Saturday, June 4, 2011
Vacuumed to cone 6! Pushing the little wood fired kiln where it hadn't been before!
A truly organised person would have the next two decades of life plotted out by the hour on a wall planner, or keyed into their palm sized electronic gadget, their raspberry... or was it a loganberry, or black currant? Whatever the device was, the truly organised person, I shall refer to them in future as a "TOP", would know on a given Tuesday that the following Sunday required them to be taking part in a Potter's Co-op working bee, and additionally, THAT was the day when a window display of pots had to be put up at the aforementioned Potter's Co-op.
A "TOP" would not, on Tuesday having arrived at the aforementioned Potter's Co-op and having seen a list on the wall, say "Gracious," or "Mercy", or "Help", in a voice pitched higher than usual and modulated by tones of surprise, because nothing would surprise a "TOP". I mean, a "TOP" would have known this, and would have planned for it ages in advance.
I am not a "TOP", in any shape or form, and my life plays out somewhere on the ground floor. I do sometimes write things on paper, and occasionally scribble odd marks on a calendar, if I can find something to scribble with, or indeed, if I can locate the calendar on which to scribble. If these jottings are ever found, then the real meaning of the aide-mémoire is often lost. A crudely circled "10.40pm" on Monday 27th, could refer to the proposed visit of a plumber, giving a talk at a school, or, most likely, the end of the world!
Anyway, to return to my exclamation of surprise last Tuesday when I saw that I was to be "in the window" of the Co-op from the following Sunday. Not only did the air reverberate with my discovery, but a little knot of panic began to form at a visceral level. I chanced a nervous glance towards my shelves at the Co-op, blinked, and looked again as I realised that the shelves were looking a little low on stock. There were only really enough pots for the shelves, and nothing left over for putting in the Co-op window as well.
On my return to home sweet home at the end of the day, I glanced at the pots that were in my studio, and tried to imagine groups of them displayed in the Co-op window..... and failed! Aggggh, recent weeks spent building, repairing, re-organising, and doing anything but make pots, had left me short of work.
Wednesday morning, some unearthly hour, saw me sit up in bed abruptly, and say, "Ah... Ha!"
Clad only in winter weight sleeping attire, I scuttled through to my studio and pulled several books about glazing from the shelf, and propelled myself back to the comparative warmth of the bed and began researching Cone 6 glazes. Cone 6, or about 1200 C (2192 F), is a glaze temperature that I have rarely explored. Lots of potters fire to that temperature and manage to get stoneware glazes to work for them. I mostly fire 1250 to 1300 C, which is quite a lot hotter.
My unearthly hour, "Ah...Ha!" was prompted by visions of some unglazed pots that I had seen in my studio, that were on mid fire stoneware clay, and a thing that had been gnawing away at me ever since I last fired my small wood fired updraught kiln. Whilst I was certain that my little kiln would never reach 1300 degrees Centigrade, I had a suspicion that the kiln might reach 1200, but had never tested that, and I thought that a quick firing of this kiln might give me one or two pots that I could add to my ones at the Potter's Co-op.
A Question of Sanity
Look, it is crazy, and most "unTOP-like" to put pots with untested glazes in an untested kiln and expect to have work to take to town, but....
... I am not quite crazy, so I also glazed up some large bowls for my electric kiln with glazes that I have at least done preliminary tests on, and I fired those yesterday as my insurance policy!
I will be unloading those first thing Sunday morning, and, with any luck, there will be a bowl or two from that firing for my display.
Now, back to Wednesday. I selected three glazes for the work that was to go in the little wood fired kiln, and spent the morning preparing them, and some of the afternoon applying them to the pots. It was a bit frustrating as I would have liked to have got the pots glazed AND fired on Wednesday, but making glazes always takes much longer than I think it should.
An Early Start
All was ready for first thing Thursday morning, and an early start was made. This time I was dashing around the house in darkness, gathering pots, props, pyrometer, and other paraphernalia, then loading the little kiln by the light of a bedside lamp that was attached to a long extension lead for power.
And.... We're Off!
We made a start at 8am, and built a small fire in the small firebox of the small kiln! (It really is a tiny kiln!). The firing went healthily enough for the first hour, and I handed over to Laura sometime after 9am. She continued the firing whilst I started preparing glazes for the bowls that I was going to fire in the electric kiln. Just after 10am I took a hot drink out to the stoker, and did a little stoking and adjusted air intakes and chimney damper. The temperature was up to 575 C (1067 F) by 10.15am.
