Ceramic Water Absorption Testing: A Practical Studio Guide

Water absorption compares the dry mass of a fired ceramic specimen with its mass after accessible pores have filled with water. The arithmetic is short. Producing dry and saturated weights that mean the same thing from test to test is the real work.

Start with the question you need to answer

A studio test might compare the same body at cone 5 and cone 6, check whether a new clay batch behaves like the previous one, or investigate bloating and over-firing. State that question on the record. If the only note says “2.1%,” the number becomes difficult to interpret once the firing and specimen are forgotten.

ASTM C373 covers water absorption and related properties for specified fired ceramic products. The official scope and significance are useful, but a casual studio procedure should not be represented as a certified ASTM test unless the current standard has been followed in full with appropriate equipment and controls.

Choose representative specimens

Use more than one piece from the same clay preparation. Test bars made alongside shrinkage specimens are convenient because they connect dimensional change with porosity. Keep thickness and forming method consistent, round sharp corners, and mark an identification code that survives firing.

Glaze changes access to the pore network. For a clay-body comparison, use unglazed specimens or leave sufficient unglazed surface and document it. A fully glazed cup can appear to absorb less simply because water could not enter through the covered surfaces during the procedure.

Record clay name and batch, preparation water if known, specimen dimensions, kiln, position, schedule, atmosphere and witness-cone result. When comparing temperature, fire specimens from the same prepared batch rather than making a new bar weeks later with different stiffness.

Dry weight must be genuinely dry

Ambient “bone dry” ware can still exchange moisture with the room. A controlled test dries the fired specimen, cools it without allowing uncontrolled moisture uptake, and weighs it promptly. The exact drying temperature and time must follow the method you are using; consistency is essential.

For studio trending, verify stable mass: dry, cool in a dry environment, weigh, then repeat the cycle until the mass no longer changes beyond your scale’s practical resolution. Label that value D. Avoid touching the specimen with wet hands between cooling and weighing.

Saturate accessible pores consistently

Different saturation procedures can produce different results. Boiling, extended soaking and vacuum methods do not automatically give interchangeable values. Choose the procedure appropriate to your comparison and use it every time. If you are working to a specification, follow the named current standard rather than a shortened internet summary.

After saturation, remove surface water in a consistent way without pulling water back out of the pores. Use a damp cloth to remove the visible film, do not leave the specimen drying on the bench, and weigh promptly. Label this saturated mass S.

The calculation

Water absorption (%) = (S − D) ÷ D × 100

If a fired bar is 250.00 g dry and 255.25 g saturated:

(255.25 − 250.00) ÷ 250.00 × 100 = 2.10%

The 5.25 g gain is the mass of water retained under that saturation procedure. Report only the precision your scale and method support. A kitchen scale reading whole grams cannot justify an absorption result to two decimal places on a small specimen.

Read the trend with shrinkage

Absorption normally falls as a clay body matures and open porosity closes. Firing shrinkage often rises over the same range. Testing both properties across several temperatures produces a much more useful picture than chasing one threshold. A curve that stops improving, or reverses while the body swells, can signal that the safe maturity window has been passed.

Do not assume lower is always better. A body formulated for earthenware can remain intentionally porous; an over-fired body can deform or bloat. The correct range depends on the material, process and use. Product specifications and applicable standards take priority over generic pottery charts.

What absorption cannot prove

• Food-contact safety: this also depends on glaze composition, fit, firing and appropriate leach or migration testing.
• Freeze–thaw performance: porosity matters, but a service claim requires the relevant product test and geometry.
• Mechanical strength: absorption can correlate with maturation, not replace a strength test.
• Glaze fit: crazing and shivering involve thermal expansion and the glaze-body system.
• Every piece in a firing: one kiln position may not represent another. Witness cones and distributed specimens help.

Printable absorption record

Troubleshooting inconsistent results

Replicates differ widely

Check specimen thickness, kiln position, surface water removal and whether dry mass had stabilized.

Absorption increased at a hotter firing

Confirm labels and heatwork. If correct, examine bloating, cracking or a body that has moved past its useful maturity range.

The saturated weight keeps falling

The specimen is drying during handling. Shorten and standardize the interval between surface wiping and weighing.

The result is zero

Check scale resolution and specimen size. “No detected gain” is not necessarily absolute zero porosity.

Safety notes

Use heat-resistant handling procedures for drying equipment and boiling water. Never place a sealed or moisture-containing object into an oven or kiln without an appropriate procedure. Ceramic dust is hazardous; do not dry-grind fired specimens without suitable engineering controls and respiratory protection. Test procedures do not override the kiln, material or equipment manufacturer’s instructions.

Sources and further reading

• ASTM C373 — scope and significance of water absorption testing
• ASTM — Standards and the Ceramic Tile Industry
• Digitalfire — Shrinkage/Absorption Test (SHAB)