Fluorspar

Fluorspar (CaF₂), also known as fluorite, is a chemical flux and opacifier used in ceramics and glaze formulation. It helps lower the melting point of glazes and contributes to opacity at low-fire temperatures. However, at higher temperatures it decomposes — releasing fluorine gas which can be toxic and cause blistering in glazes. (digitalfire.com)

Can be hazardous if fluorine gases are released during firing. Handle powder carefully, wear gloves and a respirator, work in a well-ventilated area, and avoid ingestion.