Talc is used to produce low expansion ceramics, for example- Thermal shock resistant stoneware bodies. In these, it acts as a low expansion flux that reduces body expansion by converting available quartz mineral, mainly in kaolin, to silicates of magnesia.
Talc by nature is a refractory powder. Yet, if used properly in amounts of only 1-5% in stoneware or porcelain bodies, it can drastically improve vitrification. Cone 06-04 ceramic slips, containing up to 60% talc, can often be fired to cone 6 without melting or even deforming.
Not withstanding this, other 50:50 talc:ball clay bodies will completely melt and boil at cone 6. In glazes, at middle temperature raw talc is refractory, its presence tends to create opaque and matte surfaces, yet if supplied in a frit it can create wonderfully transparent glossy glazes.
At cone 10, it is a powerful flux but also can be used in combination with calcium carbonate to create very tactile magnesia matte glazes (the MgO forms magnesium silicate crystals on cooling to give both opacity and a matte silky surface).
This being said, where transparency is needed it is generally best to source MgO from a frit (since talc loses its water of hydration quite late in the firing, after melt of most glazes has begun).