Monday, January 8, 2018

Knowing when to stop! A good Firing. Urns.

I find the last few minutes of a firing the most stressful, the Big question is knowing when to stop. I like to plan for the last hour of a firing, and think about how long glazes will need to mature, and how fast or slowly the temperature in the kiln should be rising. I try to think it out before the firing begins, but firings are dynamic, and firing with wood is something of an art as well as a science.

It would be nice to be able to open the kiln door and examine the pots like pies in an oven to see if they are "done", but the kiln is holding an atmosphere, white hot with just a faint yellow tinge, that is almost as bright and as deadly as the sun. Inside may look inviting, but it is not a friendly place, there are no happy aromas of hot pastry, braised beef, and sautéed onions!

Through welding goggles I can carefully peer at cones that are incandescent with heat, and await their bending and fall. I can sometimes glimpse the curve of a pot, the frozen D of a handle, but a glimpse is all. So I consult my graph paper log of the firing, noticing the dotted steps of each temperature reading as hour by hour the temperature rises. I time the fall of the cones, and I try to even the heat from top to bottom of the kiln, pushing in the chimney damper a little to slow the passage of heat through the kiln to the minimum needed to maintain a rise in temperature and an atmosphere in the chamber that is a little starved of oxygen.

Eventually I have to make the call to stop stoking. The top of the chamber gets hotter than lower down, so there is a compromise. If I try to get the lower part of the chamber up to an ideal temperature, than the top can be over fired. If I stop when the top is only just up to temperature, then the lower pots will be under fired. I have made some allowance for the expected temperature difference by selecting glazes that will love high temperatures for the hot part of the kiln, and ones that will tolerate lower temperatures for the cooler areas, but each firing is a living thing, and requires a judgement call.

I have some important pots in the upper part of the kiln, one is a commissioned piece, and there are some related pots as a back up in case the commissioned one turns out badly and we need a "plan B"! When the top shelf gets to cone 11 half down, I have to stop the firing for the sake of those large pots, even though the lowest pots will be under done. I know that I can re fire the pots that haven't quite got there, but it is hard to salvage a badly over fired pot!

The kiln was ready to unload on the third day after the firing, and Peter Watson, the friend who gave me my first lessons in potting and encouraged me along the way, stopped by to see the kiln unloaded.

The neighbour's -not quite our cat- cat who had helped me fire the kiln also watched with some interest.

Urn. 11 1/2 x 8 1/2 inches (292 x216mm)
The commissioned pot is a funeral urn. It was quite a responsibility to make such a special thing, and I wanted to make it simple and thoughtful. On an overcast day, the glaze seems sombre but there is a great deal of beauty when the sun lights it up.

Urn, detail of the lid.

We were very pleased to see that the urn was a success and it was well received by its owner when he picked it up yesterday.

There were some under fired pots as predicted that will go back in the kiln, but there were some treasures amongst the ones that got to temperature. In all it was a good firing!

Bowl with copper red glaze. 6 1/2 x 3 1/2 inches (165 x88mm)
Copper red bowl detail.

Vase. Ash glaze over Shino. 5 1/2 x 5 1/4 inches (140 x 133mm)

Vase. Detail. Note the crystals over the ash glaze.

Urn. Ash glaze over Shino. 11 x 6 1/2 inches (280 x 165mm)

Urn. Detail of lid.

Breakfast bowl. Tenmoku glaze with rutile over glaze. 5 1/2 x 2 3/4 inches (140 x 70mm)

Breakfast bowl. Detail. Note the pollen-like sprinkle of golden crystals.

Breakfast Bowl. 6 1/4 x 2 3/4 inches (158 x 70mm). Same glaze combination as the previous bowl.

Urn. Thrown and altered. Rutile over Tenmoku glaze. 10 1/2 x 10 x 8 inches (267 x 254 x 203mm).

Urn. Detail.

Urn. Rutile over Tenmoku glaze. 15 1/4 x 7 1/2 inches (387 x 190mm).

You may have noticed that there were several more urns! When someone commissions me to make something for them, I usually make several related pots. I think with my hands, and prefer to try to give the pot form in clay from the beginning. Other potters are happy to draw out ideas on paper, or even use 3d software on the computer to solve design problems. Really we need to find our own way to solve these things, paper or technology is good for some, clay for others, we are individuals!

