Continued Shrinkage Testing and Rate of Absorption

Upon completion of the bisque firing you will again weigh each tile as well as measure the lengths of the 100mm line. Using averages you deduce the shrinkage rate of the native clay from wet to bisque. Example, all tiles A- E began with a 100 mm. After the bisque firing to cone 05, Tile A measured 92mm, B- 92mm, C-95mm, D-95mm, E- 96mm. Differences being 8mm, 8mm, 5mm, 5mm and 4mm. Therefore, to reach the average will will add these numbers and divide by the total number of tiles. 8+8+5+5+4=30mm divided by 5=6% shrinkage rate from wet- bisque. You can also go further with the data you have collected and deduce the average shrinkage rates from wet to dry, wet to bisque, and/or dry to bisque. An average shrinkage rate under 15% will make a good clay body for continued testing.

The next test we will do is for lime blowing. In firing clay above 1632 degrees Fahrenheit calcium carbonate decomposes into lime. Clays with an excessive amount of lime or calcium carbonate will cause problems in the future. As the clay absorbs moisture from the air the lime inside will expand causing pieces of the clay to flake off. Lime is easily recognized by small pits and a white or yellowish film on the surface of the pots. There are remedies one can do to correct problematic lime blowing such as firing at lower temps or firing in a reduction atmosphere. For the lime blowing test, soak your bisque fired test tiles in distilled water for an hour and allow them to dry for a few days. If lime is present you will see a film form on the surface of the tiles.

Without certainty I believe that lime is causing a problem in this 180 year old house that I reside in. The red brick chimney we have is mortared in place and probably dates back to it's origins. Due to a slow leakage around the flashing of the chimney it might be possible that the brick has too much lime or perhaps the mortar has slowly leached into the brick causing large chunks and flakes to fall off.

The rate of absorption for the bisque clay will tell us how well the clay will absorb the wet glaze. For this test we rely on the weight measurements we have been taking. Boil your bisque test tiles in water for 2 hours. With tongs safely remove the tiles from the boiling water and allow them to cool to the touch on a dry towel. With your triple beam or digital scale take a weight measurement for each saturated test tile. The difference and average between the dry bisque and saturated bisque tiles will provide us with our rate of absorption. A percentage higher than 10% will provide a good absorption rate. Percentages under 10% will make it difficult for the bisque to absorb the glaze. If such is the case you can bisque at a lower temperature, cone 06 or cone 07, or heat the pots before glazing. Having a good rate of absorption with your bisque fired clay makes it easier to get a even coat of glaze on your vessels which will provide a uniform glaze coloring as well as a good clay and glaze interface.

Testing the wet to bisque shrinkage rate of native clay

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.