Testing

Water Absorption Calculator

Measure fired ceramic water absorption (porosity) using the standard dry/wet weight test.

Updated May 2026

Dry Weight

Weigh the fired test piece bone dry before soaking.

Wet Weight

Weigh after boiling 2hr + soaking 24hr. Pat dry surface before weighing.

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Results

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Enter your measurements above and click Calculate.

ASTM C373 Absorption Test

Water absorption measures open porosity — the percentage of a fired ceramic's weight that can be absorbed as water enters open pores. The standard test method (ASTM C373 / ISO 10545-3) boils the test piece to force water into all accessible pores.

Absorption % = ((wet_weight − dry_weight) / dry_weight) × 100

Classification by Absorption

Absorption	Classification	Examples
< 0.5%	Vitrified	Porcelain, bone china
0.5–3%	Fine stoneware	Dense stoneware (Cone 10)
3–6%	Stoneware	Standard stoneware (Cone 6)
6–15%	Earthenware	Terracotta, low-fire clay
> 15%	Very porous	Under-fired / sculptural clay

lightbulb Quick Tips

check_circle Boil test samples for 2 hours then soak 24 hours (ASTM C373). Do not short-cut — partial soaking gives false low results.
check_circle Pat the surface dry with a damp sponge (not fully dry cloth) before wet weighing to remove surface water only.
check_circle For food-safe functional ware, target absorption below 3%. Vitrified porcelain achieves less than 0.5%.
check_circle High absorption (> 10%) makes clay susceptible to freeze-thaw damage outdoors. Not suitable for garden ceramics.

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Frequently Asked Questions

How do I perform a water absorption test? expand_more

Fire and cool a test piece. Weigh it dry (dry weight). Boil it in water for 2 hours, then let it soak for another 24 hours. Remove, quickly pat dry, and weigh again (wet weight). The difference divided by dry weight × 100 = absorption %.

What absorption level is food-safe? expand_more

For functional food-safe ware, aim for under 3% absorption. Fully vitrified porcelain is under 0.5%. Note: the clay body absorption matters less if the glaze is intact and pin-hole free, but low absorption is the safest baseline.

My stoneware shows 5% absorption — is something wrong? expand_more

Stoneware fired at Cone 6 often shows 1–4% absorption; this is normal. If absorption is above 5%, your clay may be under-fired for the temperature rating. Try firing one cone higher and test again.