I think making a shrinkage ruler for each of the different clays you use would be ideal but, instead of 10cm which is only about 4", I would go with 10 inches (25cm). It would be easier to read and calculate the shrinkage. As far as the hole size...there are a couple of ways to go about that but all of them are just starting points until you know exactly how your particular clay body is going to shrink. Your primary consideration would be your boring tool and when you bore the holes. Will the clay be leather hard or bone dry? Leather hard would be easier to work with because bone dry clay is very fragile. What is your drill bit repertoire? What sizes do you have over 3/8"? Try 7/16", 1/2" and 9/16" and see what the holes shrink down to when fired. If one of them works, you have your working tool. Another option would be a 3/8" diamond boring bit to drill the hole AFTER the firing. With that, you'll have the exact size hole you want. You would want to drill those holes using a drill press and jigs to hold your work piece.
JohnnyKMember Since 10 Nov 2012
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Citrus Heights, CA
Ceramics, glazing techniques, photography, farming
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