Cone 6 glazes, bowls, tiles, and tomato sex!!
I have been doing a lot of glaze testing over the last few weeks. Whilst this has been somewhat exhausting, I have gained a lot of useful information about some glazes that are good for around cone 6 to cone 7, let's say in the region of 1205 - 1230 Centigrade 2201- 2246 Fahrenheit, all depending on how quickly your kiln climbs up to temperature.
To suit my clay I am firing until cone 6 is completely down and cone 7 is starting to bend. There is sufficient heat in the kiln to bring cone 7 down as the kiln is cooling.
|Glaze tests and a commissioned set of bowls that I completed recently.
20 Fritt 3134 (F 4108)
20 Potash Feldspar
20 China Clay
Danny uses ball clay rather than china clay. I have been mostly using a 50/50 mix of ball clay and china clay. The substitution of ball clay for china clay does slightly lower the maturing temperature of the glaze. I use fritt 4108, as 3134 is not available here. I think they are fairly similar though, essentially they are a soft borax fritt.
This clear glossy glaze fits the clay I use very well, with no sign of crazing. I did also try two other variations on the glaze that digitalfire.com mention on their site, one was a low expansion version (to further assist in glaze fit) and the other was a satin version. To be honest I found neither as good as the simple one that I have recorded above. Due to the high amount of clay in this glaze, it is possible to use it as a raw glaze on dry pots, and I have been doing this with some success.
Wollastonite is an interesting material, it is calcium silicate, and as such provides both calcium and silica to a glaze. I suppose that it is really a natural fritt. Wollastonite is used as a flux and has the advantage over calcium carbonate (whiting) in that it does not have to get rid of lots of carbon dioxide in the course of the firing, which can be a potential source of pin-holing in glazes.
|These all look very similar, but have significant changes to the base glaze.
One simple test that was fun to do (and far more rewarding), was the one in the photo above. On the right is the base glaze with 0.4 percent of chromium oxide. In the center is the base with 0.4 chromium oxide, and 0.5 cobalt carbonate. On the left is the base with 0.4 chromium oxide, 0.5 cobalt carbonate, and 4 percent tin oxide. The tin oxide is trying its best to make red from the chromium oxide, but the cobalt blue is making the red turn violet.
I also did a variation with increasing additions of dolomite. The dolomite was to encourage some little crystals to grow in the glaze to make it more interesting to look at (a bit like looking into a rock pool at the beach), you might be able to see this process in the main sequence of tiles in the photo above.
The first complete tile on the left of the photo has a green translucent glaze that you can see right through even though it is fairly dark. The glaze has 3 percent copper carbonate and two percent iron in it, but no dolomite. The tile to the right of it has 5 percent dolomite added to the glaze, which lightens it a little, and the one to the right of that has 5 percent more dolomite, notice how the glaze gets darker again, and there is a dusting of tiny pale crystals over it. By the time we get to the tile at the right end the glaze is looking a sugary and paler blue-green colour. This tile has15 percent dolomite added to the glaze. I used the 10 percent dolomite version of this glaze to put on some bowls that I was commissioned to make someone recently.
It is always good to try the effect of putting one glaze over another, and it is especially good to try this when you are doing test tiles in a controlled and orderly sort of way. When I am glazing pots I often put one glaze over another, and frequently cannot remember afterward what glazes I combined together in this way! I always mean to write it down, but forget every time! The combination in the photo above is chrome-tin red over "floating blue". On its own the floating blue came out rather a muddy grey-brown for me (the sort of colour that a child's paint box goes if all the colours are muddled up together), but this was transformed into a lovely blue where the red glaze flowed over the top!
under-glaze. The iron glaze tends to bubble up through the top glaze, and both go for a slide and create the lacy pattern that you see.
|chrome-tin red on a 9 inch bowl (230mm).
One of the most exciting cone 6 glazes that I tested, and have started to use, is the chrome-tin red that I found on June Perry's web site, shambhalapottery.com. She has posted a wonderful collection of glazes there for cone 6 and for cone 10, and it is a great resource for any potter who wants to test glazes. I have used the chrome-tin glaze in the bowl in the photo above. It is a bad photo unfortunately, but you can see from it that the glaze develops an opalescent purple where it is thick, rather like the "bloom" of a ripe red plum.
The glaze is as follows:
21 Gerstley Borate
16 Nepheline syenite
11 China Clay
5 Tin oxide
0.15 Chromium oxide
In my first test of this glaze I put in too much chromium oxide.... not sure how much, but I know it was wrong because my second batch is a much paler green when I apply it to the pot. When it is fired, the first version of this glaze comes out a darker purple-red; the second "correct" batch is a much brighter red. Nice purples can be made by adding very small quantities of cobalt to the glaze, or, if the chromium oxide is left out, good blues can be made with additions of cobalt.
I glazed up some goblets and used the chrome-tin red on a number of them. One other splendid characteristic of this red is that it does "break" nicely over detail, so the throwing rings of the goblets show up well. The blue version of this glaze also shows detail well too.
I have been experimenting with some 4 inch tiles. I have spared little thought for the tourist trade over the years, most of the time I have been fully absorbed in my task of learning to make pots, kilns, and other important things, but it is also a good idea to try to stay afloat financially, so......
I thought that sheep, cows, and kiwis would be a good start. Here I am just incising a design into a leather hard tile with a couple of wooden tools and a needle. Over the top I put a clear glaze with a bit of copper and iron in it.
I do like old Persian pottery, and I copied part of a Persian design when I decorated the tile below. I have done hardly any under-glaze type decoration before, so it seemed a good way to practice.
Laura has been helping in the studio lately, and she made tiles too, and most of the test tiles. She decorated the tile below whilst I was doing my "Persian" one.
It is supposed to be summer here, but has been quite cold and un-summery the last two days. There was even talk of snow on some of the high hills.... In spite of all that, we have a very large crop of plums ripening on the trees in our garden, and these show in the photo of the lily that Laura took recently (below).
My tomatoes are heading towards being an expensive failure. They did grow, but few tomatoes have shown any sign of setting. Now that the weather is getting chilly, we will be rather lucky to have anything from them. Anyway, in desperation I did some research into the personal habits of tomatoes, and found out that they are supposedly self fertile, but the flowers do need to be shaken around a bit, and the best time is the middle of the day! So...., I have been out amongst the tomatoes around noon each day, giving them as much encouragement as I can! I call it tomato sex, and I really hope that they enjoy it enough to make some baby tomatoes!
One useful discovery that I have made is that it is possible to use a scanner to record the glaze tests (one advantage of doing tests on flat tiles). I also was able to record the recipes by flipping the test tiles over and scanning the reverse side. The only thing to be really careful of is of scratching the glass on the scanner.
I write all the recipes on the backs of the tiles before they are fired with a paintbrush dipped in a watered down mixture of iron oxide and manganese.
Anyway, hope some of that is helpful. I have been working rather long days lately, basically from dawn to when I go to bed, so I have not been able to find much time to spend on the Internet. I am afraid that I am neglecting you all!
Kind Thoughts, P.