Make a pottery lid that still fits after firing

A new lid for an old pot is a different problem from a lid made alongside its jar. In the first case, the opening is already fired and the new lid still has shrinking to do. In the second, both parts are changing size.

I would settle that distinction before reaching for a shrinkage ruler. Automatically enlarging every lid to compensate for firing can make a co-made pair less likely to fit, not more.

Start with the surfaces that actually meet

The gallery is the ledge inside a pot that supports an inset lid. A flange is a projecting rim beneath a lid that helps locate it in the opening. The widest, decorative part of the lid may not be the part that determines whether it fits.

Sketch a little cross-section of your pot and lid. Mark where the lid rests, where it could catch, and which surfaces will carry glaze. Then use calipers, the adjustable measuring tool for transferring inside and outside widths, to check:

• The available opening: measure inside the surrounding wall at the height where the lid edge or flange will sit, and check the route it must pass through to get there.
• The support: record the gallery's usable ledge width and the opening left inside it. The part of an inset lid that rests on the ledge must be wider than that inner opening, with enough overlap to stay supported even when the lid slides to one side.
• The depth: check that a flange will not bottom out before the intended resting surfaces meet.
• The shape: measure across several directions. Record the narrowest and widest spans rather than averaging away an oval opening.

For a cap-style lid that goes over the rim, the geometry reverses: its inside diameter needs to be larger than the pot's outside rim diameter. Do not use an inset-lid subtraction for that arrangement.

Use an ordinary ruler to read physical dimensions from the calipers. A shrinkage scale's numbers are not interchangeable with ordinary centimetres or inches. On a scale labelled with predicted fired sizes, a target occupies a larger physical wet span, not a smaller one. Check what your ruler's labels mean before using it; our calibration guide explains the direction and spacing.

If you are making the pot and lid together

With the same clay, comparable moisture states and the same firing conditions, the lid and pot often shrink together. The opening gets smaller too. You do not normally take the wet opening and add the clay's full wet-to-fired shrinkage allowance to the lid alone.

Instead, establish their relationship while both are at a comparable stage. Transfer the opening measurement to the lid's fitting edge, leave working clearance, and refine the seating at leather-hard, when the clay is firm enough to handle and trim without readily changing shape. A little trimming stock is different from deliberately making the fitting diameter a full shrinkage allowance larger.

Keep the pair identified and manage their drying together, paying attention to a thin lid drying faster than a thick pot. Check the fit gently as they firm up; do not force one part into the other. For an irregular lid, a small matching mark on each piece can preserve its intended orientation. Bill Wilkey's lidded-jar demonstration shows a hand-built lid fitted directly to its gallery, including alignment marks and leather-hard adjustments.

Even if both parts shrink by exactly the same proportion, their clearance also becomes proportionally smaller. And real pots can distort: matching clay does not guarantee matching moisture history, drying behaviour or shape after firing. Check seating and lift-off, not just the nominal diameter.

If the pot is already fired

Here the existing pot stays as it is. Measure its actual finished opening, including any glaze already on it, and work backwards for the new lid only. The original pot's clay percentage is not the rate you need; you need the replacement lid clay's shrinkage through your planned process.

Choose a finished clearance, meaning the space left between the fitted surfaces so the lid can move freely. This is separate from shrinkage compensation. The appropriate clearance depends on the shape, surface finish, glaze placement, variation in your making and how the lid will be used. There is no gap that works for every jar.

A replacement-lid example

Suppose a round, glaze-fired pot has an available inside diameter of 100 mm at the fitting height. For this illustration only, choose 2 mm of total diameter clearance around an unglazed fitting edge. That means 1 mm on each side when centred, not 2 mm on each side. These are arithmetic assumptions, not recommended gaps.

The new lid's target glaze-fired fitting diameter is:

100 mm − 2 mm = 98 mm

Now suppose testing shows 12% linear shrinkage from freshly formed wet clay to glaze-fired clay, using the same body, forming moisture and drying and firing conditions planned for the lid. Linear shrinkage describes a change in length, not volume. The wet fitting diameter is:

98 mm ÷ (1 − 12 ÷ 100) = 111.36… mm, or about 111.4 mm wet.

As a forward check, 111.4 mm × 0.88 = 98.032 mm glaze-fired. That extra decimal precision belongs to the calculation, not a promise about the pot. Simply adding 12% to 98 mm would not reverse 12% shrinkage.

This calculation covers only the fitting diameter. The lid must still have adequate support on the gallery, suitable flange depth and whatever outer coverage the design needs.

Set the calipers to the calculated wet diameter using an ordinary ruler. In this example, the wet lid will not yet drop into the fired opening. Do not trim it down merely to make it fit the old pot while wet. Check against the planned dimensions instead. If you take a new measurement at leather-hard or bisque, use shrinkage data for the remaining stages, not the full wet-to-fired percentage again. The stage-specific sizing guide explains how to keep those measurements separate.

If you do not have suitable data, begin with a shrinkage test. Digitalfire's SHAB testing method distinguishes drying from firing shrinkage and records firing conditions. A useful bar result is a starting estimate; a trial lid tests the geometry too.

Leave room for glaze, and check after cooling

Glaze on a lid's fitting edge adds to its outside dimension; glaze inside an opening reduces the available space. The example above deliberately uses an unglazed lid fitting edge and measures the old pot as it already exists. If you glaze that new edge, the clay-only calculation is no longer the whole fit calculation. Test the intended glaze application rather than assuming a universal coating thickness.

For a co-made pair, ask the person running the kiln about drying support and whether to fire the lid in place. If fired together, keep the contact surfaces free of glaze and leave room for nearby glaze to move during firing. Neither firing together nor firing separately guarantees a fit. A finished old pot does not need to go back into the kiln with its replacement lid.

Wax resist helps keep glaze off during application; it is not a barrier against molten glaze. Mayco's wax-resist instructions explain that the wax repels glaze during application and dissipates during firing. Check the contact surfaces for stray glaze before loading.

Once everything is fully cool, lower the lid into place without pressure. Does it sit on the intended ledge? Lift out easily? Rock, catch in one direction, or slide far enough to lose support? I would write down those observations beside the fired measurements. “Too tight” is useful; “catches only along the narrow axis” is much more useful for deciding what to change next.