October 2021 - Obvara Firing
For October Obvara firing, I mainly used wheel thrown test pieces. Some were smoothed and given either a Terra Sig coating, then polished, while others were just smoothed with a rubber rib. 4 small vessels were given a coating of coloured slip, then burnished. The textured pots were given a treatment of sodium silicate and thrown from the inside to give the pot a crackle texture.
Surface Treatment / Glazing
No pots were given any glaze. At the bone dry stage, Terra Sig was used on some of the smooth pots and then polished with a sponge to a shine. 4 small pots were coated with coloured slip and burnished prior to bisque firing. The textured pots were produced by coating the pot at the wet cylinder stage with sodium silicate. Then a heat gun was used to dry the sodium silicate while the pot was still on the wheel and then using a rib or fingers, the pot was thrown to its final shape, careful not to touch the outside and disturb the pattern of cracking. This took some practice.
All the pots except the 4 small coloured pots were bisque fired to cone 09 either at Eversfield Studio or at my home studio. The 4 small coloured pots were fired to cone 07 to enhance the colours.
All the pots were either finish fired to 1650 F (899 C). The firing was done in either the raku kiln at Eversfield or the raku kiln at my home studio. As part of the firing process, each piece was taken out of the kiln at temperature, then quickly dipped into a brew of Obvara mixture. Then the process is stopped by placing the piece into water. This final step is very quick (seconds) and speed determines the final outcome of the piece. ( See my comments in the Results section)
All pieces were made with white stoneware, either cone 10 or cone 6. It didn't seem to make any difference which was used.
Obvara mixture : 1 Kg of flour, 2 packages of regular yeast, 1 Tbsp sugar and 8 - 10 litres of water. This mixture is left in a warm place 2 - 3 days ahead of the firing to allow the yeast / flour mixture to ferment. I usually stir the mixture twice a day.
Coloured Slip: 4 small test pots were given a coating of thin slip using 10% mason stains, china clay and groleg clay. ( Recipe to follow)
It seems like the obvara process will produce the best results when the pots are at their hottest. ( 900C ) As they cool, the obvara mixture does not get as dark, nor does it get absorbed by the hot clay as well and can leave a sticky residue. It also seems like the longer you leave the mixture in the brew, the more is absorbed and the denser the patterns. I also noticed that the longer you leave the piece in the air after dipping in the obvara mixture and before you plunge into the water, the darker it becomes. I believe this may be due to the time the carbon in the mixture is left in the air to continue to burn .
I will be testing this process on different textures, including sculptural pieces, spiky pieces and other textures. I will also be trying more coloured slips or coloured Terra Sig to see how they turn out. Might also try using porcelain clay for higher contrast results. I will also consider how this technique can be combined with other bare clay techniques such as naked Raku, horsehair, soluble salts. etc.