Mid-fire glaze tests

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.

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Glaze tests

The kiln I unloaded last week had a lot of glaze tests in it. I’ll share a few of those here…

I made a mistake by mixing my off-white speckled base glaze with zircopax, because of that the colour turned cool en the contrast between de speckles and the base colour was now too big. However ball milling it turned it into a nice, smooth creamy off-white glaze. Unspeckeled of course.

Base glaze with zircopax and FeO.

Base glaze with zircopax and FeO.

I roughly tested this ocean green colour before, and this time in my glossy base. I like the colour, but not the gloss. It has SnO, ZnO and CuO in it, and after ball-milling the colour got better. Combining it with rutile gives yellow speckles.

Sea-green glaze, the right one had been ball-milled.

Sea-green glaze, the right one had been ball-milled.

Sea-green glaze with rutile added.

Sea-green glaze with rutile added.

Up untill now I used talc and dolomite for a matte glaze surface. But this time I used MgCO3 because that’s supposed to be more suitable for low-firing. The glaze turned out matte, but also opaque.

Too much MgCO3 causes crawling.

Too much MgCO3 causes crawling.

Tests with my pink glaze showed that a wash with FeO gives black (?) and a ZnO wash does nothing which is a bit weird since ZnO and pink stain shouldn’t be compatible. But then, a FeO wash should not make it black I think…

Pink glaze turned yellow and black after FeO-wash.

Pink glaze turned yellow and black after FeO-wash.

Besides glazes I needed a slip that I can use as a glue. It doesn’t have to be neat glue like you can by in the store, but it must be usable to quickly stick small ceramic parts together. All the tests were done with paper clay mixed with different melting agents. The frits didn’t work as well as I had expected, but the boric acid with wood ash did. It did smell a bit though, and since boric acid should not be mixed with soda ash because of the chemical reaction I think it reacted with some of the K2O and Na2O in the wood ash.

Paper clay tests

Paper clay tests

I made the crater glaze this time with a slightly different frit, and it turned out differently. Now I don’t know if that is because of the frit or the different (slower) firing schedule. But since I made the tests to get other colours of that glaze it’s not really a problem.

Crater glaze colour tests

Crater glaze colour tests

Merry christmas!

Ball mill

Last week I had the opportunity to buy a second-hand ball mill and to do some testing with it.

One of my favorite glazes is a bit difficult to preserve due to crystallization. There are other solutions besides ball milling it, but apparently ball milling it works well and isn’t much trouble.

The left test tile is the glaze as it is, the right had been milled for 35 minutes, the middle one for 20 minutes. The pinholing and crawling is another problem with this glaze but only occurs when applied too thick or when there are LiCO3 crystals in it. (so it is actually not always crawling but at times little spots with too much flux).

ball mill tests

Other glazes also benefited from ball milling, for instance one with coarse SrCO3 but also my ‘normal’ base glaze. That is, if you want it to be smooth.

I fired with a new firing schedule with controlled cooling between 1050°C and 700°C to ensure the matte glazes turn out matte which worked fine for a few glazes but still there were some unexpected results. So again a good reason for more testing.

The picture at the top shows something else that came out of the kiln this morning. More on that later.