Is FAS stable at high temperature?

Solid FAS is stable at ambient temperature in the dry state, but strictly speaking it is not "heat-resistant". It begins to decompose above about 100°C, and above 126°C it decomposes exothermically, releasing SO₂, NH₃ and other gases. Its thermal stability is markedly better than that of sodium hydrosulfite — which decomposes above 85°C, can decompose explosively at 190°C, and self-ignites on contact with water; FAS has a higher decomposition threshold and behaves more controllably. As a reducing agent it will decompose on heating — and in bleaching applications that decomposition is actually part of the mechanism.