Thursday, December 30, 2010

Wood fired crystalline reduction firing

Copper red crystalline glazed pot, just unloaded from my new wood fired kiln!

A detail of the crystals which were lustrous and showed copper metal in places.

The same pot before firing in the wood fired kiln... it is the copper green one at the back!

The Video should show the firing of the kiln and some examples of the crystalline glazed testers that I fired in it.

Well I've just made another wood fired kiln. It is a small one and took less than two days to build, but the kiln is a very special one, in that I have made it for reduction firing of crystalline glazed pots.

(Reduction firing is one which is somewhat starved of oxygen. The fire is made so hungry for oxygen that it is forced to take it from oxygen atoms that are in the glaze and the clay that pottery is made of. As a result of losing oxygen atoms, the glaze may change colour, sometimes dramatically.)

My little wood fired kiln.

This kiln is number 8 in a series of wood fired kilns that I have made, and is more sophisticated than it appears from the outside.

A rough sketch of how it works.

I am pleased to say that the firebox is the most pleasant and easy to fire that I have made to date, and one of its little secrets is that all the air for the firebox enters the kiln from under the ashpit. The air passes through a simple grate and is pre heated by the ash before rising to the wood that is on a grate above it. I have complete control of air coming into this kiln, and a simple damper arrangement controls all air that leaves it. Because of the way that the ash is treated in this kiln, there was almost no build up of ash, as it was able to burn away, so I had no need to rake out ash or use a poker whilst firing the kiln.

I test fired the kiln yesterday, with a small load of crystalline glazed testers, and one copper glazed pot. The firing was wonderfully easy to do, and I fired to a top temperature of 845 degrees Centigrade (1553 Fahrenheit), raku temperature really, but enough to do what I set out to do, which was to put previously electrically fired glazed work into a reduction atmosphere, and bring out a new range of colours out of the glazes.

Brown crystalline tester... after an oxidized electric kiln firing.

The same brown glaze after having a reduction firing in the wood fired kiln.

A very pretty white glaze tester that was fired in my electric kiln.

The same glaze tester after being fired again in my wood fired kiln.

A close up showing the crystals showing up strongly with their new colours.

These tests are... just that. Some glazes don't look very nice, and others have technical problems. There is lots more work to be done to get the best out of the glazes and the kiln, but I am really excited by the possibilities of it, and these tests are the first reduction fired crystalline glazed pots that I have had to hold and to look at. I have only seen photographs on the net and in books before now. Next firing will be less oxygen starved, this one was a little too severe!

Anyway, must go to bed. It has been a long day.

Laura wearing a.... tea cosy, and why not, it looks very stylish!


Tracey Broome said...

What a brilliant little kiln, I keep thinking I would like to build something like day....

Arkansas Patti said...

Was really amazed by the before and after. What a difference.
Your cyrstals are just beautiful.
Love Laura's tea cozy hat. What a neat idea.
Happy New Year to all the Gregorys.

Elen Susan said...

these are brilliant your crystals are just wonderful.I envy your talent for this.
Wishing you a happy new year!

Peter said...

Hi Tracey,
Happy New Year!
I'll be interested to see what else I can fire with this kiln. Earthenware should be well within its capabilities currently, I was able to keep to a very steady 200 degrees Centigrade per hour climb with this until I put it into the reduction part of the firing. It will be interesting to fire it right up to find out what its peak temperature will be. It is fun having a small, economical wood kiln that is so easy to fire.

Happy New Year to you too Patti! Tea cozy hats might have to be our Old Post Office uniform this year! I'm amazed at the colour change of the glazes too....

Hi Elen Susan,
Lovely to hear from you, Happy New Year, and welcome to my site! I see that you have just set up your own blog site, and I do wish you well with that.

Dad said...

What a huge potential you have here Peter!
Well done all the participants!

ang said...

wow very lusterous...I like your baffle shelf at the top of the kiln i'll try that in my next test kiln firing, the flue end was well underfired...what cone did you fire to peter and this firing didn't change the crystals at all?

Peter said...

Hi Dad,
Happy New Year! Miss Unsworth said that potential is "existing but not developed!"
The great thing about developing this "existing" is that it is fun and smells of wood smoke and hot clay!

