The density of brass is measured against a baseline of pure water. 1,000 kilograms of pure water is 1 cubic meter and was given the baseline "specific gravity" of 1, against all other materials were measured. In this system, the density of brass was measured to be anywhere from 8.4 kilograms per cubic meter to 8.73 kilograms per cubic meter, depending on the configuration of the brass itself.
. These numbers could be further altered by the possibility of impurities, but for the most part, these impurities are negligible. You can find more information here: http://www.simetric.co.uk/si_metals.htm
