During drying, clay particles draw together and shrinkage occurs. During firing the matrix densifies and shrinkage continues. More vitreous bodies shrink more.
Clay weight loss
Average clay contains about 20% water and 9% organic matter, thus, finished pots (bisque or glazed) weigh almost 30% less than the initial clay weight.
Bibliography
“Clay Shrinkage During Drying and Firing.” n.d. Accessed August 23, 2022. https://www.lakesidepottery.com/HTML%20Text/Tips/Clay%20drying%20and%20firing%20process.htm.
“Firing Shrinkage.” n.d. Accessed August 23, 2022. https://digitalfire.com/glossary/firing+shrinkage.