Enough for now! I will be getting back to the wheel again this week. Hope to make coffee mugs, pouring bowls, a large mixing bowl for baking bread, and .... maybe some little tiles.

Monday, January 1, 2018

Christmas and in between. Firing the wood fired kiln on New Year's Eve. Happy New Year!

It is just after 5 am. The world is waking up, a few birds are stirring. Outside my window darkness bleeds into the pre-dawn sky. Now the world has willow trees, and a purple sky that is covered with a blanket of cloud that moves steadily from West to East. It is disconcerting, the clouds make believe that the earth is turning, whilst they are stationary, like a fish swimming against the current!

Waikouaiti Old Post Office Gallery in early December.

Our little gallery had a very slow start to December, but a genuine "Christmas rush" the week before Christmas. Thank Goodness!! We are so thankful to friends that made a special trip to the gallery and encouraged and cheered us, and to people that visited here for the first time.

We had our own Christmas Day a day early this year, driving through to my parents on Christmas Eve. We have to be somewhat flexible with these occasions, and it was better just to go when we were all feeling reasonably robust. We had a lovely time together, just the 4 of us this year, and it was good to enjoy each others company, some good conversation, and even a little music making. Dad and I played some Christmas carols together, he on violin and viola (not all at once!!), and me on the ukulele! I am sure that all of us were thinking of other family members too, near and far.

Mum, Dad, and myself at Christmas.

Back to Work
Laura and I were back to work on Boxing Day, with our gallery open again and me beginning glazing pots to go in the wood fired kiln. I have enough pots creating a "log jam" on my studio shelves for two or three more firings, but I also need to make a lot of small work to occupy the annoying spaces that inevitably occur in a kiln when you can fit 2 bowls on a kiln shelf, but not quite the three that you hoped would go there. Really pots are all the wrong shape, you have heard the expression "square pegs in a round hole", pots are the reverse problem... round pots in a square kiln!

Round pegs in square hole! Kiln loaded and awaiting firing.

Two days before New Year's Eve I spent all day loading the kiln and getting it ready to fire. I decided to use *wadding for most of the pots, to keep them from sticking to the kiln shelves. Wadding is something that potters who salt glaze their work will know all about, on the underside of most salt glazed pots you will see a characteristic set of marks that were the result of wadding. The wadding that I used was a blend of china clay, alumina hydrate, ball clay and grog with just enough water added to make a mixture that was very like bread dough in consistency. Wadding each pot made loading the kiln a slightly longer process, but it was oddly satisfying and enjoyable to be quietly rolling out little snakes of wadding in my fingers, nipping them off into even sized pellets of the doughy mixture, forming them into balls, wetting them, then placing them in a "fairy ring", for all the world like a circle of tiny mushrooms, on the underside of each pot!

We prepared firewood on the day before New Year's Eve, Laura did most of the wood splitting, as I did a couple of hours of final preparation on the kiln prior to its firing. Later I lit a small fire in the ash pit to dry the kiln and pots, and pre-warm the kiln.

I like firing the kiln in the **rain, and rain was forecast for New Year's Eve,firing day, so I extended the kiln shed roof with an old tarpaulin to give us some shelter.

Laura and the blue tarpaulin.

Firing on New Year's Eve
I lit the kiln at 5.30am. Rain was quite steady, but not particularly heavy. I settled into the rhythm of adding a couple of very small pieces of wood to the ash pit every few minutes, and Brian (Not our Cat!!!!) amused himself for a few minutes by climbing on the outside of the tarpaulin and pushing down at my head and shoulders with his front feet. The tarpaulin had more small holes in it already than stars in the sky, so I wasn't too sure how much cat climbing it would cope with, but Brian eventually discovered that it was wetter on top of the tarpaulin than beneath it, so came down and dried himself on my legs!

The rain went picka-pocka-plash, the birds began to sing, and trucks rumbled past on the main highway, tyres hissed on the wet road. The world slowly woke up as my little fire danced and flickered in the ash pit of the kiln.

Laura took over at half past 7, and I made some breakfast, then had a walk up the road to see if anywhere was open to buy some milk.

