A holiday in our garden! Crystalline Glaze Test Firing.

 We had a lovely walk in the garden this morning, under trees that were coloured in muted shades of red, gold, and grey-green. I had the camera with me as I had spied a large fungus growing at the foot of a tall gum tree.

In full sunlight the fungus was an alien orange, but in the grey half-light, just prior to a fall of rain, the fungus was a more homely terrestrial warm brown. I took photos, then the three of us, Myself, Laura, and Mr Smaug the cat, sheltered under another gum tree as the occasional spot of rain changed to a merry shower.

We seated ourselves on a thick cushion of dry leaves that were a mixture of gum, black wattle, sycamore, beech and rowan, and Mr Smaug rolled happily, pounced on things that rustled, hugged trees, then finally settled for a while behind me, pushed up firmly against my back purring with contentment.

Occasionally we were visited by fantails that came quite low in spite of the presence of Mr Smaug. It really was very pleasant, hardly damp at all under the canopy of trees, and the hiss and splatter of falling rain made us feel like we were in a restful cocoon, miles away from civilization, it really seemed like we were on holiday in our garden!

I had a fairly leisurely day today recovering from a busy one yesterday when I had been supervising a 15 hour crystalline glaze firing in a small electric kiln. The kiln does actually fire very well without a controller, but needs checking every hour as it climbs up to Cone 8, then every 10 - 15 minutes from when Cone 8 starts to move to when peak temperature is reached when Cone 10 is half down. From that point, there are checks every half hour for the next 5 hours as I drop the temperature of the kiln from peak to holding at about 1100 Celsius which is a good temperature for growing the crystals to a good size .

My pyrometer reads low, very low.... so I have to be prepared to start checking cones just before 1200 Celsius is arrived at. I don't really mind the pyrometer being inaccurate, because it is reasonably consistent from firing to firing. I almost always fire with cones, so I just need the pyrometer to give me reassurance that the temperature is rising, and to prompt me into action when it is somewhere near the point that I have to start peering through a spy hole in the kiln (wearing welding goggles to protect my eyes from the glare), to observe the bending of the cones.

For those of you who may not know what I mean by cones, here are a couple of photos of them in action in my wood fired kiln. The first photo shows a line of three cones arranged at a slight lean rather like teeth.

If you look very carefully at the second photo you will see three cones, and the one on the right is bending. In "real life" it is often very hard to see cones in a kiln that is at high temperature as everything is glowing so brightly, and does take some practice to make them out. The cones are numbered in sequence and formulated to bend over when their designed temperature is reached. To make things a little more complicated, they really measure "heat work" which is a combination of temperature, and the time taken to reach that temperature, and there are published charts of figures to interpret this.

Cone 10 in my kiln, taking into account its rate of rise in temperature, is around 1285 - 1290 Celsius (2345 - 2354 Fahrenheit).

I will end this post about here as I am a bit too tired to carry on with it, but hope to soon talk some more about crystalline glazing. With luck I might get a glimpse of the crystalline glaze testers that I fired in the kiln yesterday before I go to bed tonight. I was trying various tests with nickel oxide as the main colourant, and I also have a new glaze in there that I put together from a formula that I found in a glaze chemistry book from the 1950s. The book is Ceramic Glazes by Parmelee, and it is something of a classic.

The small kiln with glaze test pots.

Working from a formula, rather than from a list of ingredients is rather fun, as you have to think about what glaze materials might be needed to achieve what has been proposed. Here is Parmelee's formula for a Cone 9 crystalline glaze

0.235 K2O
0.0513 Na2O
0.088 CaO
0.0513 BaO
0.575 ZnO
0.162 Al2O2
1.70 SiO2
0.202 TiO2

In theory I am quite close to achieving those numbers with the following....

Potash Feldspar 41.8
Silica 24.40
Zinc Oxide 17.3
Whiting 4.8
Sodium Carbonate 3.1
Barium Carbonate 2.2
Titanium Oxide 6.3

I wonder if it has worked in practice? My feeling is that the Zinc Oxide looks a little on the low side at 17.3, as many crystalline glazes have Zinc at at least 20 percent, but we shall see!