Water Absorption in Pottery Clay

Generally speaking, the absorption rate of a clay body is how much water will absorb into a piece after the clay has been fired if left unglazed. This information is an important factor in determining which clay is suitable for making pieces that need to contain water or food for long periods of time. While the application of a glaze can create a watertight seal, if the glaze crazes (cracks), liquids can still absorb into the clay if left for long periods—so it’s important to know how your clay will behave.

While the absorption rate of a clay body is an objective measurement, subjective judgment plays a very important part, as you must decide what is an appropriate absorption rate for your work. There is no single appropriate absorption rate for all clay bodies or for all types of ceramic work.

If your work is primarily porcelain, you will probably want a very low absorption rate. If you are working at low-fire temperatures, you cannot hold your clay bodies to such exacting standards. 

There are three water absorption classifications as defined by the American Society for Testing and Materials (ASTM): Impervious porcelain, vitreous porcelain, and ceramic. Impervious porcelain has less than 0.5% water absorption rate, vitreous porcelain has 0.5-3% and ceramic has 3-7%.

When you are making dinnerware (plates, cups, bowls) that will come in contact with food you would want a clay with the lowest absorption rate possible(<.5%) This is because clays with a low absorption rate will not absorb water even if there is a bad fit between the clay and glaze (cracks in the glaze). 

When making work that will hold liquids but not necessarily food (vases, bird baths, fountains) an absorption rate can be slightly higher (.5-3%).  While the clays have a higher absorption rate they should not absorb enough water to seep through the piece onto the table.

Clays with an absorption rate of 3% or higher are best suited for flower pots or sculptures. Clays with this high of an absorption rate will absorb enough water to seep through the piece onto the surface below. It is important to be aware that these clays will absorb enough water that freezing becomes a concern in environments that regularly reach below freezing temperatures.

Need to figure out what the absorption rate of a clay body is? Follow the test following test.

Computational Clay Body Absorption Test:

This highly accurate test is carried out on a test tile made especially for the purpose. The test tile should be fired to maturity and not glazed.

• Make a test tile 3×3 inches or larger and fire it. Remove the tile from the kiln when it cools.
• Weigh the tile and write the weight down or on it (using a pencil or an indelible-ink pen).
• Submerge the tile in water for 24 hours or longer.
• Remove the tile from the water and use a sponge and towel to pat the surface dry.
• Weigh the tile again.
• Subtract the original weight from the weight of the immersed tile.
• Divide the difference by the original weight.
• Move the decimal point two places to the right. The result is the percentage of absorption. 

Example:

        Weight of the tile after immersion: 9.8 grams

        Weight of the tile when drawn from kiln: 9.4 grams

        0.4 ÷ 9.4 (original weight) = 0.042

After moving the decimal point two places to the right, the result is 4.2, meaning the absorption rate of the tile is 4.2%.