Monday, October 19, 2009

Cat on a Hot Tin Roof!

Have you ever been run over by a train?

You know, the huffing, puffing, hooting, fire breathing monster sort!

Have you been tied to the rails like a starlet in a movie,

as the express thunders down?

Ooops. Splat!

Ginger has!

Well, not really, but.... he loved the hot tin roof and enjoyed keeping me company as I fired the kiln yesterday.

The kiln "clammed up" after the firing. All airways to the fire box well covered with bricks, ash and sand.

I fired single handed as we had the studio open to the public yesterday. Laura kept me supplied with water and things to eat. I find that I don't feel all that hungry when firing the kiln, and eating is difficult as I am stoking almost continuously most of the time, but water is absolutely essential as it is hot work, even on a cool day, and it is easy to become dehydrated.

Weary, grimy, but still standing.... after the firing!

I fired the first part of the firing faster than I usually do and shaved a couple of hours or so off my normal time by doing so. All the work had been bisqued in the electric kiln so I didn't have raw pots to deal with. The glazes were dry, and I had pre warmed the kiln by lighting a small fire in it the evening before.

After four hours, the kiln was at about 800 degrees Centigrade (1472 F) and I went from firing in oxidation* to firing in heavy reduction* for the next hour and three quarters until we were at about 950 degrees Centigrade (1742 F). From then onwards I changed to a more efficient stoking pattern of adding enough wood to cause some, but not a huge cloud of, smoke. Wait for the smoke to clear. Then repeat! I have some copper red glazed pieces in this firing that need a good reduction atmosphere to develop the red. Hopefully I have got it right... if not, they will be green! The firing took 10 hours to cone 11* in the medium temperature part of the kiln (some parts hotter than this, some cooler). As my pyrometer is probably 100 degrees Centigrade out towards the higher end, I won't bother with temperatures, but... Cone 11 is probably around 1270 degrees Centigrade (2320 F) for the speed I was firing in the last few hours.

Useful things to watch for when firing a kiln.
Whilst stoking I do take notice of all sorts of clues as to what is going on in the kiln. Colour and quantity of smoke, colour of flame in the fire box, sound of the fire in the fire box, and, very importantly "pressure". When a kiln is starved of oxygen, as it is when firing in reduction, the flame and smoke will try and find their way out of every opening. Gaps in the brickwork and spy holes will puff smoke. Potters sometimes call this "pressure". When the kiln has lots of oxygen, it will inhale rather than exhale through gaps and spy holes. Pressure is a useful indicator of what is happening inside the chamber of the kiln, there may on occasions be pressure and a reducing atmosphere in the chamber of the kiln, but little smoke from the chimney.

Firing in oxidation means that there is more than enough oxygen available for combustion of the fuel. In oxidation, the flame in the firebox will be bright, active, and yellow. Inside the kiln visibility will be good and the colour bright. There will be no smoke from the chimney. Spy holes will suck in air.

Firing in reduction means that there is not enough oxygen available to burn all of the fuel. Carbon Monoxide will be present in the kiln, and it will greedily seek out oxygen from where ever it can find it, including stripping oxygen atoms from glaze materials and clay. There will be some smoke from the chimney, there will be "pressure" in the chamber and smoke and flame out of open spy holes. The flame in the firebox will be lazy and orange. Inside the chamber visibility will be poor.

Cone 11
A little cone shaped object of ceramic material which is formulated to flop over when at a certain temperature. Cones are one of the most useful ways of telling how things are in a kiln as, being made of similar material to the glaze on the pot, it is subject to the same variables as the glaze, namely: time, temperature, and atmosphere. Cones are numbered and can be formulated for temperatures as low as 586 Centigrade. Potters usually use a sequence of cones to give them warning when the target temperature is approaching, and a guard cone... to show if the target temperature was exceeded!


Ron said...

Great photos. Look forward to seeing the results of the firing

Peter said...

Thanks Ron. Hope the photos caused some amusement anyway. Might be able to peep in the kiln tonight. Will be away helping my mum and dad move house next two days though so kiln may have to wait until Thursday. We shall see!

ang said...

brilliant speed there pete...i wish mine would fire that fast and prob would if i used timber...!not quite the fuel for a fibre kiln though...HA
a big howdy to ginger..

Linda Starr said...

What a sweet cat ginger is, hope the firing gives you the colors you want. three more days and I am out of here, just about worn out myself and Gary even more so since he has to do all the heavy work due to my back. Love all your explanations about the firing process. Can't wait to see the kiln opening.

Peter said...

Gidday Ang! I must say that any romantic ideas I once may have had about 24 hour firings and the like do seem to have been put to rest (almost) by the actual hard slog of firing a kiln (this wee beastie is quite a demanding kiln too). It's nice to get it over with! I'll be interested to see the results as I kept the important part of the firing (when the chamber was over red heat) to a similar speed as my previous longer firings (which were about 13 hours). I'll pass on the big howdy to my friend!

Ginger is a sweet and very affectionate fellow. It has been fun having his company when I'm working. He did try to sit on my lap a couple of times when I was firing the kiln, not with any success as I'm never able to sit for more than a minute at a time when stoking, especially with some of the odd shapes and sizes of timber I was using.
You and Gary look after yourselves... Sounds like you need a sleepy sort of a holiday for a few days after you move out before you drive anywhere far. I'll be helping mum and dad move house over next couple of days so will be away from the computer then. Hope to post some pictures of pots from this firing before you head off on your adventure, but may be too late..
Best Wishes to you and Gary, P.L. & G.

Judy Shreve said...

What great info on firing a wood kiln -- great photos too -
Ginger is such an amazing cat -- he sure loves to hang out with you.

Armelle said...

I think Ginger is as beautifull as Elisabeth Taylor in the movie from Tenessee Williams's drama !!!

Great post once more Peter, take enought rest now.

Kind regards

Peter said...

Hi Judy,
lest I give the impression that Ginger is approaching Sainthood..., he does have at least one moral weakness..., his personal alarm clock goes off at 4.30am and he makes very forceful demands to be fed! He is great company though, a real potter's cat. I could probably use his tail for decorating pots with a little more training!

Armelle, Bonjour! I think that Ginger would prefer it to be said that Elizabeth Taylor is almost as beautiful as him!
Le soleil brille,
le ciel est bleu,
Je dois me réjouir
qu'il y a travail à faire !
Au revoir!

Angie said...

What an interesting post ...especially the bit about the cones.... always an interesting read.

Ginger looks so happy in the heat ..some cats love to be almost cooked. Many moons ago, my aunt worked at a school that had an urn of hot water for tea, on the go all day. There was a shelf just above it and there Timmy, the school cat, sat ...steaming his luxuriosly thick fur coat ... all day everyday ...only moving to be fed.

Peter said...

Thanks for that Angie, I can "see" Timmy the school cat in my mind's eye luxuriating in his steamy environment! I think there is still a lot of Africa in some cat's genes! They like nothing better than to bask in the heat.

Yana Out East said...

Thanks for all the details about the firing process. I have only used an electric kiln so far and I appreciate the information as it doesn't always come across in a book. The colours you achieved are beautiful.

Peter said...

Hi Yana,

Lovely to hear from you all the way from Nova Scotia, Canada! It is nice to know that the information is helpful (I worry sometimes that it might be boring for people, but I think it important to share what I can). Guess you'll be heading into winter soon so nice photos of hot kilns and fire boxes would be encouraging!

Best Wishes, P.