Q: How hot should the oven get when burning out jewelry investments? How hot should the mold be when you cast sterling silver? Why have my castings been coming out black and pitted, with little pinholes?
A: Generally you want to bring the temperature to at least 1200° Fahrenheit but no more than 1400°, letting it “soak” at the elevated temp for an hour or more, depending on the flask diameter. Then the flask is allowed to cool to the casting temperature, which for silver should be between 800° and 1000°, depending on the thickness of the finest section you are trying to cast: a filigree piece needs to be cast hotter than a solid lump.
I wouldn’t worry too much about the black firescale—that comes off with pickling and wirebrushing. Generally all sterling castings will be discolored when first broken out of the investment. I have heard of proprietary investment mixtures that contain anti-firescale ingredients, but have never tried them. Likewise there are “deox” alloys that purport to come out bright, if that is a concern.
The pinholes are another matter. If the flask was properly burned out so that no carbon residue was left in the mold, the most likely culprit would be shrinkage. When molten metal cools, it shrinks. Since it cools first at the surface, if there is no way for the interior mass to draw from a source of liquid metal, it will pull at the delicate surface crust that is in the process of forming. This causes various distortions from an overall lack of crispness in the surface detail to the pinholes you mention, with the most extreme case being an entire caved-in area that looks rough and pitted.
The way to avoid this is to have a large enough reservoir of metal in the sprue button, or if this is not practical in a particular configuration, the attachment of subsidiary reservoirs to the thickest parts of the piece—these are called “shrink balls”. Also remember to make the sprue between the piece and whatever shrinkage reservoir thick enough so that it doesn’t freeze before the shrinking action is complete.
Another culprit that causes voids in castings is gas, which can form if the alloy is overheated or is melted in the presence of air without a flux coat. Use straight borax or a proprietary melting flux, skimming it off immediately before casting.
by Andrew Werby