Barium carbonate is very stable thermally and does not readily disassociate unless at least some CO is available in the kiln atmosphere (i.e. reduction). BaCO3 is reduced to the unstable BaCO2 in the reaction:

While BaCO2 has a high melting temperature, it will break down much readily in a glaze melt (liberating the BaO for glass building). It decomposes even more readily during glaze melting in a reduction atmospheres. The dissolution process happens most quickly if BaCO3 is present in small amounts (e.g. 5% or less). Even if present in larger amounts, the glaze matrix can solidify with both types, one participating in the glass microstructure and the other acting as a refractory filler, opacifier and matting agent (especially in low temperature glazes).

Barium carbonate produces gases as it decomposes and these can sometimes cause many pinholes or blisters in glazes. 

In art ceramics barium carbonate is popular for the production of classic barium crystal mattes, BaO readily forms crystalline phases during cooling. These are dependent on adequate kiln temperatures, cooling cycle and the chemistry of the host glaze (a slightly reducing atmosphere is also beneficial). Some have observed that in some formulations, barium crystallizes so well that it will occur even with very rapid cooling. Barium can act to initiate crystal development in other chemistries, for example metallic glazes can benefit by the addition of some barium carbonate.