One of the tricky things about making pots is that clay shrinks as it dries and fires. It is not all that bothersome until you have a custom order for mugs that need to be a specific height and diameter or you drop a casserole lid while unloading a too hot kiln.
Whenever I have needed a pot of a specific size, I have sat done and done the math and then proceeded to make the pot to the size I computed. This works fine, but is time consuming when done over and over. Finally, as I walked out the door to go to Highwater Clays, Alex said, "Honey? Why don't you just buy a shrinkage ruler?" He's brilliant, I tell you!
But I got to Highwater and they were all out of shrinkage rulers. When I got home, I decided that since I could do the math, I should just sit down and make a shrinkage ruler specific to my clay.
We use Highwater's Speckled Brownstone, which has a shrinkage rate of 10%.
The first step is to subtract 10% from 100% giving us 90%. This means the size of a fired piece is 90% of the size of the thrown piece.
Convert 90% into decimal form, or .9.
Next we need to figure out what one fired inch equals in freshly thrown clay, so we'll divide 1 by .9, which equals 1.11. So, 1 fired inch equals 1.11 inches in wet clay.
Now we have enough information to create our ruler. I used a paint stick for my ruler. A) it is free. B) I have them laying around all over the studio. C) Did I mention free?
Now, I find it hard to figure out on a standard ruler exactly where 1.11 inches would fall, so I converted to centimeters. 1.11 inches equals 2.8cm. I marked my ruler down to quarter inch increments (every 7mm) using an ultra fine tip Sharpie marker.
Considering that most shrinkage rulers cost about $13 and this only took about 5 minutes to make, I think this was certainly worth my time.