Vacuum casting is a method of encasing glass objects (typically lampworked) in clear glass -- without bubbles.
The process begins by heating the object to be encased to the temperature at which point it can no longer thermal shock -- about 850F for most soda-lime glass. The tranparent glass being used to encase the object is heated to the point that it runs freely.
The object is placed in a metal cup that is being kept warm to maintain the temperature of the object. The molten glass is then "poured" over the object while a vacuum is created by drawing air out of the cup. Done properly, the glass replaces the air and creates a bubble-free encasement around the object.