Monday, 30 June 2014

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.

Friday, 11 April 2014

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.

Friday, 14 March 2014

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.

Saturday, 22 February 2014

Searching for a good crackle

With ceramics, it's exciting when serendipity leads you down new and interesting paths. When I tried formulating a matte ash glaze using mainly china clay and wood ash (based on a recipe by Carlos Versluys, which I read about in the 2nd edition of  Phil Rogers' excellent book, Ash Glazes), I discovered that when applied thickly, it crawled and split, creating interesting crackle patterns. The high proportion of raw ash in the glaze gives them an unpredictable element, which I like, but it also means that they are extremely sensitive to firing temperature, thickness of application and the type of clay body or slips underneath.

In my latest electric firings, I've tried adding a little magnesium carbonate to encourage crawling. This works up to a point, but at higher temperatures, it begins to act as a flux and the crackle effect is lost as the glaze begins to melt and fuse back together. Peeling is also an issue where the glaze is very thick..although fairly robust, curls of glaze can crack off if enough pressure is applied. Okay for sculptural work perhaps, but would not be suitable for most functional ware.

Here are some of the recent test tiles and experimental pieces:

Tea caddy,
matte copper ash glaze over black slip

Sculpture, "King for a day",
matte copper ash glaze

Decorative bowl,
matte ash glaze with magnesium carbonate

matte ash glaze with magnesium carbonate
Vase, 15 inches tall
Matte copper ash glaze

Vase close-up

Test tiles
The white test pieces are a matte ash glaze with very high clay content over black slip decoration. Develops some nice cracking, but again, thickness of application over the bisque is critical to avoid peeling. In an electric firing, only black slips containing manganese and cobalt show through clearly .. the test on the left in the photo below has a dark slip containing red clay and iron oxide only, and it's barely visible through the glaze:

Test tiles
I particularly liked the test tile second from the right, which is a matte ash glaze with red iron oxide added .. works nicely over the Potclays grogged pink body. I'll be trying this again with a higher clay content to try to achieve a more pronounced crackle.

Thanks for reading.

Monday, 27 January 2014

First glaze experiments of 2014

I did a test firing yesterday with the electric kiln, trying out a more matte copper ash glaze over various clays as well as a dolomite glaze. This is a first for me..I've had a large bag of dolomite languishing on a shelf for years and never used it.

This glaze really shows the cool and hot spots in my kiln. I fired lower this time, to around 1260 centigrade  (the temperature recommended for the dolomite) and a couple of pots came out a very nice satin-matte white, whereas on others the glaze was over fired, turning almost transparent and running off the pot onto the kiln shelf. The kiln was soaked for half an hour and maybe this created too much heat work. Fortunately, not too much damage to the shelf, but two pots were completely ruined. I also tried the dolomite over a black slip containing cobalt oxide and manganese and it didn't like that too much..crawled and ran alot, although some of the effects were quite interesting, especially where the glaze stayed on top of the black slip:

The two perfect pots:

Vase (only bottom section has black glaze, brushed on)

And some pieces coated with the matte copper ash glaze:

I'd added more china clay by eye, plus a little magnesium carbonate (to encourage crawling) and a couple of teaspoons of soda ash. I hoped the soda ash would react with the copper oxide to produce more greens, and it did in places, especially on the small pot in the last photo. Unfortunately, not a very attractive shade of green..I may re-formulate the glaze with smaller amounts of copper carbonate rather than the oxide, maybe that would create more delicate colours. But I do like the rusty browns it produces over the darker clays such as Potclays grogged pink (left pot in the last photo).

Thanks for reading!