How I learned to fire my red glaze so it comes out red – Part 2

Last time I did not explain how I learned to fire my red glaze so that it came out red. This time I will.

To start with it was Janice Tchalenko’s red glaze recipe that I had got from a book. It became my red glaze when I re-mixed it making little alterations to see how it would behave. Then I began to understand it a little.

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Red glaze amongst other coloured glazes. #reduction

Copper red glazes fired in reduction are like magic, an alchemy happens in the kiln which is barely under the potter’s control. When Chinese potters first discovered a way to make red pots I imagine it would have been a well guarded secret that passed down the generations of a small band of craftsmen for many years before it became more widely known. Like all industrial secrets if someone else wants it badly enough they will be able to find it, either by their own experimentation or by stealing the knowledge somehow. The technical ability to make bright red is not the whole answer though, it is what you do with that technical ability which makes great pots or not.

To start with it was difficult to get the red glaze to be consistently red. It would come out transparently white -ish  over my base glaze, or dirty grey if I reduced too much. By asking around and reading as much about it as I could (this was a little before the internet and Google) I had worked out that you need to get the reduction in early for red. But sometimes it was beautiful red and sometimes it just wasn’t there, or not at the back of the kiln or near the spy holes.
I became obsessed with trying to understand what was happening in the kiln. Red glazed draw trials pulled from the kiln during firing were never red but somehow red would appear when the kiln had cooled. I started to have strange dreams in which I was small enough to walk into the kiln through one of the burner ports, wandering through a glowing yellow cathedral unharmed by the flames.
Eventually, after months becoming years of observation and adjustment of firing schedules, I was managing to get a consistent strong red. Then I decided I did not like it. The softer half lost reds were definitely more beautiful!

This leads to what I love about craft. The wholeness of the enterprise. You cannot divide the technical from the aesthetics or the function. You get to understand all of it slowly and each part is enmeshed with the others. You might love the look of a jug but if you find it awkward to use or it dribbles you love it less.

Technical stuff:

Janice Tchalenko’s Copper Red Glaze: Batch.
Potash Feldspar – F.F.F. (Potterycrafts P3296)
300
Borax Frit (Potterycrafts P2957) 45
Whiting 45
Copper Oxide 1
Tin Oxide 3

As you can see there is very little Copper in the recipe, in fact there is more Tin Oxide. Although the glaze comes out red in reduction because it has copper in it, it seems to be the tin that helps to fix the colour. When the glaze goes into the kiln it is off-white and powdery because it has no clay in it. We put clay in glazes to control the tendency of glazes to craze as they cool on the pot. Copper red glazes need to have little or no clay in them to get bright and clean colour. Adding clay to help control the crazing or keep the ingredients in suspension in the glaze bucket makes the red dull.

The other things that copper red needs to work  apart from early reduction and little or no clay in the recipe are:

  • separation from the clay body.
  • separation from the kiln atmosphere – the red develops best where the glaze is thick.
  • slow cooling from 1020°c – 950°c.
  • good fluxing and glaze transparency because the red colour is an optical effect.
  • protection from too much re-oxidation – glaze is often redder inside foot rings.

Here is a table of events that I have found should lead to a good firing:

Pyrometer Reading
 Action  Cones
Orton cones 09, 8-9-10
0 – 900°c Oxidising with the damper in the flue wide open. Temperature rising at about 100°c/hour.
 900°c  Adjust the damper in the flue to about ¼ closed to start gentle reduction. Start to notice smell of reduction.
 950°c  Heavier reduction – damper almost ½ closed. Flow of primary air at burners reduced.  Orton cone 09 fully bent.
1080°c Oxidise for 2 minutes with damper & burners open. This is supposed to brighten up all the colours, and it does.
1100°c While oxidising the temperature shoots up. Then reduction resumes as before.
1180°c I look at the cones at the top and bottom of the kiln to judge how even the firing is. If there is a big difference in the bend in cone 8 viewed through the top & bottom spy holes I will slow the firing to try to even things up. Orton cone 8 just flexing.
Up to about 1225°c Firing is slowed by easing off the gas pressure whilst maintaining reduction by use of the damper, to even up the cones.

Eventually the gas is turned off and the damper and burner ports are sealed up.

Cones fall one by one, about 30 minutes between each.

Cone 10 is left standing.

As every potter knows, the final temperature read out is not as important as the cones, they are the deciding factor in judging when to stop the firing.

#pottery #copper_red #craft #ceramics #glaze

 

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How I learned to fire my red glaze so it comes out red – Part 2