Long shrinkage ruler for larger pots and slabs

A ruler that stops halfway across a platter leaves you with another job: carrying the measurement across without losing the line or shifting the starting point. One clear measurement from end to end avoids those awkward joins.

A longer shrinkage ruler can help, but it isn't the only answer. An ordinary long rule and a calculated wet dimension, a full-size template, or calipers that reach across the pot can be more useful than simply buying a longer version of the same tool.

Physical length is not scale capacity

There are two things to check in a ruler listing:

• Physical length: how long the tool itself is.
• Scale capacity: the largest dimension its usable markings can represent, at the chosen shrinkage percentage.

On a compensated scale that reads the expected fired size when laid against wet clay, the intervals are physically enlarged. At a hypothetical 12% wet-to-glaze-fired linear shrinkage (change in length, not volume), an interval labelled 20 cm needs about 22.73 cm of physical space: 20 ÷ 0.88. It does not measure an ordinary 20 cm.

Even if a tool had a full 24 cm of usable measuring span, that span would represent only 21.12 cm glaze-fired at this rate: 24 × 0.88. End margins could reduce the usable span further. This is an illustration of the maths, not a description of our ruler's markings or its maximum reading.

The PoodlePotter ruler is 9.45 in physically long. A long or custom-length version is not confirmed, so don't plan a larger span around one.

If you're considering another ruler for larger work, ask for the usable span and highest reading at your percentage, not just its overall length. The ruler buying guide covers the other choices without assuming that longer always means better.

Measure the span you actually need

For a flat slab, length and width are straightforward, provided the clay stays supported while you measure. For a pot, deciding where the endpoints belong can be harder than finding a tool long enough to reach them.

A broad rim: decide whether you need the outside diameter or the clear opening inside. Measure straight across the centre, not along the curved surface. Check another direction too; one reading can hide an oval rim. Large calipers can transfer the gap to a rule on the bench without laying a heavy ruler across soft clay. Touch lightly rather than squeezing the wall.

A tall pot: height is the vertical distance from its support surface to the chosen top point, not the length of its sloping side. A vertical rule beside the pot and a light horizontal reference held level with the top let you read that height without following the profile.

A curved slab or platter: a flexible tape laid along the surface measures the curve, not the straight span across it. Both can be useful, but they answer different questions. If the finished piece must fit a shelf, measure its widest straight span, including any projecting handles, rather than just the rim. If you're planning clay to drape over a mould, also check the surface path and any trimming allowance. A flat cutout's width is not automatically the formed platter's width.

One large slab, without a long shrinkage scale

Suppose you want a flat slab to finish 40 cm long after glaze firing. Assume tests using your clay, comparable forming moisture and the intended drying and firing process give 12% wet-to-glaze-fired linear shrinkage.

Wet length = target glaze-fired length ÷ remaining fraction
40 cm ÷ 0.88 = approximately 45.45 cm wet

You can mark that wet length with an ordinary rule whose usable span exceeds 45.45 cm. The decimal places show the calculation, not a promise of that precision in clay. It does not need a shrinkage scale: the calculation has already done the compensation. Applying a compensated scale again would allow for shrinkage twice.

For repeated pieces, transfer the wet dimensions to a full-size template. Label it with the clay, percentage, wet starting stage and glaze-fired target, and check both axes with an ordinary rule. Before making a batch, measure a representative slab through drying and firing in both directions; one test-bar length doesn't establish how the whole slab will behave. Keep a paper template dry or transfer it to suitable water-resistant material before using it against damp clay.

The 12% here is an example, not a recommendation for your clay. A leather-hard slab has already shrunk partway; use a measured leather-hard-to-glaze-fired rate if that's when you're sizing it, rather than applying the full wet-to-fired rate again. Measure your own shrinkage before relying on a template for a fitted project. Digitalfire's shrinkage test method explains why drying and firing measurements have separate starting lengths and why firing conditions belong in the record.

When you have to join measurements

Moving a short ruler along a long edge can work, but every move is another chance to leave a gap, overlap a section or turn slightly off line. Two transfers that each add 1 mm make the wet layout 2 mm too long. Under the same hypothetical 12% shrinkage, that would still be 1.76 mm extra after glaze firing, before any clay variation.

If a short ruler is all you have, make the layout on a dry template surface beside the clay. Draw one straight baseline, use fine endpoint marks, and align the ruler's actual zero mark rather than assuming the physical end is zero. Record each section instead of keeping a running total in your head. Keep the units consistent. Use ordinary distances throughout, or the same compensated scale throughout; don't mix the two. If you're calculating several sections, keep the unrounded values until the final total rather than rounding each section separately.

An overlapping check can reveal a misplaced join, though it won't expose every error shared by the same tool and method. An independent full-span check with an ordinary long rule or suitable tape is better. Keep the tape straight and flat, not bowed or following a rim.

For a template printed across several sheets, align its registration marks rather than simply butting paper edges together. Secure the sheets, then check the assembled overall length and width. Correctly printed individual pages can still make an incorrectly joined template.

For the print-scale check itself, Liz Denys' printable ruler notes recommend printing her files at 100% and checking their ordinary reference scale. Our calibration guide explains how to check compensated distances too.

For one big platter, I'd start with a full-span ordinary measure and a labelled template. For frequent, varied large work, a longer compensated ruler may save repeated calculations. Either way, the useful improvement is fewer transfers and clearer endpoints. It is not a promise that a wide slab will stay flat or that a fired pot will land exactly on the planned dimension.