Good pots and Things that go Bump... in the Kiln! A Cautionary Tale!
Before I go further with my Cautionary Tale, here are a few photos of pots that worked well in firings I did earlier in the week. The amphora and the vase with the copper red crystals will be part of a group exhibition in August. I will give more details of that in a post slightly nearer the time.
|Amphora with reduction fired crystalline glaze.|
|A detail of the amphora.|
|Vase with cobalt blue crystals.|
|vase with copper red crystals.|
|Here is a side view of the vase showing one of the handles.|
Back to my Cautionary Tale!
Kiln elements don't last for ever, and they wear fairly quickly when doing firing to high stoneware temperatures. Elements gradually get thinner as they wear, and it gets harder and harder to push electricity through them. Towards the end of their life the kiln takes longer to fire, and struggles to reach peak temperature. This is not too serious for an ordinary stoneware firing, but it can make crystalline glazes impossible to do well, or even impossible to do, as they generally need to climb quite quickly to the peak temperature.
I changed the elements in the larger of my two electric kilns two firings ago. There are nine elements in all, at about $45 each, they are fairly expensive to replace, but it is part of the cost of producing pots, and has to be done. When it is time to replace them, it is best to do them all at once. I had kept the old ones going until the poor kiln could not even manage to climb at 60 degrees C per hour for the last 100 degrees or so. (140 F/per hour), and because the kiln was so slow, I had not been able to use it for crystalline glaze firings for quite some time.
I use Orton Cones to tell me when my pots have reached the right temperature. Cones use similar materials to porcelain, or simple stoneware glazes, and really indicate "heat work" rather than just air temperature, as they are affected by temperature, and the time that it took to get there.
Cones all have a number, and I fire my porcelain to cone 10. When my elements were old and tired and the kiln was climbing slowly, the kiln would reach cone 10 when the pyrometer (which measures the air temperature in the kiln) was reading about 1275 C (2327F). After changing the elements, the kiln climbed much faster, and the pyrometer was reading almost 1300 degrees C (2372F) when cone 10 was down. That is a 25 degree difference (77F) between firing with old elements and with new ones.
Potters who do not use cones, but just rely on the kiln pyrometer, can easily over fire or under fire work if they make a change to their firing schedule and reach the peak temperature more quickly or more slowly than usual. I found it more than ever essential to use cones after changing the kiln elements as the kiln was firing like a new and unfamiliar acquaintance, with a lot more "get up and go"!
I have used cones for nearly every glaze firing that I have ever done, and they have always been reliable. Sadly, one let me down on Wednesday morning near peak temperature on my second firing with new elements.
I loaded the kiln on Tuesday afternoon with just 3 pots. Two small ones, and a large amphora shaped pot that had a narrow base. The pot was glazed with an alkaline copper glaze that I knew to be fluid, and somewhat ash-like so the pot was placed on a thickly thrown porcelain ring, and this was positioned in the centre of a stoneware saucer that was designed to catch any run off glaze. The narrow base made the pot somewhat unstable, but it seemed OK when in place in the kiln. The pot took up most of the kiln space, but I squeezed in two small pots to go in with it.
I fired the pot through Tuesday night, and was nearing what I thought should be peak temperature around 7 in the morning. I slowed the kiln down at 7 so the kiln would take the last part of the firing at a gentle pace and give the glaze time to mature, and temperatures to even out over the pot. At about half past 7 I thought that I should start to see cone 9 begin to bend. My aim with this firing was to fire to cone nine down, but not bend cone 10 as the body of the large pot was a fine white clay that I knew to be mature at cone 9, but over fired and likely to slump and distort above that.
I noticed no sign of movement in cone 9 at half past 7, and was back in the house recording the temperature in my firing log, when red splashes of blood appeared on my graph paper, and my nose began to bleed with some enthusiasm! This was decidedly inconvenient, and it was a difficult chore to bend down to peer through the spy hole of the kiln 10 minutes later whilst clamping my nose with finger and thumb, and a fist full of paper towels. Poor Laura assisted in this macabre scene, by slipping welding goggles over my head for me, and helping me pull on gloves.
I was concerned to see no change to cone 9, as the pyrometer** was already reading slightly above where I expected cone 9 to be down. With only one large pot on board and no kiln furniture in the kiln, I expected cone 9 to go over at quite a high temperature, but I was feeling distinctly uneasy not to see movement at least.
With anxiety levels keeping pace with the rising temperature of the kiln, I checked 5 minutes later, and 5 minutes after that, and 5 minutes after that... Whilst checking the kiln just before 8am I heard and sensed a muffled thump in the kiln. Finally at 8 o’clock, I noticed cone 10 that was behind cone 9 start to droop, and cone 9 started to bend with it. I shut off the kiln with the pyrometer saying 1295 C. Cone 9 and cone 10 should have been about 25 degrees C apart (77F), and there they were bending together!
|In theory cone 9 should have been completely collapsed by the time cone 10 started to bend, but here the tips of both bent at the same time. Later cone 10 caught hold of cone 9 and held onto it.|
I was able to lift the kiln lid on Thursday morning, and I saw what I had kind of expected. The large pot had fallen.
By a great stroke of luck, it had not hit the new kiln elements, and only did superficial damage to a kiln brick in the side of the kiln, but it did join itself to one of the small pots that was low down in the kiln.
It is possible that the small pot actually did some good, as it prevented the large pot from rolling further, but the two are now well joined!
|An unintended union!|
There are always lessons to be learned when firing a kiln. I had to make a choice between believing gut instinct, or what the pyrometer was telling me, or what the cones said. I chose to believe the cones, because they have never let me down in the past, where as the other two have been unreliable!
In future I will change how I set my cones and I will angle them so they clear each other as they fall, and I will have a better view of ones at the back of the pack if they fall before the ones at the front.
I will now have to regard all my remaining cone 9s as unreliable. It is more than likely that others in the same batch were faulty. Of course I cannot be sure as to whether or not all the cone packs that I have prepared in advance, were out of the same box.... Agggggh!
Clay can move and distort when it is fired to near vitrification. It can bend like rubber. The distortion of the pot as it fired past its safe top temperature, will have been enough to destabilize it. To add to the problem, the stoneware saucer on which it was standing, also began to flex, and could no longer provide a firm platform for the pot to stand on. I think I would have just got away with firing the unstable pot had the temperature not gone so high, but not by a very safe margin. I need to give further thought about how to fire pots that come to such a narrow base.
Anyway, life goes on with all its occasional dramas!
*Smorgasbord.... of course this is a word that is not really intended to use in connection with sound! I love the wikipedia entry for smorgasbord. Here is a quote from it....
"The Swedish word smörgåsbord consists of the words smörgås (open-faced sandwich) and bord (table). Smörgås in turn consists of the words smör (butter, cognate with English smear) and gås. Gås literally means goose, but later referred to the small pieces of butter that formed and floated to the surface of cream while it was churned. These pieces reminded the old Swedish peasants of fat geese swimming to the surface. The small butter pieces were just the right size to be placed and flattened out on bread, so smörgås came to mean buttered bread. In Sweden, the term breda smörgåsar (to butter open-faced sandwiches) has been used since at least the 16th century."
**Pyrometer comes from two Greek words, pyro, meaning fire, and meter, meaning to measure. Firemeter, isn't that nice!
Really I should be using the more correct term, thermocouple to describe this useful instrument, but I think that most potters seem to refer to such things as pyrometers in this part of the world at least.., well, I do anyway!