Measuring Shrinkage

I talk sometimes about measuring my work so that I can account for shrinkage during firing. Clay shrinks during the drying and firing process but metal and plastic bike parts do not shrink, obviously. I need to build my ceramic sculptures a bit larger than I intend them to be when fired. The same issue comes up when building lamps, water fountains, sinks and more lamps.

Bike gear set on top of a fired shrinkage tile. The clay impression of the bike gear is visibly smaller than the gear itself.

At school I tell students we have about 12-13% shrinkage from wet to glaze fired. We tested it quite a while ago, but my clay at home I haven't tested. I suppose I could look up Seattle Pottery's listed shrinkage rate, but I'm firing to a lower temperature and mixing old clay bodies together, so it's best to check shrinkage myself.

Shrinkage tile after firing

The easiest way I know to test shrinkage is to make a shrink tile. I rolled a slab out of my clay (in this case I am using Seamix with sand from Seattle Pottery) and cut it into a rectangle at least 12 centimeters long. I used a ruler and a needle tool to mark a 10 centimeter line in the clay. I labeled the tile with the clay name and punched a hole in case I want to hang the tile later. Then I let it dry slowly before firing to prevent warping. Shrinkage can still be measured accurately with a flexible ruler on a bent tile, but it's easier if the tile stays flat.

Shrinkage tile with ruler to compare measurements before and after

  

I marked each centimeter just in case I made a mistake on one of the marks. With more marks, I can check shrinkage for 10 centimeters or each centimeter individually. (For those of you who enjoy a math challenge, go ahead an use inches for your marks, but calculating shrinkage rates and double checking measurements is much easier in a base ten system.) After firing I remeasured the original line. The new length is 9.4 centimeters. I lost .6 centimeters during firing. My shrinkage rate is 6%.

Metal rod standing inside wet clay form. Notice the 6% gap.

Knowing the shrinkage rate is helpful to give me a sense of how much the clay changes. I could buy or make a shrink rule to help with measurements (we have one in the YVCC clay studio), but my usual approach is to guess and estimate. This week I decided to guess during building and then use the fired shrinkage tile and my ruler to check my guessed measurements. 

Measuring the bike rod against the shrinkage tile measurements.

I built a form into which I will insert a tall black metal rod (the part that holds up the seat on a bicycle). I measured the diameter of the rod using my shrinkage tile. The diameter was about 3.5 marks on the tile, which translates to 3.5 centimeters before firing. (I didn't bother to check the actual diameter of the rod because it doesn't exactly matter using this method.)

Measuring the wet clay opening with standard centimeters

Interestingly, the interior diameter I had built into the form using the highly precise art of guessing was almost exactly 3.5 centimeters. Go me!

Pressing a bike part into the wet clay

For the smaller attachments, I pressed the plastic piece into the wet clay to mark the location and general shape--in this case a circle with a little bit sticking out at the top. Then I measured the plastic on my shrinkage tile and used my ruler to mark the clay with the appropriately enlarged dimensions. The result was very similar to what would have happened if I had pressed the plastic into the clay and wiggled it around a bit (my method for the metal rod) or cut out around the plastic piece using a thick cutting tool.

Impression of the plastic bike part outlined with dashes marking the scaled up measurement 

Six percent shrinkage is almost insignificant at a small scale, but being able to measure correctly becomes more important at a larger scale, such as when I am planning for large gears to be inserted into the clay. I am drying the work slowly, again, since any warping during drying will obviously result in the measurements being inaccurate. After firing I can check the ceramic openings against the actual bicycle parts to make sure my process is efficient.

Round shrinkage tile with impression of bike gear (I made this to have a visual of the size change in relation to a larger bike gear.)