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.