How to Run a Clay Shrinkage Test That You Can Reuse

A shrinkage percentage is only as useful as the test behind it. The practical goal is not to discover a universal number for “stoneware.” It is to build a small record that belongs to one clay body, one forming condition and one firing.

Why a bag-label percentage is not enough

Manufacturers usually publish shrinkage for a defined test and firing temperature. Your result can move with water content, forming pressure, particle alignment, drying, peak heatwork and clay batch. A thrown wall, an extruded strip and a slab rolled in one direction do not necessarily behave identically.

Drying shrinkage happens as inter-particle water leaves and clay particles pack closer together. Firing shrinkage follows as the body sinters and vitrifies. Digitalfire’s references on drying shrinkage and firing shrinkage describe why these values are comparative process indicators, not fixed properties independent of preparation.

What you need

• Clay from the batch you actually intend to use
• Two straight guides and a rolling pin, slab roller or extrusion method you can repeat
• A rigid ruler or digital caliper readable to at least 0.1 mm
• A pointed marking tool and a permanent identification code
• At least three test bars; five is better when a production decision depends on the result
• Your normal drying arrangement and the firing you plan to use

Prepare a bar that represents the work

Wedge the clay as you normally would. Make a bar long enough to carry a 100 mm gauge line with space beyond both marks; roughly 140–160 mm long, 25–35 mm wide and 6–10 mm thick is convenient. Consistent thickness matters more than a particular dimension.

Compress the surface and avoid stretching the bar after it reaches its final size. If your production method creates strong direction—slab rolling or extrusion, for example—mark the direction on the back. To examine anisotropy, prepare a second set at 90 degrees rather than pretending one bar represents both axes.

Incise two narrow marks exactly 100.0 mm apart. A 100 mm baseline makes a quick visual check easy, but still record the actual wet distance instead of assuming the marks stayed perfect while you made them. Add a code that survives firing: clay, batch, test number and intended cone. Do not rely on a loose paper label.

Measure at three defined stages

1. Wet measurement

Measure soon after forming, once the bar can be handled without distortion. Place the caliper points at the same edges of the marks each time. Record the distance, thickness, forming method and any unusual softness. Very soft clay contains more water and can show more drying shrinkage than the same body used at a stiffer consistency.

2. Bone-dry measurement

Dry all specimens in the same orientation with airflow reaching both sides. “Looks dry” is not a definition. The reliable studio check is stable mass: weigh the bar, wait, and confirm that another weighing shows no meaningful loss. Let it return to room temperature before measuring. Record cracks or curvature instead of discarding inconvenient specimens without a note.

3. Fired measurement

Place the bars where their firing conditions can be identified. Record the programmed schedule, atmosphere, witness cone and kiln position. Pyrometric cones respond to heatwork, not just the controller’s final number; Orton explains the effect of heating rate, hold time and atmosphere in its pyrometric cone resources.

Measure after the kiln and specimen are fully cool. If the bar is warped, forcing it flat changes the distance. Record the warpage and use a specimen that remained measurable for the linear result.

The three calculations

ClayCalc uses the conventional ceramic shrinkage relationships:

Drying shrinkage (%) = (wet length − dry length) ÷ wet length × 100

Firing shrinkage (%) = (dry length − fired length) ÷ dry length × 100

Total shrinkage (%) = (wet length − fired length) ÷ wet length × 100

Do not add drying and firing percentages to obtain total shrinkage. The stages use different denominators. For a bar measuring 100.0 mm wet, 94.0 mm dry and 89.0 mm fired:

• Drying shrinkage = (100 − 94) ÷ 100 × 100 = 6.0%
• Firing shrinkage = (94 − 89) ÷ 94 × 100 = 5.32%
• Total shrinkage = (100 − 89) ÷ 100 × 100 = 11.0%

Use replicates, not a favourite result

Calculate each bar separately and look at the spread. Three total-shrinkage results of 10.9%, 11.0% and 11.2% describe a controlled process. Results of 9.8%, 11.3% and 12.1% are a reason to examine preparation, measurement and kiln position before averaging.

For a fitted lid or tile order, use a conservative allowance informed by the spread and build in a practical tolerance. A calculated wet dimension is not a promise that every handmade piece will land on one exact fired dimension.

Printable clay test record

Common reasons the number changes

Clay stiffness changed

More forming water generally creates more room for particles to move together during drying.

The measuring line is not in the forming direction

Rolled, thrown and extruded particles can align. Compare axes deliberately.

The firing delivered different heatwork

A hold, slower final ramp, atmosphere or kiln position can change maturation even when the controller peak appears similar.

The specimen was not fully dry

A remaining moisture gradient can distort the “dry” dimension and increases firing risk.

Only one specimen was used

A single bar cannot show whether the method is repeatable.

When to repeat the test

Repeat when the clay batch changes materially, when you change forming or firing, when fitted work begins missing tolerance, or when absorption data suggests the body’s maturation moved. Pair shrinkage with absorption across two or three firing temperatures to see a useful maturity trend. Keep the physical bars or clear photographs with the record; six months later, the notes are far more valuable than a percentage with no context.

Sources and further reading

• Digitalfire — SHAB shrinkage and absorption test
• Digitalfire — Drying shrinkage test
• Orton Ceramic — Pyrometric cones, heatwork and cone charts