Has been used in glazes as a substitute for red iron oxide.
Historically it was just a naturally-occurring red iron oxide containing impurities (i.e. clay minerals) or an ultra-high iron clay. Today, Crocus Martis sold by ceramic suppliers is more likely to be a soluble synthetic impure red iron sulphate or a calcined-at-900C version of the sulphate (it is insoluble). This material is not normally highly processed and therefore can produce iron specking in glazes. Normal iron oxides can be used as a substitute (although less will be needed since this material has a significant weight loss during firing).
Sulphur is released during firing and it does not decompose fully until as high at 1200C.