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Kiln firing of clays with jade


#1

Karen just asked a question about kilns. Orchid mostly discusses
gemology but if you google on “micaceous clay” and related subjects
you see that micas, magnetite etc. are used to enhance the appearance
of kiln fired clay. In effect these minerals are tiny gems set in the
clay and some potters set more precious stones in clay. The finished
products can be as valuable as any jewelry ring or necklace.

I am taking a beginners course on clay modelling but I am not the
artist in the family. A family member sculpts excellent dinosaur
figurines which I would like to fire in a kiln. Can anyone recommend
a small starter kiln, 110 v electric or propane?

Also I have some pieces of BC nephrite jade which I want to set in
clay tiles or sushi plates. I could embed the jade as grit or set
larger stones, eg one inch pieces cut on a tile saw. Has anyone else
tried jade in clay?


#2

Peter, I haven’t specifically tried nephrite in clay but I have
tried jade in a kiln to see how it reacts. At 1200 F it turns gray to
brownish and crumbly. Had similar experience with jadite. Would not
recommend it.

Cheers from Don in SOFL


#3

The minerals you mention are added to a clay to give it certain
properties upon firing. Iron oxides give a body colour that can be
red or black depending upon the oxidation state of the kiln. Mica is
sometimes added to a clay body tha is being fired at lower than
normal temperaturs such as a large figural piece. Many materials are
used as glaze additives, including amazonite, jade, malachite,
magnetite etc. They give the glaze colour, cause it to melt, flow and
stick to the body of the clay. I have never heard of anyone firing
gemstones in situ, they would just react and become part of an
amorphous mass. The particle size of these crushed gemstones is in
the 5-10 micron size genrally, sometimes as coarse as fine sand but
still invisible to the naked eye. You would be better off setting
your pieces of jade after firing and glazing your ceramics.

An electric kiln is the easiest to useas they are available with a
decent thermocouple and programmer from about $400 for a portable
1.1 cu Ft model. This will do you nicely for most items you are
likely to make and will also allow you to do glass fusing and casting
if you have the correct thermocouple/controller. Just specify when
you buy. Gas kilns tend to be larger, harder to control and require
more space and time to use and are mor expensive to buy. Also you
cant do glasswork in them as they heat up too quickly and you cant
hold a temperature so easily. They are better for certain types of
ceramic and you can do reducing atmosphere firings though.

Nick Royall


#4

Thank you for the reference to “glaze additives”. Googling suggests
quite a long list too and I have asked the library to get me a copy
of Hamer and Hamer’s pottery dictionary. Cutting nephrite with
diamond blade saws gave me a lot of nephrite dust and I thought I
would try it as an additive after I get my first kiln.

I also want to try firing the nephrite itself because of my field
observations. Close to the deposit of nephrite there are a couple of
major faults shown on the geological maps and an adjoining
outcropping seems to be a kind of lava. Did it heat and glaze the
nephrite… sort of a natural kiln effect? Some surfaces are glazed
with a transparent coating which is a fraction of a mm thick. It
enhances the colour of the stone a lot and kicks the hardness up at
least a notch on the Mohs scale. Any suggestions as to what it might
be chemically? It is too thin to assay.

I read your comment about jade reacting in a kiln and becoming part
of the amorphous mass which I think is also what Don (“coralnut”)
was saying. At 1200 F kiln temp he says the jade turns brown/gray and
crumbly. But maybe I can somehow “bake” that glaze onto jade stone
and simulate what I see in nature, perhaps by keeping the kiln
temperature low enough.