After I saw some glaze tests that were fired above 1200ºC I wanted to try some glazes for higher temperatures. I like my earthenware glazes, especially the colours, but looking at these glazes it just seemed that a lot more had been going on during firing. Which is probably the case.
After reading some more about glaze ingredients and making a list of the ones that seemed sensible to use, like spodumene. SrCO3, ZnO, dolomite and CaPO4, I mixed the different glazes and applied them to the test tiles. Each glaze was tested three times: The base, + one colouring additive and + 2 colouring additives. The tests were fired very slowly to 1150ºC and then underwent a controlled cooling. I cheated a bit, to speed up the process of mixing the glazes I didn’t weigh the ingredients but mixed them by volume.
Without a specific goal in mind, it was merely curioustity, there were some promising results. My favourites were the ones with copper. To combine FeO with TiO2 turned out to be a bad idea.
Alltough I must say that my favourites are still my earthenware glazes with their soft colours, the colour of CuO in some of these tests looks a lot better. Maybe because turning CuO into turquoise at earthenware temperatures takes ingredients that have a tendency to craze or LiCO3 which I find difficult to handle due to the crystallisation. Besides the copper-colours I like the matte surfaces and am curious to see how those will look on the outside of a wheel thrown cup.
Probably because of the slow cooling I finally managed to fire a tin/chrome red in my own kiln. But I will try this more often before using it, in my experience this glaze is very unpredictable and maybe it’s a better idea to use stains for red and pink.
The glazes had been running, something I forgot about after only firing predictable earthenware glazes for quite a while, so now there’s some work to do to clean up the kiln props.
I can’t yet quite figure out which ingredient caused which effect, but the tiles are fresh and I’ll look at them again sometime this week to see if I can figure out how to finetune the best results, probably by focusing on the copper-blues and mattes.
Besides these tests I fired some ‘old’ glazes to test the firing-schedule.
Here are some of the results.