In dentistry we line our flasks prior to investing with strips
of thickish asbestos prior to investing. As memory serves this is to
lessen the shrinkage in the cast crown or bridge. I have never used
this in casting any jewelry. It was a must in dental school.
It's not so much that the lining itself lessens the shrinkage of the
casting, its that you're using high expansion investments (often,
crystobalite based, for golds, others for higher temp metals).
These investments expand more on heating than the normal jewelery
investments, and are intended so that the expansion, which makes the
mold cavity larger than the original wax model, then equals the
amount of contraction the casting will experience as it cools, so
that the finished casting is the same size as the wax. The high
expansion investments can either distort a casting flask, or if not
that, crack from the stresses, if the flasks are not lined. These
dental investments, while not commonly used by jewelers due to
higher cost and a bit more complexity in the investing process (the
lining, among other things), are still good for jewelers to keep in
mind, for those occasional casting tasks where this same exact match
of wax size to casting size is needed, such as some types of inlays,
and the like. Jewelers should note that the expansion of the
investment is not automatically right for your metal. You have to
match the thermal characteristics of your alloy, to those of the
investment. Calibration amounts to determing the correct flask
temperature for casting, which is then chosen as much to decide the
temperature of the flask when the metal solidifies, as anything else.
this of course, complcates the usual determinants for flask
temperature, that being casting quality and porosity, etc.