This is an important concept in glass casting.
Specific gravity is the relative weight of a substance compared to water. For example, a cubic centimetre of water weighs 1 gram. A cubic centimetre 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 to determine how much glass is needed to fill an irregularly shaped mould. If the mould holds 10 ounces of water then it will require 25 ounces of glass.
If filling the mould with water isn't practical (many moulds will absorb the water) then any material for which the specific gravity is known can be used.