At 12.15am, the kiln became more tricky to fire, the pyrometer was showing 960 C (1760F), and the kiln began to feel like it was "running out of puff". This was way short of what I thought the kiln capable of doing, and I was a bit mystified. By dint of careful stoking, the kiln was coaxed to a reading of 1015 C (1859 F) by 1pm, and I was preparing to give up. The pyrometer, I knew from experience, was very inaccurate, and we were probably 60 degrees or more hotter than what it was saying, but we were using lots of fuel, and trying very hard, but were going to be more than 100 degrees short of target.
Oiling the Works
About that time Laura said "what about cooking oil??" and rushed off to the kitchen. Returning with a small bottle of sunflower oil, she applied it to some of the wood and took over stoking. At 1.15pm, powered by sunflowers and Laura's efforts, the kiln had crept up to 1050 C (1922 F) on the pyrometer, and a glimmer of hope opened its fragile wings and fluttered them experimentally in the balmy breeze. A reading of 1050C probably suggested that we were in fact well over 1110 C (2030 F), and were slowly beginning to chase down our target.
A Partial Vacuum
About this time Laura said, "How about the vacuum cleaner?!" I cantered away at speed, and returned with a very old Electrolux that a good friend of ours had given us many years ago. Vacuum cleaners are electrical gadgets that are designed to suck! The Electrolux had the peculiar property of being more effective blowing than sucking, and even came with a neat little spraying system with a little glass bottle and an atomiser to allow the house-holder to respray children's bicycles, second hand furniture, bedroom walls, or even the car. It was the product of the swinging 60s, a time when a real man read Practical Mechanics, and his ideal woman, showed tantalizing glimpses of shapely lower calf whilst expertly preparing healthy meals in her 350 horsepower oven with 5 forward speeds and chromed steam extractor that her man had rewired for her in his DIY basement workshop.
The Electrolux was plugged into the power, and began to emit a jet-like howl as its hose directed a blast of cold air, beetles, spiders, and autumn leaves towards the fire box of the kiln. I hastily arranged bricks around the kiln's under ash pit air intakes to direct the blast from the Electrolux into the firebox, and... away we went.
An hour later cone 4 was down, 1168 Centigrade (2134 F), and cone 6 was softening.
At 2.20pm cone 6 was down which is about 1200 Centigrade ( 2192F). At this point the pyrometer was showing an error of about 80 degrees, but it had done a great job of encouraging us when the temperature was going upwards, and steering us to a temperature where I was then able to judge progress by the cones that I had set in the kiln.
At 2.30pm we switched everything off and closed up the kiln to prevent cold air rushing through it as the fire died down.
I was able to unpack the kiln first thing the next morning.
What did the pots turn out like?
Mmmmmm..... To be honest, I'm not sure that any were quite good enough for the Co-op window. Pots on the top shelf of the kiln were a bit under-fired as the temperature there was only approaching cone 4. In spite of this we got a copper red vase that was fairly acceptable, it would have been more lively if it had got hotter, but it was OK.
There was also a nice little bowl. The bowl had picked up more heat than the vase, and the glaze was more glassy and less like paint!
In the bottom layer of the kiln, where things got to cone 6, there was quite a nice greyish glaze on a vase that had some lively movement and variegation in the glaze where got hottest.
Sadly, this vase was rather let down by the "reliable" glaze that I had put inside it as a liner glaze. This was a glaze that I have used many times before, but it had come out as rough as sandpaper and was discoloured pink in places from volatilised copper from the other glazes.
I have not had this problem before, but this kiln is very small, and any heavy metal that decides to start wafting around at high temperature may well have an effect on other pots.
We will have to hope that some bowls have come through the electric firing OK. I will find out tomorrow!
Cones measure heat work. Potters place them where they can be easily seen in the kiln when it is being fired. Carefully formulated from similar materials to what the glazes are made of, cones start out straight, but flop over when a certain temperature is reached. The amount of time taken to reach that temperature has an effect too, and cones usually give the potter a far better idea of what is going on in the kiln than a pyrometer, which merely measures air temperature. These cones have been pushed into clay. The clay acts as a base for the cones, and helps them stand up in the kiln. Just like putting a fork into a baked potato before putting it in the oven, I have pushed holes into the clay to help all the steam get out when it is fired.