There are times when a potter needs to work backward, beginning with the finished product and deciding how to make a copy, also known as reverse engineering. Perhaps one of these scenarios has happened to you:
1. A customer brings in a casserole with a broken lid (1). Or maybe the fragments of a whole pot. Can you mend it (no), or could you make another one like it (I’ll try).
2. You once made a series of a dozen attractive pots, and they’ve sold well. There’s only one left, and you’d like to make more, without having to re-invent the whole thing.
3. You picked up a stunningly attractive pot for a dollar at the junk shop. Beautifully made by an unknown potter somewhere in the past, and good enough to copy. This is a case for R & D (that’s pottery-speak for ripoff & duplicate).
Let me share with you a method that works well for me. All you’ll need is a black pen, a red pen, a millimeter ruler, and a simple calculator. But first some homework: a shrinkage test.
Simple Shrinkage Test
Because clay shrinks during drying and firing, of course you have to make the original wet pot bigger to account for this. You must do a shrinkage test on the clay you plan to use. If you haven’t done this already, here’s one way to do it:
Roll out a slab of clay, the same consistency that you normally use, with a thickness of about 7 mm (¼ inch, near enough). Trim it to a suitable shape, and on it make two marks that are 100 millimeters apart. Leave the bar to dry, and measure the length between the 100-mm marks. Expect it to have shrunk, usually by about 6 mm. Make a note of this, and set the bar aside until your next bisque firing. After the bisque, measure it again, and (surprise, surprise!) usually find that it’s still close to the same size. Next, put the bar through a glaze firing, and measure again. Expect more shrinkage this time.
A typical shrinkage bar, made from a white stoneware clay body (2):
wet length: 100.0 mm
dry length: 94.0 mm
bisque-fired length: 94.0 mm
cone 9 fired length: 87.5 mm
In those long-ago days before calculators, the potter would make a shrinkage ruler with the scale enlarged by the right amount to match the shrinkage of whatever clay was in use. Different clay, different shrinkage, different shrinkage ruler. I’m not a big fan of these rulers, messy things. It’s so much easier now with a calculator. Example:
wet size÷fired size = shrinkage number/factor
100÷87.5=1.14 (rounded to 2 decimal places)
This is the magic shrinkage number for this clay. Remember it. Write it down.
The Reverse Engineering Bit
Let me explain using a real example. A customer brought in a broken celadon sake jar (see figure 1). It was part of a set, and the customer (of course) was hoping for a replacement, same shape, same size. Where to begin?
The first thing to do is to make a sketch of the pot, and on it write measurements of height, width, etc. in black. Now multiply each measurement by the magic shrinkage number (1.14 in our example), and write the new value in red (3). Look carefully at the numbers. Those in black are measurements (in millimeters) taken from the broken sample. The numbers in red are the same measurements multiplied by 1.14. These red numbers are the dimensions to use when making the replacement pot. That’s all there is to it. Draw the sketch in your log book, with the fired size written in black, and the wet sizes in red.
As the replacement pot was under construction, a throwing gauge was set to 57 mm wide and 182 mm high, and a lower pointer was set at 94 mm wide and 74 mm high, marking the middle of the belly, the same as the red numbers in the sketch-book diagram (4)
How Much Clay to Use?
You can just guess, and probably get it right. Or maybe not. This becomes easier if you’ve done a little more homework first. Weigh out a 100-gram sample of your usual clay. Let it dry, and weigh it again. Bisque fire it, and weigh once more. Then fire it to the usual top temperature, and weigh again. Digital scales make this so easy. Divide the wet weight by the final weight to get the weight change factor.
The following numbers are using a sample of my most-used clay (5):
wet weight: 100.0 grams
dry weight: 78.5 grams
bisque-fired weight: 72.0 grams
cone 9 fired weight: 72.0 grams
In this example, (wet weight)÷(fired weight)=100÷72=1.4 approximately.
Weigh the desired pot that you want to copy, and multiply the resulting number by 1.4 to estimate how much clay to use. Add a bit to allow for trimming.
Summing it up: For the clay I use most, I just multiply the final fired size by 1.14 to get values for the wet size. Multiply the final fired weight by 1.4 to get height and width measurements for the approximate weight of clay to use. That took ten times longer to explain it than it does to do it. You can reverse engineer a pot in just a few minutes.
Did we get the arithmetic right? Not perfect, but really close.
Roger and Pauline Graham have operated a small cottage-industry pottery for 27 years near the village of Gerringong, Australia, just south of Sydney, making mostly domestic stoneware. To see more of their work, check out http://members.optusnet.com.au/~rogergraham.