This black body is made by Seattle Pottery Supply. Surfaces like this are obviously not functional, but for decorative ware? Yes! How does this happen? This body contains a material that is adding to its LOI (likely raw or burnt umber). Not just that, but the gases are being expelled at the wrong time. How is that? The glaze is fluid at cone 6 and begins melting way down around cone 04. It is melting long before the gases of decomposition from the body are finished being expelled. So they have to bubble up through the glaze, creating the effect you see here. This body is actually over-mature and brittle at cone 5, but at cone 01 its strength is fairly good.
Suspended micro-bubbles in ceramic glazes affect their transparency and depth. Sometimes they add to to aesthetics. Often not. What causes them and what to do to remove them.
Loss on Ignition is a number that appears on the data sheets of ceramic materials. It refers to the amount of weight the material loses as it decomposes to release water vapor and various gases during firing.