The Problem with Eyeballing
A lid that does not fit its pot is not a lid; it is a flat disc. The frustrating reality of pottery is that clay shrinks as it dries and fires, and shrinks differently depending on how thick it is. You simply cannot estimate your way to a fitting lid. You need a caliper.
What Is a Caliper?
A caliper is a measuring tool with two adjustable jaws. In pottery, we use a simple bow caliper or vernier caliper to measure the diameter of an opening so we can throw the lid to match.
The Shrinkage Problem
When you throw a pot and a lid, both will shrink by the same percentage during drying and firing, typically 10–15%, depending on the clay body. As long as both pieces are made from the same clay and fired to the same temperature, they shrink at the same rate. This means:
Measure the opening when wet, throw the lid to match that wet measurement, and both will end up the same size after firing.
The key insight: measure both at the same stage. If you measure the pot opening when it is freshly thrown and throw the lid immediately to match, both start at the same wet size and shrink to the same fired size.
What You Are Measuring
Different lid styles sit in different ways. Before you measure, you need to know what type of lid you are making:
- Drop lid: Drops into the opening. Measure the inner diameter of the opening.
- Cap lid: Sits on top of the rim. Measure the outer diameter of the rim.
- Gallery lid: Sits on a ledge (gallery) thrown inside the rim. Measure the inner diameter of the gallery.
Measurement Discipline
For reliable fits:
- Measure twice before throwing lid
- Keep caliper setting unchanged until lid form is set
- Label measurements if making multiple lids
Tiny measuring errors become visible after firing.
Dig Deeper
The shrinkage problem is rooted in how water leaves kaolinite particles during drying and how the clay body contracts further during vitrification in the kiln. A caliper is one of the oldest precision measuring instruments, used in metalworking and woodworking for centuries, and its application in pottery ensures that the relationship between pot and lid survives the unpredictable journey through the kiln.
