The test tiles I made from clay sourced from my local area in North Wales were fired at different temperatures between 1000C and 1280C. It’s possible to see just from the images below that this clay is low-firing since it even begins to warp and turns a dark red/purple at 1100C. Any higher than this and it begins to melt to fill the tray, bubbling and becoming metallic. Visible on the tile fired to 1000C are white deposits on the clay which are probably sodium and potassium salts. These act as fluxes and indicate the clay may start to melt as it gets to higher temperatures, which is proven on the higher fired tiles. Clays high in silica tend to puddle at higher temperatures. My raw clay is a dark green-grey colour which suggests the presence of carbon.
To test for porosity I boiled the test tile fired to 1000C for 1 hour. I weighed it before and after boiling. The tile turned out to weigh exactly the same before and after (80.2g), showing when fired to this temperature it has a porosity of 0. The clay has reached it’s maturing temperature at just 1000C so it’s a very low firing clay.
After air drying the 10cm line on the tiles shrunk to 9.4cm showing a shrinkage of 6%. However, after firing to 1000C it shrunk further to 8.5cm which is a shrinkage of 15%.
I also weighed out a 100g ball of the clay and fired it to 1000C. The fired ball weighed 76.3g – a 23.7% decrease in weight.
To test for the presence of limestone (calcium carbonate) in my clay, I placed a small lump in dilute hydrochloric acid. No fizzing (carbon dioxide released) took place which proves there was no limestone in the clay. This is supported by the fact geological survey maps show no evidence of carbonate materials in the area I sourced it.
The three tiles above show slip made from my clay brushed on the left hand side. Different amounts of glaze materials have been mixed with the clay along the tiles. The first tile shows my clay as it mixes with wood ash. A ratio of one part clay to two parts wood ash produces a warm khaki colour. Equal measure of both creates a dark brown while a ratio of two parts clay to one part wood ash makes a dark purple/brown. The wood ash on its own is on the right hand side of the tile.
Below this is the clay mixed with potash feldspar. A small amount of this feldspar mixed with my clay produces very reflective brown glazes, however a very small amount of clay produces a subtle and attractive pale blue-green like a blackbird’s egg. Potash feldspar (also known as Orthoclase) is the commonest of the 12 types of feldspar. It’s an important glaze material and is used as a flux in bodies.
Below this is my clay mixed with whiting creating different shades of green. My application of the glazes is a bit splotchy, next time I know to paint on more than one layer and aim for a more even coating. I like the effects of mixing two parts potash or whiting to one part clay and I plan to make up a batch of these greens to use on vessels.