# How to measure your own clay shrinkage with a test bar

Studio notes · Measuring clay

Make repeatable clay test bars, record wet, dry and fired lengths, and calculate a shrinkage rate with room for real studio variation.

By Zoë · The Poodle Potter

Published 2026-09-20 · 7 min read

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![Close-up of wet hands shaping the rim of a terracotta-coloured clay pot](https://potteryruler.com/images/guides/forming-clay.webp)

A test bar is a small piece of clay with two marks you can follow through drying and firing. The useful result is not just “12%”. It is “12% from wet to glaze-fired, for this clay and this firing”, with the measurements still there to check.

## Set up a test you can repeat

We are measuring **linear shrinkage**: change in length, not volume. Decide which stages you need before making the bars. For planning wet work to a finished dimension, measure from freshly formed wet clay to the cooled final firing. If you usually measure your pots at leather-hard instead, record a leather-hard starting measurement too. It cannot stand in for the earlier wet one.

Gather your usual clay, an ordinary millimetre ruler, a fine marking tool, a flat drying surface and a notebook. Calipers can help with later measurements if you can locate the marks consistently, but avoid pressing their jaws into soft clay. Use an ordinary scale for this test, not a compensated shrinkage scale.

Make three bars as a practical starting set, not a guarantee of statistical reliability. Use clay from the same batch, at the working consistency you normally use. For slab work, prepare the clay as you would your slabs. For thrown or extruded work, treat flat-bar results as an initial reference and also check a sample made by your actual forming method.

A bar roughly **120 mm long and 25 mm wide**, with an even thickness suited to your work, leaves room for a 100 mm measuring span and a label. Keep the dimensions and preparation consistent across the set. These are convenient studio dimensions, not a claim that every pot behaves like a small slab.

## Make the marks before drying starts

First scratch a short identifier outside the planned measuring span, where it will survive firing: A, B and C will do if your notebook connects them to the batch. Add an arrow showing the rolling or forming direction where relevant. Finish labelling before making the measurement marks, so it cannot disturb them.

With each bar flat and still wet, make two narrow, shallow marks **100 mm apart** along its length, away from the ends. Choose a clear reference on each mark, such as its centre, and always measure between those same references along the same straight line. Do not alternate between mark edges and centres, or measure the bar's outer ends later.

Check and write down the actual wet span immediately. If you made it 99 mm rather than 100 mm, use 99 mm in the calculation; do not quietly call it 100. Avoid moving or stretching the bar after this measurement. If handling changes it while still wet, measure again and replace the starting record.

In your notebook, beside those identifiers, record:

- Clay manufacturer and body, batch or bag reference if available, and whether it is reclaimed or mixed.
- Date, forming method, bar thickness and a brief description of the clay's wet working consistency.
- Wet mark-to-mark length in millimetres, measuring tool and chosen mark references.
- Intended drying routine, bisque firing and final firing.

“Wet” covers a range of moisture conditions. A repeatable moment, such as immediately after forming, is more useful than comparing freshly rolled clay with another bar left uncovered while you finish the rest.

## Follow the bars through your usual process

Dry them evenly on a flat support, using the drying routine you want the test to represent. Avoid forcing one bar dry while treating the others differently.

If you need a leather-hard rate, measure the original marks when the clay reaches your usual working firmness and describe that condition in your notes. Leather-hard is a working description rather than one exact moisture content, so this checkpoint is harder to reproduce.

When fully bone-dry, measure the marks again. Do not assume a pale surface means the inside is ready for firing; follow your studio's drying and kiln-loading practice.

Put the bars through your normal bisque and final firing, within the clay supplier's recommended range. Measure after each firing once the bars are fully cool. If you single-fire, record that path instead of inventing a bisque checkpoint.

For a clear **clay-body** measurement, leave the bars unglazed, including the reference marks, but send them through the final glaze-firing schedule alongside the work. “Glaze-fired” here names the firing stage, not a coating on the bar. A separate glazed project sample is still needed to check coating thickness, distortion and actual fit.

Record the firing date or load identifier, schedule including holds, target cone or temperature, kiln atmosphere, shelf position and how the bars were supported. If [witness cones](https://www.ortonceramic.com/pyrometric-cones) are used, note their result: these small ceramic indicators bend during firing to show heatwork, the combined effect of time and temperature. A controller's peak temperature alone does not describe the whole firing.

