Last summer I started exploring the characteristics of highly crackled shino type glazes. This line of investigation was prompted by the surfaces of some old kiln bricks which had been used to build brick saggars inside a gas kiln. The bricks had been bedded in a fire clay mortar which had in places adhered to the brick and cracked when the saggar was dismantled.
My aim was to find a glaze that would provide me with a highly textured glaze which could be applied over slips, engobes or directly onto my high flux clay bodies. The glaze needed to remain crackled through the long duration of a fifty six hour anagama firing and the crazes strong enough to allow the colour of whatever was beneath them to be visible through the cracks. The glaze which I was after although being white should hopefully have a little iron from which the flame could generate some colour. Another of the attributes I was looking for was simplicity, containing no more than four materials.
All tests were fired to Orton cone 11 in a 6 cubic foot down draught natural gas kiln. Although this was not ideal it was the most efficient way of testing in between wood firings. All recipes are shown as parts e.g Grams, oz's, kilo's etc.
I started by testing a 50/50 blend of potash feldspar and standard china clay. This feldspar and china clay mix gave an attractive soft satin shino which crawled into attractive cracks, however, the edges of the crazing were a little softer than I was after. As I was also intending to add oxides to the final base glaze to produce a black version of the glaze, I needed the base to be matter, so that any added fluxing action of the oxides in the kiln’s reduced atmosphere would not cause over fluxing.
To the 50/50 mix I then added 25% nepheline syenite. (test1)Not a logical step to matt a glaze but my next step none the less. This created a very interesting heavily crazed glaze when applied thick. Where the glaze was applied thinner smaller cracking appeared with the glaze exhibiting some flashing from the high iron body beneath. If applied too thickly however, it tended to come away from the surface. This was a good outcome in many ways. The same glaze depending on the thickness of application would give an extreme range of results. Very thick and it produced a severe crackle almost like a crackle slip, when applied thin however it gave very delicate splitting of the glaze surface not dissimilar from pealing paint.
50 – Potash feldspar
50 – China clay
25 – Nepheline syenite
I also tested this glaze with a 3% addition of Red iron oxide (test 2)
This was more like what I had been looking for although too brown. So, to the 50/50/25 base mix above, I added 5% red iron oxide, 2% cobalt carbonate and 4% manganese oxide (test 3). The resulting glaze was not uninteresting however the additional fluxing of the metal oxides produced too much melt in the glaze and the crazing was lost.
The next stage was explore whether changing the proportions of flux made any difference to the melt, so the next mix substituted half of the original Potash feldspar quantity for 25% more nepheline syenite. The results were very similar to test 1 however the crazing had a slightly softer appearance.
The next test substituted the china clay content of the above mix to AT ball clay (high iron). This gave a soft crawling surface with a rather unattractive light brown colour. Not really what I was looking for. So on to another test batch
So for this next test the AT ball clay content was lowered by a half and it was replaced by china clay. By adding a higher alumina material, it was hoped that this would matt up the glaze but also keep enough iron in the glaze to develop a bit of colour. This mix was then:
AT ball clay - 25
China clay - 25
Potash feldspar - 50
Nepheline syenite - 25
This glaze is very good in my eyes. It has a very pleasing crackle which is deep enough to expose the body beneath. The glaze also displays some good colour potential. In the gas kiln tones of peach were generated, this will be a good glaze to test again in the wood kiln. After this I decided to strive for the mat black glaze which I had also set out to look for. I decided to go basic again so the next test was:
China clay - 85
Potash feldspar - 40
To this, was then added 5% Red iron oxide. 2%Cobalt carbonate and 4% manganese. This produced a very interesting glaze which was actually what I was looking for.
Several other tests were then carried out including a 50 / 50 mix of AT ball clay and Potash feldspar. This had no body adhesion and fell away from the body after the firing.
The final successful test which gave a good off white pealing crackle glaze was:
China clay - 5
Cornish stone - 25
Spodumene - 25
Although I have not managed to test all of these samples in an extended wood firing, the two which have been tested fulfilled my expectations. They both give a flexibility of coverage as well as the ability to respond to the atmospheres and volatiles which are present throughout the firing. These are not glazes which I will use every day but occasionally, a piece of work requires a bold statement, and these glazes certainly provide that.