A successful April firing with gas and wood

Earlier this year I built another experimental gas/wood kiln, this time on a larger scale (8 cubic feet) and using a much more powerful gas burner (A Stedmark No.100 kiln burner with flame failure valve). I was hoping that being a larger kiln, it would lose heat less rapidly and be easier to fire to stoneware, despite it being a simple updraft design. In fact it proved harder to reach temperature than with the smaller kilns .. after getting stuck at high-earthenware temperatures with the first firing, the design was modified and capacity of the ware chamber reduced slightly. For the second firing last week, I tried to encourage a more even flow of the flame through the ware by positioning the first kiln shelf further over the burner's "fire box". I hoped this would ensure more heat could spread through the first layer of pots. I also placed a second shelf on props, which, being level with the top of the kiln, formed part of the roof .. this increased heat insulation and helped to reflect more of the heat back into the kiln. It also acted as a support for the double-layer ceramic fibre roof which lay on top of the second kiln shelf, overlapping the top of the walls.

The kiln was fired with lower gas pressures than last time, as I had been advised that the optimum for this burner is likely to be around 4 Psi ( less than a third of a Bar). At higher pressures you are also more likely to suffer from tank freeze (frost appearing on the outside of the tank up to the level of propane remaining) and this will slow down the outflow of gas to the burner. However, even at 3 to 4 Psi frost still appeared on the tank as it got down to around half empty .. perhaps this was what caused the temperature to stall but I was unable to get close to 1280 degrees C (cone 10) according to the cones and pyrometer.

As a result, I was expecting the results to be rather dismal but I was very pleasantly surprised! I suspect the cones and thermocouple were in a cold spot on the same wall as the burner port and had given a false impression. Further into the kiln and closer to the "fire boxes", the majority of the pots were well fired, with the ash and celadon glazes completely melted. None of the pots was over-fired and it was pleasing that there was no evidence of bloating, despite the fact that some of them must have been fully blasted by the gas flame. Levels of reduction were also very promising, no doubt partly due to the wood which I threw in several times during the firing.

After ten hours, some heavy reduction cooling was induced by putting wood and charcoal into the side stoke hole and as much charcoal as I could stuff into the fire box through the burner port. The kiln was then clammed up around 8pm (this does generate a fair bit of smoke!) and left to cool overnight. At lunchtime the next day it was still too hot to open, but by 4pm I was prepared to risk a peek as it was a warm and sunny day.

Here are some of the pots which came out well:

Ash-encrusted vase

Ash-glazed tea bowl

Lidded container

Sake cup

Sake cup

Carved vase

Bottle vase

Vase

Vase with wood ash glaze

Vase

Iga style vase with crusty ash glaze

Round bottomed bowl

Vase with slip glazes

Pot, approx. 3 ins tall (n.f.s.)

I am now considering moving to a downdraft design, mainly as I would like to fire more quickly and efficiently. The updraft experiments have been interesting, but gas is rather expensive these days at £63 for a 47kg tank!

Thanks for reading.

Some new pots and glazes

Gosh, doesn't time fly? I can't quite believe it's been over two months since I posted anything here. Well, my only excuse is that I've been pretty busy, continuing to try out different methods of making, developing new glazes and testing them, all with varying degrees of success.

The matte ash glazes, based on an original recipe by Carlos Versluys, have proved to be particularly interesting, but also very challenging, as their textures and colours are affected by so many factors. So far I've produced some attractive, crackled textures by applying them thickly, but to achieve a good colour response from the clay body, I am realising that they need to be applied fairly thinly, since the glaze, especially if calcined alumina is added, is highly refractory.

But, at the risk of repeating myself, it's only by experimenting and "failing" a few times, that one can discover what the critical factors are, and narrow down the parameters within which the potential for success lies. And of course, the more things you try, the more chance there is of serendipity stepping in and offering up pleasant surprises like this one from Saturday's firing:

Vase, electric fired, 4 ins tall

This vase was coated with with white slip, and after the bisque, was glazed with a dolomite glaze ( my thanks to Rachel Wood for the recipe, found in Techniques Using Slips, by John Mathieson) dipped half way over a Japanese-style Oribe glaze. I am guessing the white dolomite has picked up some of the copper oxide from the Oribe, just enough to turn it this wonderful orange colour.

Another exciting result came from putting the same dolomite glaze over one of the matte ash glazes containing iron oxide. Fired in the electric kiln to around cone 7/8, it came out fairly crawled, revealing the cracked texture of the matte glaze beneath:

Bowl, electric fired, 4 ins wide

Someone said they thought this looked like a thin layer of snow over dry earth..

Anyway, here is another selection of pieces I've fired recently, either by electric or gas and wood:

Vase, electric fired

Close-up of above vase

Vase, raw ash glaze, fired with gas and wood

Two vases, fired with gas and wood

Container, fired with gas and wood

Squared vase, Nuka glaze, electric fired

Squared vase, Nuka glaze, electric fired

Bottle vase, dolomite glaze, electric fired

Vase, dolomite glaze, electric fired

Carved guinomi, matte ash glaze, fired with gas and wood

Carved guinomi, matte shino glaze, fired with gas and wood

Carved guinomi, matte shino glaze, fired with gas and wood

Guinomi, dolomite glaze, electric fired

Guinomi, dolomite glaze, electric fired

Sake cup, dolomite glaze, electric fired

Sake cup, dolomite glaze, electric fired

Thanks for reading.