## Keep the lengths, not just the answer

Here is a **hypothetical record for one bar**, not a suggested shrinkage pattern for your clay. Copy the structure into your notebook, with a separate record for each bar. Add a leather-hard row if you measure at that stage, and leave any missed measurement blank rather than guessing it.

| Measurement stage | Same mark-to-mark length (mm) | Record alongside the length |
| --- | --- | --- |
| Freshly formed wet | 100 | Working consistency, forming method, date |
| Bone-dry, unfired | 94 | Drying routine and any distortion |
| Bisque-fired, cooled | 93 | Bisque load and firing conditions |
| Glaze-fired, cooled; bar unglazed | 88 | Final load and firing conditions |

For any pair of stages:

> **Linear shrinkage (%) = (starting length − later length) ÷ starting length × 100**

**A small test. A useful number. (mm)**

Marked length, wet: **100 mm**.

Same marks, fired: **88 mm**.

12 mm shorter over a 100 mm starting length = **12% shrinkage.** An illustrative example, not a clay specification.

**A small test. A useful number. (in)**

Marked length, wet: **3.937 in**.

Same marks, fired: **3.465 in**.

0.472 in shorter over a 3.937 in starting length ≈ **12% shrinkage.** An illustrative example, not a clay specification. Inch measurements are rounded.

Keep both lengths in the same unit. The record and calculations below use millimetres so that a 100 mm starting span makes the percentages easy to follow:

- **Wet to bone-dry:** (100 − 94) ÷ 100 × 100 = **6%**.
- **Wet to bisque-fired:** (100 − 93) ÷ 100 × 100 = **7%**.
- **Wet to glaze-fired:** (100 − 88) ÷ 100 × 100 = **12%**.
- **Bone-dry to glaze-fired:** (94 − 88) ÷ 94 × 100 ≈ **6.38%**.

That last calculation starts at 94 mm, not 100 mm. Do not add drying and firing percentages calculated from different starting lengths. Use the two recorded endpoints for the journey you actually need.

The 100 mm span makes the arithmetic easy, but it is not essential. If a bar actually started at **99 mm wet** and finished at **87 mm glaze-fired**, its 12 mm loss would mean **12 ÷ 99 × 100 ≈ 12.12%**, not 12%. The [stage-specific sizing guide](https://potteryruler.com/guides/stage-specific-pottery-sizing) explains how to apply your measured rate to a new piece.

## Decide how much confidence the bars deserve

Measure each span more than once, lifting and repositioning the ruler between readings. If the readings disagree, check the reference marks and alignment before calculating more decimal places. A 1 mm difference in the final reading, with a fixed 100 mm wet starting span, changes the result by **one percentage point**. Uncertainty in the starting measurement matters too.

Calculate each bar's percentage from its own starting length. Keep the individual results and their range, not only an average. For example, three hypothetical results of **11.5%, 12% and 12.5%** average 12%, but the spread still matters. Those decimals are useful only if your measurements support them; extra digits on a calculator are not extra evidence.

If a bar cracks across the measuring span or bows enough to change the straight-line measurement, label it as damaged or distorted and repeat the test. Do not bend it flat, follow the curve with a tape, or quietly drop it because the number looks inconvenient. Keep the observation: it may reveal a drying or firing problem that the percentage cannot solve.

Repeat a set in another normal firing before relying on the result for a close-fitting project. If you want to compare kiln positions, record where each bar went rather than pooling all the numbers without explanation. A few bars show the variation you observed, not a guaranteed limit for future pots.

For a wide slab or tile, measure two perpendicular directions on a representative sample as well. One line on a narrow bar cannot prove that a whole piece will shrink equally in every direction or stay flat.

## Put the record where you will use it

Keep the bars with their notes, and label the working rate with the clay body, **starting and ending stages**, firing conditions and observed range. Retest after a meaningful change in clay, moisture preparation, forming or firing, or when finished sizes stop matching your expectations.

When you're ready to choose a ruler, [choosing your shrinkage percentage](https://potteryruler.com/guides/choosing-shrinkage-percentage) helps you match it to that record. If the result falls between whole-number rates, the [decimal-rate guide](https://potteryruler.com/guides/decimal-shrinkage-rates) looks at whether rounding matters for the size you are making. There is no need to make the test result neater than the clay allows.

## Further reading

[Digitalfire's Shrinkage/Absorption Test (SHAB)](https://digitalfire.com/test/shab) is a detailed reference for marked test bars and systematic clay-body records. Its [firing shrinkage explanation](https://digitalfire.com/glossary/firing+shrinkage) is useful when interpreting dry-to-fired figures and changes with firing conditions. This guide covers the length measurements only, not the separate absorption test.

A little testing now. Fewer surprises after the kiln.

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