Compounds with clays or glazes that burn away during firing. For example, calcium carbonate decomposes during firing to produce CO2 gas and loses almost half its weight. Other common volatiles are sulphur, carbon, water, fluorine, nitrogen. The term "volatilization" refers to the burning off of volatiles, it is not the same as "vaporization" which refers to the loss of desired oxides because they vaporize as a product of being fired toward or up to their boiling points.
Specific oxides needed in ceramic glazes can be subject to vaporization (and therefore lost up the chimney of the kiln) if firing proceeds to long or too high.
In ceramic manufacture, knowing about the how and when materials decompose during firing is important in production troubleshooting and optimization