Various material used in ceramics can generate fluorine gas during firing. This gas in hazardous, for example ILO says: Avoid all contact and in all cases contact a doctor.
For example, fluorspar contains alot of fluorine has a TLV (threshold limit value) of 2.5 milligrams per cubic meter of air breathed. By comparison iron oxide is considered a safe-to-use material at 5.0, kaolin is 2.0, barium carbonate is 0.5, quartz is 0.1-0.05.
Cryolite also contains a very large amount of fluorine and thus poses a specific hazard. Even Cornwall Stone, a widely used glaze material, can contain up to 2%.
Good kiln ventilation is essential. Over a period of time, fluorine gas will even etch windows in the kiln area until they are opaque like frosted glass. This is ample evidence of its presence.
Fluorine Hazards at ilo.org
Fluorine at Wikipedia