Some recent results from the electric kiln

I've been meaning to update my blog for some time, and now that Tim Lake (thanks so much, Tim!) has very kindly mentioned it in a recent article in Ceramic Review, I figured it was high time to upload a new post.

Weather wise it wasn't the greatest of summers .. a constant procession of windy days has stopped me firing the gas kiln outdoors more than a couple of times, so I returned to using my electric kiln for bisc and glost firings for a while. Incidentally, I don't view electric firing as in any way inferior as a method of producing pots .. the results are different, but can often be stunning in their own way. However, I do think it is more difficult to create visually arresting glaze surfaces in oxidation, and I enjoy the challenge of trying to produce something new and exciting using this method.

Lately, I've been playing around with applying thick textures to my pieces and am now working on developing new glaze and slip recipes which will work well with more random and irregular surfaces. One of the things I want to create is a greyish glaze, probably fairly matte with some variegation in hue. I managed to create a grey slip easily enough, using black iron, manganese and cobalt (also one with nickel oxide instead of the manganese) but the same combination in a more fluid glaze has proved unpredictable to say the least. This is the closest I've come so far, but the black iron and/or manganese in the glaze gave it a rather brown tint:

Interestingly when applied thinner, the cobalt begins to dominate and more of a blue-grey appears with specs of brown. As is so often the case, I rather like the result but it's still not really what I'm aiming for. On another test piece I used equal proportions of cobalt oxide, nickel oxide and black iron oxide and less wood ash than the recipe above to make the finish more matte. The surface texture of the glaze turned out perfect but the colour reminded me of greeny brown cowpats, not quite what I had in mind! Funnily enough, where it overlapped with a white shino glaze on the inside it was actually a wonderful charcoal grey in places! Such is the chemistry of glazing .. complex and sometimes impossible to fathom. I plan to try reducing it in a saggar and see if this changes the colour to something easier on the eye. If anything interesting occurs, I'll post a pre- and post- reduction photo in my next post.

Update 12th Nov: actually I managed to take a shot of the piece today, so here is the before photo:

I am very taken with the texture and feel of this glaze .. and I suspect that the colour would be darker if applied more thickly. You can see the small area of greyer glaze around the inner rim at the back of the cup. 

Anyway, the search goes on ...

Oh, and one other thing which really amazed me recently. The celadon coffee cup below had been fired in the gas kiln to earthenware temperatures with a little reduction .. and the glaze came out completely unvitrified but one could see that it had already taken on a distinctly grey colour. The piece was later refired to cone 9 (around 1260 degrees C) in the electric kiln and of course, I expected the glaze to oxidise and come out a shade of amber yellow. Incredibly, it stayed this beautiful pale grey with some iron bursts, so even at a temperature well below 1200 degrees C, the glaze surface must have been sufficiently sealed to prevent the iron oxide molecules from re-oxidising. That may not be surprising to some people, but I for one didn't know it was possible.

Here is a selection of pieces from my recent firings, a few of them are now available in my folksy store:

Round bottomed cup with dolomite glaze

Vessel with maroon slip glaze

Cup or bowl, approx 3 ins height

Bowl with dolomite glaze, width approx 4 ins

Carved bowl, approx. 3 ins height (sold)

Sake cup with Oribe glaze

Cup or bowl with slip and Oribe glaze

Vase with Oribe and dolomite glazes, height 5 ins

Saggar-fired vase with crackle shino glaze (sold)

Textured bottle vase with dolomite glaze

Coffee cup with blue-grey glaze

Coffee mug with Nuka glaze (sold)

Bowl with dolomite glaze, approx 4 ins wide

Thank you for looking!

Reduction of re-fired iron glazes

Here is a sake cup which was electric fired recently to around cone 9. I used a satin matt, wood ash glaze containing around 5% yellow iron oxide .. it's based on an excellent recipe by John Jelfs and the only change I made was to use raw wood ash (rather than washed) and marble stone powder instead of normal whiting. For me, this glaze goes on quite thinly as the raw wood ash can make it deflocculate a fair bit, but the thicker clay on this piece meant more of the glaze was sucked onto the textured surface.

Usually it comes out of the electric kiln dark yellow and rust red where thinner, sometimes with a hint of greeny grey where thicker. What is interesting about this glaze is that it can be reduced after having been fired to cone 9 or 10 in an oxidising atmosphere.

Sake cup, height 2.5 inches

I think I'm right in saying that most highly vitrified glazes (such as glossy shinos) cannot be reduced once their surfaces are sealed in oxidation. This piece was only re-fired to high earthenware temperatures in a gas kiln (with some addition of wood through a stoke hole) and the glaze has turned a much darker, mossy green. I'm still not certain whether the colours appeal to me, but I do like the mottled effect .. what do you think?

