Stage-specific forward and reverse sizing with a pottery shrinkage ruler

A 100 mm bone-dry tile and a 100 mm glaze-fired tile give you the same reading on an ordinary ruler, but they are not the same target for making a match. The dry tile still has firing ahead of it.

The calculation needs two stage names: where the new clay starts, and where you want it to match. It also needs a direction: are you predicting a size from the clay in front of you, or planning clay to reach a target?

The stage map

For a usual bisque-then-glaze process, the route is:

Wet → leather-hard → bone-dry → bisque → glaze-fired

• Wet: workable clay at the moisture state used for your initial measurement. A freshly rolled slab and a partly dried slab need not have the same shrinkage left.
• Leather-hard: stiff enough for work such as trimming, but still holding moisture. It is a working description, not a fixed percentage of drying completed.
• Bone-dry: dried, but still unfired. Reaching this stage does not mean the piece has reached its final fired dimensions.
• Bisque: the first firing in this route. Record which firing it was; “bisque” alone does not specify its temperature or dimensional result.
• Glaze-fired: the later firing used to finish this piece. Measure after cooling, and keep the firing conditions with the result.

Some pottery follows a different firing route. Label the stages you actually use rather than forcing them into this sequence. Lakeside Pottery's drying and firing explanation is useful background on why drying and firing are separate changes.

A wet-to-glaze-fired rate describes the whole journey. A bone-dry-to-glaze-fired rate describes only the remaining firing change. Neither can stand in for the other, and a wet-to-fired scale should not be applied unchanged to a leather-hard piece.

One hypothetical clay record, three destinations

All the numbers below are invented teaching examples, not recommended rates for a clay type. Imagine a test made from one clay body, measured at a consistent wet forming state, dried fully, bisque-fired to cone 06 and glaze-fired to cone 6 in oxidation. Those are the imagined test conditions, not a firing recommendation; use your clay supplier's guidance for your own body.

Suppose the same marked span measures:

• 100 mm wet
• 94 mm bone-dry: 6% smaller than the wet span
• 92 mm after cooled bisque: 8% smaller than the wet span
• 88 mm after cooled glaze firing: 12% smaller than the wet span

Each percentage here starts from the original 100 mm wet measurement. The 8% is not an additional loss after drying, and the 12% is not an additional loss after bisque.

These are linear measurements: lengths, widths or diameters, not volume or capacity. To collect your own evidence rather than borrow these figures, follow the test-bar guide. Keep the clay body, moisture state, forming method, drying conditions and firing schedules with the measurements.

Forward: what size will this wet piece become?

Forward sizing starts with a dimension you already have. The calculation is:

Later size = starting size × (1 − shrinkage percentage ÷ 100)

Using the hypothetical record, a straight span measuring 150 mm wet would be estimated at:

• Bone-dry: 150 × 0.94 = 141 mm
• Bisque: 150 × 0.92 = 138 mm
• Glaze-fired: 150 × 0.88 = 132 mm

Those are three predictions from the same wet measurement, not three successive instructions. If you want the bone-dry size, stop at the wet-to-bone-dry calculation. Applying the full 12% would predict the wrong stage.

The estimates also assume that the dimension behaves like the test span. A flat test bar cannot promise that a rim will stay round or a slab will stay flat.

Reverse: how large should I make the wet clay?

Reverse sizing starts with a desired later dimension, perhaps measured from an existing object. Use the rate for the new clay's journey to that stage:

Starting size = target size ÷ (1 − shrinkage percentage ÷ 100)

With the same hypothetical clay record, a 100 mm target calls for different wet sizes:

• To match 100 mm at bone-dry: 100 ÷ 0.94 ≈ 106.38 mm wet
• To match 100 mm at bisque: 100 ÷ 0.92 ≈ 108.70 mm wet
• To match 100 mm at glaze-fired: 100 ÷ 0.88 ≈ 113.64 mm wet

The decimal places show the arithmetic, not a promise of that precision in clay.

Adding the shrinkage percentage to the target does not undo shrinkage. For the 12% example, 100 × 1.12 gives 112 mm wet, which would become only 98.56 mm after a 12% reduction. Division gives the correct starting estimate because the percentage loss is measured against the larger starting size.

Is the existing target staying as it is?

If you measure an existing bone-dry tile at 100 mm and want the new tile to match it when the new tile is also bone-dry, use the wet-to-bone-dry rate. That calculation does not establish either tile's eventual fired size.

If both tiles will later be fired and must match then, consider the remaining shrinkage of both. Different bodies or firing histories can make an equal dry size an unequal fired size.

An already-fired object that will not be fired again is different: its measured dimension can be a fixed fired target for the new piece. For a replacement lid, first allow for clearance, the space between fitted surfaces that lets the lid move, and any glaze on the new fitting surfaces. Then work backwards from the intended fired clay dimension, not directly from the pot's opening. A co-made same-clay lid and pot often shrink together, so do not automatically enlarge every lid by the full shrinkage allowance. The lid-fitting guide separates those situations.

What if the piece is already dry?

Return to the hypothetical test span: it was 94 mm bone-dry and 88 mm glaze-fired. The remaining shrinkage is:

(94 − 88) ÷ 94 × 100 ≈ 6.38% from bone-dry to glaze-fired

It is not 12% minus 6% = 6%. Subtracting the two wet-based percentages gives a difference relative to the original wet length, not a percentage of the now-smaller dry length.

You can use the measured lengths directly, without rounding that percentage first. The fraction remaining from bone-dry to glaze-fired is 88 ÷ 94:

• Forward from 150 mm bone-dry: 150 × (88 ÷ 94) ≈ 140.43 mm glaze-fired.
• Reverse from a 100 mm glaze-fired target: 100 ÷ (88 ÷ 94) ≈ 106.82 mm bone-dry. That is a dry-stage checking dimension, not the wet size to make.

These use the same hypothetical record. Notice how different the dry starting size is from the 113.64 mm wet needed for that same fired target.

Separately measured drying and firing percentages should not simply be added: their starting lengths may differ. Digitalfire's shrinkage test method records dry and fired lengths and uses the dry length as the reference for firing shrinkage. Check the reference stage before using a supplier's figure labelled “firing shrinkage”.

Put the right journey on the ruler

A shrinkage ruler puts that calculation onto a physical scale. Liz Denys's printable-ruler instructions describe measuring wet clay to read its expected fired size directly. The scale still needs the right stage-specific percentage.

On a compensated scale whose labels represent later sizes, the distance from zero to a 10 cm later-size mark for 12% shrinkage would be 10 ÷ 0.88 ≈ 11.36 ordinary cm. That larger physical span is the starting size. In reverse, you use it to lay out the target; in forward use, you compare a starting span with the scale to read its predicted later size. Do not assume a shrinkage-scale “10” is an ordinary 10 cm, or assume every ruler uses the same layout. See how to check a shrinkage scale before relying on unfamiliar markings.

The hypothetical 6%, 8% and 6.38% stage rates here are not selections on the PoodlePotter preview, which lists whole-number rates from 10% to 18%. For an unmatched rate, use the calculation with an ordinary ruler rather than substituting an unrelated preset.

Beside a planned dimension, I would keep a note like: “113.6 mm wet → estimated 100 mm glaze-fired; 12% wet-to-glaze-fired; clay and firing record attached.” It leaves much less room for guessing which part of the journey the number describes.