Blackhills Entry 3: Triaxial Glaze Testing with Ash
Plant ashes are useful glaze ingredients. Trees, shrubs, grasses, weeds take up minerals from the earth during their lifetimes to support their growth and reproduction. The minerals in a plant will vary based on the type of plant, the soil and environmental conditions where it is growing, and based on the specific part of the plant (e.g. the minerals present in bark will likely vary from heartwood or new growth). This is what draws me to ash glazes – they are directly linked to the plant and the landscape where it grew.
When John and I planned my visit in March, he told me about a specific glaze he wanted to develop. In the studio there was a bucket of oilseed rape straw ash glaze that produced the most beautiful surface – a buttery matt pale blue which pooled slightly in the throwing rings and in the bottoms of pots which produced a transparent cracked ice. I bought a large fruit bowl glazed in it from John a few years ago and it is gorgeous. The problem was that the recipe for the glaze was a mystery – the bucket of glaze was mixed up a long time ago and it had run dry. So we needed to recreate it.
Triaxial blends: Some of John’s ash glazes use three simple materials: ash, feldspar, and clay. If this mystery glaze was made from these three materials, it should theoretically be possible to find it by combining these three materials in all possible proportions. I planned a triaxial blend to do this, to test all combinations of the three materials in 10% increments, from 0% of a given material to 100%. This gives 66 possible combinations to test. A triaxial diagram is a way to keep the structure of these tests organized – each corner of the triangle below corresponds to 100% of each material. As you move further away from point C (100% clay) and closer to point A (100% ash), for example, the samples contain less and less clay and more and more ash. The line connecting point C and point A is equivalent to a line blend of clay and ash. To give a few examples: point B is a sample containing 10% clay, 80% ash, and 10% feldspar; point D is a sample containing 40% clay, 20% ash, and 40% feldspar; point E is a sample containing 20% clay, 20% ash, and 60% feldspar. I used Phil Roger’s book, Ash Glazes to get my head around the basic principles of triaxial blends and The Self-Reliant Potter by Andrew Holden to help me think through how to measure out materials for each sample.
Measuring using small amounts of materials: Ash is precious – it takes a lot of wood or a lot of straw to produce a tiny amount of ash. I didn’t want to use any more than needed to create these 66 samples and I wanted to be as precise as possible when measuring small amounts. Most of what I’ve read about line blends or triaxial blends describes measuring by volume. I decided to measure by weight instead in order to be more precise using smaller amounts. I decided each of the 66 samples would contain 45g of dry material – enough for glazing a few test rings and bowls.
Then came the measuring, lots and lots of measuring. 66 samples x 3 materials = setting a small plastic tub on the scale and hitting the tare button 198 times. Well actually not quite because three of the samples were pure ash/feldspar/clay and 24 samples contained only two materials, but still, it was a lot of measuring.
In and Out of the Kiln: After I measured each material out into the containers, we gave each a good stir and dipped our labelled test rings in. The rest of the work was up to the kiln. We fired our first batch of tests in the gas kiln, up to about 1275C. When we first took them out, they were in tight rows on small kiln shelves and nothing looked overly exciting. It wasn’t until I set them up into the triaxial triangle that we could start to read them. It was incredible to see how much texture and color variation there was from different proportions of just three simple materials. Within the 66 tests, there were a few that jumped out at all three of us as special glazes – they weren’t necessarily what we were looking for but they had qualities that kept drawing us back to them again and again.
Second round: We identified a sub-triangle of 28 points within the 66-point triaxial blend that held a good number of promising (functional) glazes and we wanted to see what these samples looked like using rhododendron ash. In addition to John’s test rings, I made up a full set of tests for myself using little bowls that I had biscuit fired before I arrived at Blackhills. I also used these little bowls to test some of my favorites from the oilseed rape straw ash triaxial on an actual form. Into the kiln again.
The kiln overfired slightly which made our job of reading the tests more difficult. The little test bowls were beautiful though, and they gave much more information about a glaze compared to the test rings. The next time I do a triaxial blend, or any glaze testing, I will use small bowls. For me, the extra effort of throwing them is worth it.
Final round: The final round of testing was for the wood kiln. We dipped 28 little bowls into 28 samples from the original 66-point oilseed rape straw triaxial blend. John placed them into a careful area of the kiln – a spot that doesn’t get too hot, too cold, nor too much (if any) soda. 29 hours of stoking up to cone 10/11, followed by 34 hours of cooling.
And out they came, on my final day at Blackhills. There was a lot to do that day. We unpacked the kiln slowly, giving time to each pot as it came out. Then we carried all our work into the studio, table surfaces covered in white cloths to help us see the pots more clearly. I wish I had had more time to give the tests. Some of them looked incredible – but they also looked very different from what we saw come out of the gas kiln. I really needed to spend about a week with them afterwards to compare the two sets of tests, but that is for a future trip.
In hindsight, trying to recreate a glaze in two weeks was slightly overoptimistic. But it gave us focus. In the process of trying to find one glaze, we identified many more worth pursuing and learned more about the base materials and how they interact. We still think the mystery glaze is in there somewhere – but chances are it lies somewhere in between our 10% increments. That’s part of the beauty of a triaxial blend – once you identify a promising area within it, you can look in finer and finer detail using smaller and smaller increments until you find the specific thing you’re after. We really needed another couple of weeks to do this. However, I also learned that this precise way of working will take you so far and then after that, it’s up to the firing.
