Glazing

I glazed these pink bowls in a different way then I did before and am very happy with how they turned out. I like the look of unglazed ceramics but the texture can be uncomfortable using cutlery. I like people to use my ceramics and I hope, now that they have a more pleasant feel to it,  this way these bowls will become someone’s favourite.

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Terra Sigillata

I love terra sigillata. There’s something about the subtle sheen that no glaze can compete with. The first time I saw a picture of it, and realising it was something different than a glaze, was in the book “The ceramic process” from the European Ceramic Work Centre (EKWC http://sundaymorning.ekwc.nl/ ). It was a sculpture, if I remember correctly made by Wolfgang Laib, with terra sigillata sprayed on it.

My teacher explained how to make it but at that time the process seemed too intimidating. About a year later she had a new recipe which gave terra sigillata in 3 hours and my fellow students had beautiful results using it. There even was no need to polish it, but polishing did enhance the sheen. It was made with Dolapix PC 67.

In my studio I then only had sodium silicate and sodium carbonate so I tried this recipe with success thanks to Vince Pitelka http://iweb.tntech.edu/wpitelka/professional/terra_sig.htm

  • Clay: 680 g
  • Water: 1,6 l
  • Sodium carbonate: 2 ml
  • Sodium Silicate: 2,5 ml

Mix water with deflocculants – add dry clay – stir – let it settle for 20 hours – poor the most fluid part in another container.

I use terra sigillata on bone dry and bisque, preferably on the latter. When bisque ware is dipped in a container with terra sigillata for 12 seconds there’s an even layer on the clay. When it is almost dry I polish it with a soft cloth or with cotton balls wrapped in clingfilm.

Terra sigillata makes the texture of the object stand out. Because of the small particles it doesn’t hide the skin of the object but shows every detail. For exemple this platter shows my fingerprints and so the way I made it, which I like. IMAG1140 02 klein

You can also use it under a glaze. the glaze in this picture is normally a clear glaze but this time terra sigillata was applied before glazing the red clay.

terra sigillata

My white terra sigillata fits my red clays and lets the red colour shine trough a bit. You should test the terra sigillata on your clays, it tends to flake off when it doesn’t fit the clay or when it has been applied too thick. For example; my red terra sigillata flakes off when used on my red throwing body and I have also experienced troubles when using wax resist on unfired terra sigillata.

You can also use it for functional earthenware to seal the clay at the bottom. When the fired earthenware clay isn’t able to absorb moist its glaze tends to craze less. I fire to 1050°C. Sometimes higher firing results in less sheen .

At the time that I experimented with colours I did not yet have a ball mill and so my coloured terra sigillata had less sheen than the natural one. I make it with clay powder but also to process different clay left-overs. Then the colour will be a bit of a surprise. There’s a large portion of the clay that you started with that will appear as a residu at the bottom of the container. Apparantly it can’t be used for anything, so I let it dry and make the plants in the garden happy with it. The last residu I had, had colouring oxides in it because it was made from left-overs. I let it dry, smashed it, fired the pieces and that way I had little rocks to build small heaps in the garden as a shelter for insects, hoping they’d rather go live there than in my studio.

If you want to start using terra sigillata, don’t let yourself be discouraged by the making process or the idea that it will need a lot of polishing. It is fairly easy to make. I have never used a hydrometer or siphon, those are things with wich you are probably able to work more precise, but it is possible to work without. There are many recipes on the internet and a lot more information, and you will have to find out which recipe works best for you and your clay. Make small amounts of terra sigillata from the different materials in your studio (different types of clay, ball clay and other clay powders) to see wich colour or shade you like best and wich one fits your clay best. Once you have been working with those for a while, you will notice it when the suspension needs more or less water. I have also never bought demineralised water for this process, but sometimes I use cooled down boiled water.

When you are using a brush to apply it, the sheen depends a lot on the kind of brush you use. The brush needs to have very soft hair, it doesn’t need to be natural. Just try al the brushes you think are ok and find out which one works best.

I do polish the surface, but it takes very little time (maybe one minute for a cup). Mainly to make the red clay colour shine trough the white terra sigillata.

Give the terra sigillata time to dry properly, otherwise the sheen might get lost.

Good luck experimenting!

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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