Some of the nicest things in life are also some of the naughtiest, and this is also true of ceramic glazes! Yes, if you want hygienic craze free, non- toxic, dishwasher proof oven ware, you will probably go for a clear, shiny, thin, faultless, sprayed-on boring coating that is best produced in a ceramics factory... Yawn!
Many of the more interesting glazes are also very practical, a good Tenmoko glaze will generally fit well, are likely to be non-toxic, and they look great with food. Sadly, they are not the height of fashion these days, but in the 1960s you probably weren't a potter if you didn't have a good Tenmoko glaze to slosh over your mugs, bowls, teapots, and casseroles.
Ahh, Tenmoko...! But it is "brown" you say, as if "brown" were a disease! Ha! Just imagine a bowl this colour brimming full of pumpkin soup! Or with a green salad with some tomatoes! Or tumbling full of oranges! Actually this tenmoko bowl would be a bit large for soup, it is nearly 12 inches in diameter and stands 3 inches tall (300mm x 75mm). I got this out of the kiln about 3 days ago, and was amazed at the colour that is in it when you tilt the bowl around in the sunshine. Sadly the rusty flashes of light off iron crystals, and pinpoints of violet-blue don't show up in a photo, but it was a joy to behold.
The glaze is one I usually refer to as "BTM" (Black Tenmoko) in my high fired glaze page. The glaze was passed onto me by my friend, Peter Watson. And it may be a Dartington Hall recipe.
I have put the recipe in my High Fire Glazes page, and I often "tweak" it to suit my firing temperature.
I was firing to cone 10 (almost 1280 Celsius in my electric kiln) and this time round I used;
Nepheline Syenite 55
Ball Clay 15
and red iron oxide 8.
You can also use Potash Feldspar instead of Nepheline Syenite, and China Clay instead of Ball Clay. This will raise the maturing temperature a cone or so.
Here is a bowl with a nice pool of thick crazed and bubbly glaze. The bowl is just over 11 inches wide by 2 inches high (280mm x 50mm). I used the same glaze combination in a firing that I did a couple of days before this one. There are two alkaline glazes, both chemically the same, but using slightly different ingredients. The glaze that I use in the middle has calcium carbonate as a flux, and I find it gives me a bluer glaze where it is thick, but can cause problems with bubbles which are not nice when they occur around the rims of bowls, but can be rather splendid in the centre of a decorative bowl. The glaze that I used around the rim and sides of the bowl uses Wollastonite to provide the necessary calcium. Wollastonite is calcium silicate, and it doesn't release bubbles of carbon dioxide, whilst the glaze is firing, where as calcium carbonate does. The two used together like this are rather nice I think. You can find the recipes for both these green glazes on my High Fire Glazes page.
And, finally, crawling...
I am doing some tests of a glaze that I have formulated to have abnormally high shrinkage whilst it fires. I am testing this glaze over various other glazes, and some rather interesting results are starting to emerge!
Here are some tests that I did some time ago. These were 50/50 mixes of Nepheline Syenite and Magnesium carbonate over an iron red glaze. I have written about that on my High Fire Glazes page.
I must head off to my studio to glaze some more pots!
The sun is out today..... Amazing to see it after lots, and lots, and lots, of rain!