Joseph Herbert on tue 13 dec 05
There are many ways to go on this, Mimi Obler's book, "Out of the Earth,
into the Fire." gives lots of insight into how various earth materials
interact with the fire. Ian Currie's grid method can be a great aid in
evaluating earth materials. Phil Rogers book about ash glazes also talks
about getting from raw materials to glaze. In each case, the chemical
analysis of the material is not much of a consideration.
There are some general guidelines that I am just making up as I go along.
One of the most common rocks you can find in many parts of the country is
some sort of shale. Shales are almost entirely clay minerals often with
lots of iron along for the ride. Many shales have been dug directly from
the ground and fired as bricks. Not much of a glaze often, although some
iron rich clays will melt to a dark brown glass, witness Albany slip. Most
of the time, however, a shale will have too much alumina in it to be a glaze
by it self.
Another common rock is Sandstone, many of these are more than 90% silica.
not many glaze chances there, however, one of the problems that shales have
is not enough silica. Humm.
Limestone is usually largely calcium carbonate with some small fraction of
clay minerals thrown in . But wait, there is a flux here. when the Carbon
Dioxide cooks off, there is calcium oxide. One of the problems with the
shale is not enough flux to melt the alumina.
Wood ash is a traditional source of flux for glazes.
Igneous rocks come in a wide variety of compositions, some of which look a
lot like a cone 10 glaze. There is a pumice layer around here (new Mexico)
that might just need a little clay to make a decent cone 10 glaze. On of
the reasons I think that is because I was able to find a chemical analysis
in geologic literature. But that is not the point.
The alkaline glazes of the North Carolina potters are composed of the clay
body that the ware is made of, some wood ash, window glass, and limestone.
all ground together.
How to proceed?
Make some small bowls or tiles with indented areas. Assume your tests will
run off of the test pieces and take precautions.
Most of the material you can easily collect, either in your back yard or
along some stream bank, will contain too much clay to make a successful
glaze. A line blend of your collected material with wood ash or whiting,
adding 5 percent increments to 20 percent, gives you five samples to fire.
Probably none of them will be very good looking but one will look more like
a glaze than the others so make a line blend with that clay/ash composition
and 5 percent increments of flint or feldspar up to 30 percent. that will
give you 6 samples to fire. Probably two of those sample will look
interesting enough to fool with. you can then experiment with changes
around the composition you find the most interesting, find if it fits your
clay body.
You might have made line blends of various combinations of materials and
fired 15 to 50 samples. If you have kept careful notes, marked your samples
well, and didn't change more than one ingredient at a time, you should be
close to a cone 10 glaze using found materials, mostly. and with no
chemical analysis
I have a theory, based on not much more than an idea, that American Folk
Potters arrived at their higher temperature firing because the ingredients
for lower temperature glazes were expensive and difficult to get. With fuel
(Wood) essentially free for the cutting, the lower cost of found glaze
materials more than made up for the extra fuel required to make them melt.
Using found materials entirely may require some more thought or special
situations. For Example, in Texas there are granite quarries that produce
building stone. The slurry of rock particles from one of their saws will be
close to the composition of a cone 10 glaze. There are lots of barbeque
places in Texas that smoke meat using various hardwoods and producing lots
of wood ash. There are also any number of romantically named shale
formation that outcrop along the road. With these three materials, an
acceptable National cone 10 glaze of Texas could be found, probably in fewer
than 25 tests. like five sets of five line blends.
One of the things to remember about this, even if you secure an analysis,
you really need to get a fairly large amount of stuff because the next batch
of stuff you are going to use will probably produce different results. If
you are going to pay for an analysis, make sure you have a lot of whatever
it is and the whole mess is thoroughly mixed. Kind of painful to have an
analysis of that really unusual little corner that doesn't represent the
rest of the pile at all.
Last but not least, grinding. The grain size of your materials has a very
large influence on how they melt and how they look after they melt. If you
can choose all of your materials so they are relatively fine grained, you
can do testing without a ball mill. A native clay, screened wood ash, and
commercial feldspar powder can make a decent glaze with out extra treatment.
Other materials will present various levels of problems.
Anyway, the thing to do is get some materials and start testing.
Good Luck
Joseph Herbert
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