To quote Michael Cardew from his fantastic book, Pioneer Pottery,"An unknown clay should be presumed guilty until proven innocent." Commercially available clays often have the shrinkage, porosity and absorption rates listed in there catalogs, the work has already been done. But what about a unknown native clay?
In the near future my husband and I will be moving to our property in western New York state. Our parcel is filled with native terracotta clay and shale. As a potter this is a potential gold mine for me but I don't know much regarding this particular clay body. Is this a clay body I can use for throwing on the wheel, what temperature does it fire to? One thing I am certain of is that there were local potteries in the old days so chances are pretty good that I can utilize this clay.
After finding a nice pure vein of clay I was able to do a simple plasticity test by rolling out a wet coil and checking for cracks, it passed. Refer to my previous post- "Native clay processing on a large scale". My next step was to make a couple of small balls and bisque fire them in my kiln to cone 05. I fired these small balls of native clay inside a commercial clay bisque fired bowl as a precaution. I did not want the clay to melt and ruin my shelving, I could sacrifice the bowl. The clay survived and fired to the same rich red coloring as my commercial terracotta clay body. Now it's time for some more testing.
To test the shrinkage rate of a native clay works on averages and does not involve complex formulas, thankfully. To take from Tom Buck, Leon Nigrosh and Michael Cardew begin by rolling out a slab of native clay that is 1 cm thick. Allow the slab to set up for a few hours before cutting out your test tiles. Take a piece of box board and cut a rectangle 13 cm long and 4 cm wide. This box board will be your template for the tiles. Using a fettling knife cut out as many tiles as you can, preferably 10. Next draw a line 10 cm long down the middle of each test tile. Set them aside to stiffen up to the point of handling.
Thinking ahead to porosity and absorption testing were going to kill 2 birds with one stone. Label each test tile A, B, C, D and so on. Using a triple beam or digital scale weigh each tile in grams in it's wet state and write it down, always keep notes. Completely dry the tiles flipping as needed and/ or weighting them to prevent warpage. After the tiles have completely dried, measure the 10 cm lines (100 mm) and weigh them again. Example- Tile A is 96 mm (-4 mm), Tile B is 97 mm (-3 mm), Tile C is 95 mm (-5 mm). The average difference of A+B+C+D+E+F+G+H+I+J =X divided by the number of tiles equals your wet to dry shrinkage in percentage. Example- in mm- 4+3+2+3+4+3+2+3+4+3= 31 divided by 10 = 3.1% wet to dry shrinkage.
Next we are going to fire the test tiles in the kiln to bisque temperature, cone 05 or cone 04. After the kiln has cooled and you have removed your tiles measure each 10 cm line again and weigh the tiles again. Tile A is 89 mm (-11 mm), Tile B is 90 mm (-10 mm). Again, take those averages and divide by the number of tiles. This percentage will give you the average wet to bisque shrinkage rate. Somewhere in the range of 10- 15% is good and worth investing the extra time for glaze firing and absorption and porosity testing which will be part of my next post.