Good pots and Things that go Bump... in the Kiln! A Cautionary Tale!

I put this in for Arkansas Patti, who misses the romance of  wood firing!  I must do some more again soon. This photo was taken back in 2007 and shows Laura and our friend Dave Sharp firing a wood fired raku kiln. Ah the good old days, the crackle of a fire and the smell of wood smoke!
A smorgasbord* of sound accompanies the firing of a pottery kiln. If it is a wood firing, there is the crackle, spit, and roar of the fire, and probably some frantic chopping sounds when the potter realises that he is running short of wood! A gas firing will have the somewhat alarming, hissing white noise of gas burners sucking air and squirting gas and flame. An electric kiln firing could be thought to be silent, but it is not. My kiln makes a quiet buzzing sound when electricity is pushed through the spirally wound elements. There are audible metallic clicks when the simmerstats switch the current on and off, and there can be occasional ticks, clonks, and creaks as the outer metal casing of the kiln expands with the heat. These sounds are part of the personality of every kiln and indicate that it is alive, just like the snuffles, snores, mutters and sighs of a fellow human show that life continues! Sounds can also red flag health problems. A chesty cough or a wheeze, a rolling rumbling tummy, can let us know all is not as it should be. An ominous thump from a kiln that is near peak temperature, may also be a sign that it is time to call the doctor!

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.[3] These pieces reminded the old Swedish peasants of fat geese swimming to the surface.[citation needed] 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!


cookingwithgas said…
Great read, I'm going to suggest my students read this. Such a wonderful shape but really hard to figure how best to hold it up.
Peter said…
Hi Meredith,
Hope the students find something that helps. If I had a bigger kiln I would be tempted to fire 3 pots like this leaning in together with alumina wadding in between them (like tumble stacking a wood fired kiln). I may have to make some sort of a stand out of similar clay to the pot, and use wadding so I can release the pot from the stand after the firing? Maybe someone else has a better suggestion??
Linda Starr said…
oh wonderful results Peter especially the first one I love that pot, sorry about the unexpected unions. Are there any other crystalline or wood fire potters you can ask what their system is?
Peter said…
Good to hear from you Linda,
It was a great relief to have the good results from the kiln, as well as the unexpected union later in the week!
Not sure really who I would ask about firing pots with such narrow bases. I'll probably just have to work away at it and sort something out! If I make them with good strong rims, I could fire them upside down with not such a fluid glaze.
raindrop said…
Great post Peter, I learn a lot from each of your 'episodes'! Would putting both your saucer and your pot on a thin layer of Silica sand help with your large pots moving? While I have not fired pots of the same shape as you, I have found that the sand acts like ball bearings and allows movement for bases with concentrated weight levels.
Melissa Rohrer said…
I use cones even though I can program in an end temperature. I feel a little more in control doing this. Not so sure about that now.
gz said…
My friend Peter Goodridge is working with Wally Keeler on a salt glaze kiln that fires and cools in 18 hours...
kiln experimenting is fun!!
I like the idea of a smorgasbord of sounds ... why not ... and your unintended union makes for an interesting piece. Pottery seems to require a bit of the inner mad scientist ... :)
Peter said…
Hello Raindrop,
Thanks for sharing about silica sand with the "ball bearing" effect, it certainly can be useful at times, particularly under something big and heavy. Not sure if it would have solved this particular case, but it is a thought.

Hi Melissa,
Do continue using the cones, they are mostly fantastically useful things and I have used them hundreds of times without problems. This was the one exception!

Hi Gwyenneth,
Lovely to hear that Wally Keeler is still going strong, he makes an appearance on the wonderful TV series, "The Craft of the Potter", that Mick Casson fronted in the 1970s.

Hi Teresa,
Yes, sounds often have a sumptuous quality that elevates them to food status! You are right about the "inner mad scientist", when I am glazing pots I almost feel the need to wear a lab coat and have a geiger counter in a shady corner of my studio for appearance sake at least! I must say that pottery has made science more interesting for me.
Arkansas Patti said…
Thanks for that picture Peter. I do miss seeing you sipping a drink into the night watching your fire. Didn't know the electric kiln required so much watching also. Just not as "romantic."
It is a shame about that wedded pair. I'd put the combo on Ebay and watch the bids fly for that "one of a kind" pair.
Linda Starr said…
Oh Peter, I just remembered you might be able to separate those two pots by using a propane torch to heat up the glaze till liquid and then pull them apart, I have done that with a couple of pieces that stuck to one another before.
Peter said…
Hi Patti,
Lovely to hear from you, glad you saw the photo of the firing. I really must get the wood fired kiln going again. It has been far too long. I have good news about the wedded pair.... I managed to get them separated yesterday! I know that divorce is not usually cause for celebration, but this one went far better than it might have, and I now have the large pot looking reasonably pleased with itself, and not too scarred by its brief partnership!

Hi Linda,

Thank you so much for saying about the propane torch. I remembered reading that someone had done that, and I couldn't remember which blog I had read it on... Must have been you! :)
I wondered about propane, but wasn't sure if I would end up breaking something, or if I could direct the flame well enough. I also considered hammer and chisel... In the end I bought a set of small diamond files and sawed away at the joint. It took about an hour, but worked in the end. The large pot was hardly marked by the file, and only a little scarred by gluing to the other pot. The small pot is not in good shape, as I had to grind into that a bit in order to get at the place where the two were joined.
Not worried about the small pot as it was mostly a glaze tester, but very pleased to have freed the big one as it might have a buyer already.
Many thanks for getting back in touch about this. P
Armelle Léon said…
Bonjour Peter,
Sorry you have had such a problem with your cone 9 !!! I am please to know the new elements of the kiln are safe. Anyway, to lose two pots, it's a lot.
I also see I have missed one post, the Spoonbills are really beautiful and look like our egrets. I have seen some of them this winter, it's probably the same family, though the beak is sharp.
The flight is very elegant too and it seem the way of eating is the same.
A bientôt
Peter said…
Bonjour Armelle,
Good to hear from you. It was most unexpected having cone 9 fail like that, as they are usually so reliable! I may start setting a cone 8 to give me warning that cone 9 should be bending soon! I enjoy seeing the spoonbills and black swans at the lagoon, it is always a wonderful sight to see them in flight, or landing on the water. We are lucky to be living near a lagoon and the open sea.
Linda Starr said…
Perhaps the propane torch can heat up the spot on the big pot and make it invisible, might try on the small pot to get the feel of it.

I remember now, Gary held the torch and heated the two pots and I held one of the pots and could 'feel' when it was ready to pull apart.

Glad you got them apart.
Kate said…
Recently I had trouble with two cones (7 and 8) going down together but I have a bad habit of not setting them at the correct angle (too upright) so must be more careful of that in future. Could your 9 have been a little too upright? Fun blog to read. Thanks.
Peter said…
Hi Kate,
Good to hear from you. Interesting about you having 7 and 8 going down together.

One thing about 7 and 8 is that they are both quite close in temperature, only about 10 degrees C apart, so there is not always a lot of time between them. If I have done my maths right, you would have only 4 minutes between those cones if the kiln was climbing at 150 degrees per hour! Indeed, there is so little time between 7 and 8 in kiln climbing at 150 per hour, that someone checking the cones every 5 minutes could easily miss the "magic" moment that the cones went down!

9 and 10 are a little different, in that they are about 25 C apart, so there should be much less chance of them falling at the same time, even if set wrongly.

I have my 9 and 10 from that firing right beside me as I write, and both look OK for the angle that they were set, so I really do think the formulation was wrong.

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