Shrinkage occurs because clay loses moisture during drying and its physical structure becomes denser during firing as particles vitrify.

Shrinkage is a crucial consideration for potters, as it can affect the final dimensions and fit of pieces, particularly in functional ware like lids and bases or tiles that must align.

Shrinkage rates vary depending on the type of clay, with earthenware typically shrinking less (5–10%) compared to stoneware or porcelain, which can shrink 10–15% or more. Understanding and accounting for these rates helps ensure consistent, well-fitting results in ceramic work.

Shrinkage is made up of two stages:

1 - Wet to dry shrinkage
2 - Dry to fired shrinkage

The amount of wet to dry shrinkage is directly related to the overall moisture content of the formed piece. 

Typically for slip cast items, the  moisture content will be around 28 – 30%. 

For pieces made from extruded clay the moisture content will typically be 19 – 22%. 

During drying, water surrounding the clay ingredients is drawn towards the surface of the formed piece through capillary action. The clay particles move towards one another as water is removed and the clay shrinks.  As drying continues, the leather hard stage will be reached when the clay particles become locked in place and are unable to move, (approximately 12 – 14% moisture content). 

No further wet to dry shrinkage will take place after the leather hard stage.

During the firing stage, some of the ingredients containing oxides of calcium, magnesium ,sodium and potassium will begin to melt at a lower temperature than other ingredients formed primarily from silica or alumina which have a higher melting point. 

As the temperature increases beyond 1000C a glassy molten phase is produced which gradually becomes mobile and fills the pore structure of the ceramic article.   As temperature advances further all available pore space is filled with the glassy phase and the ceramic article can be described as fully vitrified. 

At this point, the dry to fired shrinkage will have reached its maximum value.

You can find Shrinkage / Contraction rates for all clays here