These two samples demonstrate how different the LOI can be between different clays. The top one is mainly Redart (with a little bentonite and frit), it loses only 4% of its weight when fired to cone 02. The bottom one is New Zealand kaolin, it loses 14% when fired to the same temperature! The top one is vitrified, the bottom one will not vitrify for another 15 cones.
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.