Have you ever broken the lid to a fully finished teapot and needed to make a replacement? If you simply measure the lid with calipers, the measurement won’t be accurate. Some clays shrink 8% or even 14% from freshly formed to the fired state. That can be enough of a difference to sabotage attempts to remake the broken piece, unless you accurately calculate how much larger it was before it shrank.
While helping a student with this exact problem, I had an aha moment. I had recently used a copy machine to enlarge a small image. What if I photocopied the broken lid, then used the copier to enlarge it by the correct percentage?
This solution involves knowing how much your clay shrinks, then doing some math. Most clay companies list shrinkage rates on their websites. If you mix your own clay, you can figure this out by making a shrinkage ruler. (Several great Ceramic Arts Network videos pop up on YouTube if you search for “determine clay shrinkage.”) As for the math, use the following equation, where X equals the shrinkage percentage:
(100 ÷ (100 – X )) • 100 = unfired equivalent size
To simplify this process for my students, I created a Clay Shrinkage Calculator on my website that allows you to enter the percentage your clay shrinks, click the “calculate” button, then the “unfired equivalent copy size” is determined. Punch this into the photocopier settings to enlarge by that percentage, tape your broken lid back together, photocopy it, then take a measurement off of the printout using calipers. Voila! Now you can remake the piece with fresh clay using the copy as a guide.
To use my calculator, visit www.AudryDealMcEver.com and click on “Clay Shrinkage Calculator” in the menu.