Second of a Four Part Series: Designing for Metal Clay
Last time I discussed how to calculate the shrinkage of clay.
Now, we want to take that information and use it to figure out how to design a layout for a piece and create templates. We need to know how much to enlarge the design so the piece will shrink to the desired size after firing.
When I do this, I don’t need to measure anything. I just need to know the shrinkage rate of the clay I’m working with.
I can do an enlargement calculation one time for each clay type and always use it for sizing my templates for that particular clay type.
Here how I do it: (When calculating enlarging, I always use the number 10 to make it easy). Let’s say we are going to work with sterling clay, which we calculated the shrinkage for in my last post. It is 15%. If 100% (100) is the size we are starting with, and we shrink it by 15% (15), then 100-15=85. 85% is the size you are left with when something shrinks by 15%.
So, if our target size is our easy-to-use number, 10, we divide it by the percentage that we have after shrinkage; 10÷85=.1176. We need to enlarge our art by 118%.
I like to add an additional 2 to 3% to that number to accommodate the additional shrinkage of the clay from drying and the bit that I lose through sanding and refining. So when I work with sterling clay I enlarge my designs 121%. The pieces, when fired, shrink to a size that I expect. This makes all the difference when I’m designing something critical, like a bracelet that I want to be a certain size. It is also useful when working with creating settings for stones.
Another time I use these calculations is when making a series in different clay types. I made a group of earrings in silver, copper and bronze and I wanted them to all be the same size.
A quick guide to calculations:
• Divide the fired size by the unfired size. Subtract this number from 100.
•Subtract the shrinkage rate from 100. Divide the number 10 by this number.
Next time we’ll discuss using the enlargement rate to make templates of a design.