working with the proper torch settings
	Maintain your torch and equipment
	Replace worn or contaminated crucibles
	Preheat crucibles before casting to burn off any residue or moisture
	Be sure that you are achieving a complete burn out
There is no way to remove gas that has been absorbed. Set aside suspect buttons, and start with fresh metal.