The Technique of Getting an Accurate Read
A caliper is only useful if you use it correctly. Sloppy measuring leads to lids that are slightly too wide or too narrow, and after firing, slightly is enough to ruin the fit.
Setting the Caliper to the Pot
- Place the caliper jaws inside the pot opening (for a drop lid measurement) or on the outside of the rim (for a cap lid).
- Open or close the jaws until they just touch both sides of the opening. Light contact, not pressing.
- Lock the setting if your caliper has a lock mechanism. If not, hold it firmly and do not adjust it.
Transferring the Measurement
- Without changing the setting, move the caliper to your freshly centered clay on the wheel.
- Hold the caliper above the clay, jaws pointing downward, and use it as a guide as you open the lid.
- Stop opening when the inside of your lid just touches both jaws.
This gives you a lid that starts at the same diameter as the pot opening, and since both shrink at the same rate, they will fit after firing.
The Tolerance Game
For a well-fitting but easy-to-remove lid, aim for the lid to be 1–2mm wider than the opening measurement. This small tolerance accounts for slight variation in how you handle and fire the pieces.
A lid that is exactly the same diameter as the opening may fit perfectly or may stick slightly. A lid that is 1–2mm wider will always fit loosely, which is actually preferable for functional ware.
Pro Tip
Write the caliper measurement on a scrap of paper or on the bat with a pencil. Once you lose the setting, it is very hard to re-establish exactly.
Fit Tolerance Checklist
Before wiring off the lid:
- Confirm target diameter plus 1-2mm tolerance
- Check flange or seat depth
- Check that knob placement does not distort lid center
This avoids sticky lids and loose wobble fits.
Explore More
The tolerance game in lid fitting is a practical lesson in precision that echoes the use of calipers in engineering and machining, where small measurement errors compound into unusable parts. Different clay bodies shrink at different rates; porcelain typically shrinks more than stoneware, so knowing your clay's shrinkage rate is essential for consistent lid fits.
