A fluoride of aluminum and sodium associated with granite. It is a valuable source of insoluble sodium used in enameling and sometimes in frits and glazes. This material is a very active melter.
Because cryolite lacks oxygen it is useful in creating artificial reduction glazes for electric firing.
Since this material is looking for oxygen to satisfy vacancies resulting from the gassing away of fluorine, it will take it from the kiln atmosphere or from neighboring molecules. The net effect of this take up is that cryolite has a lower than expected LOI.
The reaction is likely 2 Na3AlF6 producing 3 Na2O, 1 Al2O3 and 12 F. 2 Na3AlF6 has a weight of 419.8, 3 Na2O. Al2O3 has a weight of 287.8 and 12 F is 228. In a theoretical reaction 419.8 grams goes in and 515.8 comes out (assuming F stays in the elemental state and oxygen is absorbed from elsewhere as needed). However various factors can play to effect a partial gassing of fluorine (an incomplete supply of oxygen to fill all vacancies and the fact the fluorine can form silicates by supplanting oxygen and thereby releasing it as a gas). The complexity of the situation can be demonstrated by mixing it half-and-half with kaolin, weighing, firing to cone 6 and weighing again. In one such test we did the cryolite lost 13.9% of its weight. Considering that the kaolin component loses 13% there still should have been a net gain. The mixture was an incredibly active melter (it needs to be fired in a deep crucible), this suggests that F was gassing.