Porosity: is the amount of water fired clay can absorb. Clay in the plastic state has different particles filled with water and when fired to 600ºC it is transformed and becomes ceramic and is known as bisque. The spaces left where the particles have burnt away make the bisque very porous.
As the firing temperature increases, it continues to shrink so there are fewer spaces and the body becomes less porous. Each type of clay has a set temperature at which it completes its vitrification and if this is exceeded it can deform and melt. Porosity is measured by weighing the piece, boiling it in water, weighing it again, and calculating the increase in weight.