Specific gravity is the relative weight of a substance compared to water. For example, a cubic centimeter of water weighs 1 gram. A cubic centimeter of soda lime glass (includes most window and art glass) weighs approximately 2.5 grams. Therefore, the specific gravity of these types of glass is 2.5.
Specific gravity is a very useful concept for glass casting artists who are trying to determine how much glass is needed to fill an irregularly shaped mold. If the mold holds 10 ounces of water then it will require 25 ounces of glass.
If filling the mold with water isn't practical (many molds will absorb the water) then any material for which the specific gravity is known can be used. If, for example, the mold was made from a wax original and the weight of that original is known, the amount of glass required will be approximately 2.5 the weight of the wax. Why? Because most waxes have a specific gravity equal to that of water.