Not sure exactly how high it went as it was placed close to the exit flue of the gas kiln so it may have been well blasted by the flames. Nonetheless, it's interesting that the iron oxide has changed colour at much lower temperatures than the previous electric firing.

Incidentally, this is how the same glaze and texture combination looks when it's reduction fired in a saggar with practically no oxygen present:

Thanks for reading.

A tip for glazing pots without a footring

Many of the pots I make these days don't have a footring, and if the pot is a rounded form, then finding a way to dip-glaze them without leaving finger marks can be challenging. As an alternative to dipping tongs, one possible solution is to use old pieces of kiln shelf to create a foot for the pot.

First I take a piece of kiln shelf which already has batt wash fired on to it. Using a hammer or a small rock, the shelf is then broken into smaller and smaller pieces, until I have one of suitable size and shape to fit onto the base of the pot to be glazed. Next, the base and the chunk of kiln shelf (a side with a robust layer of batt wash) are coated with PVA, glued together, and left to set. If the batt wash is flakey or powdery, the pot can easily work loose and drop off.

I find that on a bisqued pot (fired to around 1050 C ), the clay absorbs moisture from the glue so it dries very quickly, often in a matter of  minutes. Once the "foot" is firmly fixed the pot can easily be dipped very close to the bottom edge.


So far I have tended to use this method with small to medium sized pots, but there is no reason why it couldn't work on larger pieces (perhaps using several glued supports). However, handling and moving them onto the drying shelf could no doubt become more problematic.

It's important to make sure the pot is stable when stood on the fragment of kiln shelf, as it may move slightly in the kiln as the glue burns away during the glost firing. Also precautions should be taken regarding the fumes which will be given off by the burning glue, so safe ventilation of the kiln is essential.The same technique can be used with shelf supports or pieces of wadding from previous firings, but again, one needs to be careful that the pot will remain upright when fired.

Another advantage of this method; no need to add wadding when loading the kiln, and it doesn't annoyingly drop off down the side of the shelf when you start rearranging the ware!

And now a shameless commercial plug;  I've added a link to my online shop, Birchcroft Ceramics, in the sidebar of the blog, and just this morning listed a wood-fired tea caddy or food storage jar:

Tea caddy, height approx 4.75 inches

I hope you might like to to have a browse through the pots some time, and please don't hesitate to e-mail me at mark_smalley@yahoo.co.uk if you have any comments/questions or wish to enquire about other pieces which are shown here or elsewhere on the internet.
 
Thanks for reading.

Here comes the sun .. soon hopefully!

Summer hasn't quite arrived here and the very poor weather recently has made it impossible to fire my gas/wood kiln outdoors. The BBC is even predicting a frost in parts of England tonight which is pretty unbelievable for this time of year. In the mean time I have fired the electric kiln again, mainly to test some new glaze and slip combinations and a glaze recipe I found in the book Dry Glazes by Jeremy Jernegan. This is a matt copper glaze ( Taffy Matt by V.Cushing ) with a small addition of tin oxide, which is recommended to be fired to cone 10 in a reduction atmosphere. Quite intriguing as the photo of the test tile shows the glaze to be a dark reddish brown where thicker and light green where thin, whereas I would have expected it to be shades of pink where thinner. I will try it in the gas/wood kiln soon, but in oxidation (fired to cone 9) it has come out like this:

Sake cup

Small vase, approx. 2 ins diameter

Quite an attractive glaze, with some subtle variegation even within the grey areas. It highlights the carved areas of a pot in exactly the way I had hoped.

A couple of pieces were fired with the combination of Oribe and dolomite glazes I have used before. It came out really well on this heavily textured vase where the second layer of glaze was poured rather than dipped. Next time I intend to leave more of the green glaze showing as the contrast has worked rather nicely on the underside:

Vase, approx 6 inches tall

Vase, underside

Bowl, height approx. 3 ins

Interesting that on the above bowl the top layer of dolomite was dipped over the rim, but has dropped down over half an inch in the firing. It hasn't done that on the other vase, presumably because the top of the pot is more horizontal..

Another pot glazed with Oribe was this tea bowl, this time layered over a dark iron oxide slip:

Tea bowl, height 3 ins, diameter 4.5 ins

And finally something a little different .. a textured pot, which was initially thrown as a closed form, then laid on its side. The matt copper ash glaze was brushed on, hence the variation in thickness and hue:

Ikebana vase, height approx 4 inches

I feel this vase form could work well for flower arranging.

Thanks for reading!

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.