During drying, clay particles draw together and shrinkage occurs. During firing the matrix densifies and shrinkage continues. More vitreous bodies shrink more.

(“Firing Shrinkage” n.d.)

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.

(“Clay Shrinkage During Drying and Firing” n.d.)

References

“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.