Wednesday, November 17, 2010

DIY Shrinkage Rule

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.

10% Shrinkage Rule next to a standard ruler
 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.

I converted to metric to more easily mark my ruler.
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.