This is perhaps due to the elimination of moisture and water of crystallization
that may be present with any hydrated constituent of the mineral.
Magnesium hydroxide releases its 31% water of crystallization
when heated to above 325[degrees]C as water vapor.
Black iron oxide and some other metal oxides are used in some applications, but care has to be taken not to choose materials that will lose water of crystallization
and/or change their crystal form at service temperatures, as this results in a change of color and possible damage to film integrity.
USG LP Filler #2, the dihydrate form of calcium sulfate, is made by fine grinding select gypsum containing 17-20% water of crystallization
. Because of the water, LP Filler #2 can provide fire retardancy in a room-temperature curable system such as unsaturated polyester spray-up.
Accordingly, the temperature is increased to desorb bound water, such as water of crystallization
, until the water content falls to the range required for optimum product stability.