Hi Ang,
A Happy New Year to you!
Sadly... no cones were used. My trouble with cones for this firing was that the low temperature ones that I have all have iron oxide in them, which makes them useless for reduction firing as the iron fluxes them. For this extra firing of crystalline glazes, you really don't need a very high temperature. In fact, reduction applied at too greater temperature, above about 900 - 950 C, if my understanding is correct, will cause a dramatic and disastrous boiling and blackening of the glaze. This is due to the high zinc content of crystalline glazes. I have refired a crystalline glazed test in the electric kiln to 1020 degrees centigrade with no loss of the crystals. In fact there was some improvement, as the outline of the crystals became slightly darker.

At some stage in the not too dim and distant future, I will try firing the kiln quite a bit higher with some earthenware pots in, just to see how easy 1100 is to get, and how evenly the kiln fires. I might manage a cone or two for that one, as I can do cone 03 upwards in non iron bearing cones.

srgb said...

Hi Peter
Best wishes for 2011 to you and Laura.
I was very impressed with the glazed cups you showed in the last blog Peter you go from strength to strength and now the kiln what are we going to see during 2011?
I have torn myself away from the shed today to update the blog, I'm not sure what this means for the new year probably that I worked too hard for the previous 3 days on the house roof.
Feast or famine.

Pat - Arkansas said...

Amazing! While I don't quite understand all the techical details, the results are wonderful! You do good work, Mr. G.

Happy New Year to you, your menagerie and the pretty lady in the funny hat. (very clever)

Peter said...

Lovely to hear from you Bob,
Happy New Year to you and yours too! Just hopped over to your blog and see that you've been up on the roof attending to a possible leak..., we need your tender ministrations down here! We have a ridge that needs attention as we found in 24 hours of rain that we had recently! We could claim to have brought the outside inside, or to have improved the outside inside flow in our property!! Very trendy!

Hello Pat,
Happy New Year to you too, and big purrs from our feline friends to yours!
I fail to understand all the technical details myself, which keeps things interesting! (wouldn't it be boring if we "knew all that there is to be knowed"... to misquote from Wind in the Willows) Pottery seems to have endless vast horizons of territory to discover.

Yana Out East said...

Just catching up on your blog since before Christmas. What a treat to see the before and after shots on your test balls, I had no idea you could wood fire crystals and have them still be crystals. Happy New Year.

Linda Starr said...

Happy New Year to you and Laura and the cats. This kiln you have built is very intriguing to me. What a great idea to refire your crystal glazes. Two chances to see how the glaze will turn out. Everytime I see some reduction work I get re-excited about it wishing I could do some of it again. I love how the fire has changed the crystal glazes and enhanced the edges of the crystals. How long did it take you to get up to the temp you wanted? and how much wood did you need? Can you see where these questions are going. I need to build myself just a wee one in the back yard. Hope all is well with you.

Anonymous said...

I really like this look. I'm new to the potters world so would you be willing to share the recipe for the last piece on this posting (the green/white/purple one)? Thank you.

Peter said...

Hi Anonymous,

I don't mind sharing the recipe, but I am a bit concerned where you say that you are "new to the potters world", as crystalline glazing is technically demanding and you really need your own kiln and lots and lots of testing of things to get such glazes to work. Most of the success of the process depends on the firing rather than on the glaze formulation, so there is a lot to learn if you want to do such things. The glaze that you mention had two firings, one to its full temperature of cone 9 in an electric kiln. The first firing formed the crystals, but gave white crystals on a white background. The second firing was to just over 800 degrees Centigrade in a specially made wood fired kiln that I describe in this post, and it was subjected to a reduction atmosphere. It was the reduction atmosphere that developed the green and purple colours.

The glaze is simple,
51 fritt 4110 (same as 3110)
24 silica
25 zinc oxide (calcined)
+ 6 titanium dioxide
+ 2 bentonite

You will need to fire this to cone 9 quite rapidly. Cool to about 1100 Centigrade, then hold that temperature for a minimum of 4 hours.

You will need either porcelain or a good white stoneware clay. Preferably one that vitrifies around cone 9.

The pot must be fired on a stand with a glaze catching saucer underneath, because there will be a heavy run off of glaze when the pot is firing in the kiln.

The glaze is probably best brushed onto the pot. About 3mm thick for the top third and 2mm thick lower down. You will need to experiment with glaze thickness.

A helpful site that you should visit is
This is a forum for crystalline glaze potters. There is lots of practical helpful information on it, and I highly recommend having a look. There is some lovely work on there, but also information about the process that will help you get started with crystal glazing if this is what you really want to do.

Good luck with this, and do report back with your progress. I am happy to help where I can, but I do feel that, if you are really a beginning potter, it is probably best to concentrate on more basic things first, because the road to crystalline glaze heaven is paved with many disasters and frustrations! Best Wishes, P