lili krakowski on wed 28 jan 04
Slip glazes of all sorts seem happier applied to leatherhard clay. It =
ain't necessarily easy because some slip glazes shrink more than the =
clay underneath and need modification. Nevertheless the slip goes on =
smoothly and can be applied with the pot on the wheel. I glaze mine =
after trimming. Glycerin makes the slip more flowing. =20
As to leaking weeping c. 6. This has zip nothing nada to do with c. 6. =
It has a lot to do with the clay body.
Ok. Many suppliers list a clay body as from c.04-2 or from c2-6 or =
whatever. That means that it fires satisfactorily at c.04, can, let us =
say, stand some moisture (flower pot), occasional washings (sculpture on =
bookshelf) (tile) but it DOES not make it suitable for food, or actual =
storage of liquid. Will it make a ring? In all probability.
For the historically minded: those huge water jugs that go back into =
history and still are used today were MEANT to leak. Evaporation acted =
as coolant.
At c.2 the body should do better. It should not really leave =
rings...Why? Because as a body is fired it shrinks and vitrifies. But =
that is a progression, not an instant transformation. Think roast. =
Vegetarians, think winter squash. You put the thing in the oven and as =
its temperature rises inside it gets softer etc. This is called =
cooking. In a kiln the temperature rises IN THE KILN ITSELF taking the =
clay with it. The temperature in the clay goes up too, and the clay =
gets cooked. In an oven--softer; in a kiln, harder
Furthermore I have encountered often enough that for some reason the =
suppliers idea of 2% vitirification at c.6 and mine are NOT the same. =
Either they have not tested the body in 500 years, or they are not that =
accurate.Whatever.
BUT PLEASE, do not blame others in cases where there are so many =
variables. As the cone charts will tell you the rate of heat increase =
affects the actual temperature at which cones deform. As the charts =
ALSO tell you junior cones (the cute little guys for the kiln sitter) =
and standard cones do not deform at same temp. Also where a pot is in =
the kiln, how closely or loosely a kiln is packed all can affect when =
the cones deform. (How this works on digital kilns I do not know.)
As a routine precaution. When you set out to buy clay from a new =
supplier, or a different body from same supplier, start out with a =
sample if they will give it to you, or the smallest amount they will =
sell. Not only test the body and see how it works for you, but make a =
shrinkage bar and also test tiles. Test your glazes on the new clay. =20
For absorption. Make a tile or pot, not too small. Bisque. Weigh it. =
Fire it. Cover with water, boil at least half hour. (I put these tests =
in pot with potatoes, killing two birds etc.) Fish the pot or tile out =
of the water, pat dry, weigh. The difference between dry weight, boiled =
weight is absorption. Figure out the %. My standard is no more than =
1.5% for functional ware.
GLAZE IS JUST A RAINCOAT,NOT WATERPROOFING. A glaze covers the pot =
surface and makes it resistant to liquid, easier to clean, and so on. =
It does not waterproof. =20
Make the absorption and glaze tiles tests everytime a new batch of clay =
comes into the studio. That is why clay body producers have lot numbers =
on the boxes. Because life happens, even at the factory.
Lili Krakowski
Be of good courage
| |
|