A grey and dripping day at 7.45am

At 8.30, with the kiln a shade over 300 Celsius (572 F), I took over stoking and started the gradual transition to building a fire in the main fire box. This involves keeping the ash pit fire going, but occasionally adding a small log at a time to the fire box. The temptation is to put too much in the firebox, and then have the kiln suddenly climb too rapidly, so patience is required.

9am and 385 on the clock (725 F).... a rhythm of regular light stoking of firebox and ash pit has been established, and the kiln is climbing at a brisk, but controllable 175 - 180 degrees (347 - 356 F) per hour, which we maintain on average until the last hour and a half of the firing.

10 am and 590 degrees (1094 F), Laura stokes. I need a rest, but am anxious about abandoning things altogether, as it will not be all that long before we have to think about reduction atmospheres and the like, so I don't want to go inside and fall asleep. I bring out a nice blue lounger chair, and fit it under the tarpaulin. I can rest on that, but still give directions if needed.

Brian making it difficult for me to read the kiln log!

11.15 and 820 degrees (1508 F) or so, time to push the chimney damper in a little and deliberately burn the wood somewhat inefficiently. Ideally, when making a ***reduction atmosphere, stoking and damper settings arrive at a balance where the kiln still can gain temperature, but the fire burns with a lazy, hazy flame. Every time new wood is added, oxygen is depleted from the kiln chamber, and smoke starts to issue from cracks around bricks and spy holes as the pressure builds in the kiln. It is easy to rush this stage when firing alone, to try to do too much. To choke fire box, to fiddle, to over stoke! With Laura feeding the fire, and me observing, it was easier to get things right, I could keep an eye on fire and smoke from the front of the chamber and call for "two more lumps of wood" at appropriate times, and Laura could watch for chimney smoke and let me know the colour, "black and thick", "brown", "grey", or "grey and thin", and so on! 

I made lunch at about 12.30, then took over stoking at 1am when Laura headed inside to open our gallery for the afternoon. The temperature was now indicating 1130 Celsius (2066 F). After half an hour I slowed the kiln's rise, so that there would be more time above 1200 degrees (2192 F) for time and temperature to work their magic with glazes. It would have been possible to have "spiked" up to temperature quickly, but I do not think that results from such firings are as good.

The ****cones that I had set to give an indication of temperature and heat work, started to bend from just before 2pm. On the top shelf, in the hottest part of the kiln, cone 10 was down at about 2.20pm. I continued stoking until 2.40pm, when cone 11 was half down on the top shelf, and cone 10 had a good droop on it in the middle of the kiln.

By 3.07pm I had the fire burned out and the kiln clammed up and the temperature down to just over 1000 degrees (1832 F).

And now we wait!
I will probably unload the kiln on Wednesday morning.

Happy New Year Everyone!

(From a recipe by Phil Rogers from his excellent Salt Glazing book.)
Alumina hydrate    2.0
China Clay             0.5
Ball Clay                0.25
Grog                      0.25

Some potters believe (me included!) that moist air may actually improve the colours of the pots due to the extra hydrogen that is available as the extreme heat of the kiln splits off the H from H2O! This is not so far fetched an idea as it may seem, after all "water gas" was, and probably still is, manufactured by spraying steam over incandescent coke. This produces a potent blend of carbon monoxide and hydrogen that is a useful fuel. All air coming into my kiln is pre warmed over a bed of glowing charcoal, so humid air could well produce water gas.

***Reduction atmosphere
The kiln is fired with a surplus of fuel and an insufficiency of air. Oxygen is desperately sought, and even oxygen atoms that are part of the metals in the glazes are fair game! The result is that glazes change colour, copper green becomes copper red (or even lustred with a thin layer of copper metal), a honey coloured iron baring glaze becomes green, and so on. Even the colour of the clay that the pot is made of changes. Reduction atmospheres are one of the joys of firing with a fuel fired kiln, rather than an electric one.

I have written about these elsewhere, but cones are made of a ceramic material that is similar to porcelain. These the cones are numbered and are made to bend over when their designed temperature is met. They are affected by temperature, and the time taken to get to that temperature. They measure "heat work".