A temporary kiln fired with gas and wood

This week I built and fired another gas kiln, this time with the addition of a side stoke hole for wood. The interior dimensions were 18 by 18 inches wide, by 15 inches tall, which I think equates to around 2.8 cubic feet. The bottom and sides were assembled dry from a combination of heavy-duty fire bricks and light, high-temperature insulations bricks and the roof from a section of double-layer kiln fibre held in place with fencing wire. The exit flu was simply a slot left in the roof of the kiln at the opposite end from the burner port and extra pieces of kiln fibre were used as a damper.

This kiln proved quite tricky to fire in terms of getting the temperature up to 1285 degrees C but according to the pyrometer, I finally got there after 10.5 hours. At various points the temperature would stick and I had to make minute adjustments to the burner position and damper to get it rising again. No doubt the firing may have been alot quicker had I been able to use HTI bricks throughout. Cone 10 didn't quite drop but 9 was well over on the bottom shelf, where I think it was slightly cooler.

After the kiln reached 900 degrees C I started stoking occasionally with 2 or 3 pieces of pine kindling and each time this would cause a temperature drop of around 30 degrees. Over 1200 degrees this became quite exciting as large flames would shoot back at me out of the side stoke hole as the wood combusted instantly. I would like to have used alot more wood but it was so difficult to get the temperature up that I had to hold back on creating more reduction. At the end of the firing, the gas was turned off and six or seven small pieces of wood were pushed into the burner port and side stoke hole in an attempt to achieve more reduction during cooling. I felt this was necessary as the walls of the kiln and the roof are far from air-tight, and sure enough, the following day there was nothing left of the wood, barely any ashes at all.

A couple of shots of the kiln as it was dismantled:

Kiln built on wooden pallet, burner port on right.

Kiln with spy hole and stoke hole for wood (right) on facing wall

One problem with this kind of kiln assembly is that the light-weight bricks are incredibly fragile .. they break easily when being moved around, and a few crack during each firing. The heavies take much longer to heat up but are at least fairly durable!

I think the addition of wood helped in creating intense bursts of reduction throughout most of the kiln, although it's hard to know exactly how it affected the glazes, as many of the pots already had raw ash added to them before the firing. Pots placed near the exit flu were more oxidised (example in the second photo). As usual, I was testing quite a few new glazes so the results were very mixed .. here are some of the pieces I felt turned out okay:

vase, 2 ins tall

vase, 3 ins tall

vase, 9 ins tall (re-fire from wood kiln)

vase, 5 ins tall

tea caddy, 4 ins tall

vase, 3 ins tall

vase 3.5 ins tall

Vase, 2.5 ins tall

whisky cup, approx 3 ins tall (re-fire from wood kiln)

whisky cup, underside

And below are a few test pieces I put in with my own shino-type crackle glaze. I was pleasantly surprised by these results as in the electric it was quite a disappointing glaze with a very dry and sandy finish.

The little bowl on the left was given a coating of groggy black clay (when leather hard) and this has added an extra dimension to the glaze in terms of texture and colour. Can't wait to try this one on a larger pot!

Thanks for reading.

A gas-fired, glaze test kiln

Yesterday, I built and tested a very small gas kiln, partly to see how fast I could fire it to cone 9 temperature! The kiln was built simply using high-temperature insulation bricks, nothing else..even the roof was made using bricks:

Only two small pots were placed in the kiln:

This was meant to be a crackle shino over a dark, iron-oxide slip. A little (1%) iron oxide added to the glaze has made it flux, losing most of the cracks. It has also bubbled rather alot, possibly a sign that the atmosphere was not reduced sufficiently. This is probably because it was positioned directly under the exit flu..unavoidable in a kiln of this size.

This piece was more successful. The glaze was made with kaolin, AT ball clay and raw wood ash and has come out rather like a traditional, Japanese shino. Wood ash was also aplied to the top half of the pot (glue was sprayed on and the pot dipped in a pile of ash). It actually looks very much like an anagama-fired piece with a variety of surface colours created by the melting ash. But this kiln was fired to approx 1275 Centigrade in two hours. Some wood was introduced to the gas flame by placing bamboo into the burner port..interestingly, each time this was done, the temperature would drop very quickly by 30 to 42 degrees C. It would then take about 10 minutes for the temperature to get back up as the bamboo ash burned away. The kiln was so small, I had to pull the weed burner head about 7-8 inches away from the kiln to get the hottest part of the flame inside the chamber. Optimising this distance was critical to achieving a rapid temperature rise.

Here are a few other pieces I fired recently in the electric kiln:

Carved tea caddy, fired to cone 8/9

Carved tea caddy, approx 4 ins tall

This tea caddy was carved from a solid block of clay. It took a very long time to finish so I was relieved there were no firing cracks!

Carved incense burner, fired to cone 8/9

Black tea bowl, fired to cone 8/9

Thanks for reading.