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Re: [teacherartexchange] melted marbles etc.


From: Marvin Bartel (marvinpb_at_TeacherArtExchange)
Date: Tue May 24 2005 - 09:19:48 PDT

Nearly everything has its melting point. Water melts at about 32 F,
but many things melt at kiln temperatures. Even a piece of wood
leaves ashes. Wood ash (soda and silica) melts if you fire it hot
enough. Wood fired pots often have nice glazes that come solely from
ashes flying through the kiln with the flames. Borax that is sold
for washing is an active flux that makes glazes melt easier. I often
use it in raku slip. Unlike many glaze ingredients, it is soluble in
water, making some interesting complications when doing the post
firing smoking. Many things that would not work alone can be added
on top of regular glaze materials or mixed into regular glazes.

To find out if something melts at your firing temperature, a student
can try it in a shallow bisque container. Try it in a glazed dish
and an unglazed dish to see the difference. When runny things are
used on the sides of pots, potters set the pots on bisque or
firebrick stands in bisque fired catch basins so their kiln shelves
are not ruined. Side fired pots get nice sideways runs on them.
They are set on three bisque points that hold the piece above the
catch basin. After firing, be careful of the sharp points where the
pot is broken off of the support points. Carefully rub these spots
with a piece of kiln shelf or a an abrasive stone to make the pot
safe to handle without cutting your fingers.

Our students are often trying things like thin copper wire, nails,
oil paints, tempera, etc. just to see what will happen. When a
student asks me what will happen if I . . . , I resist telling them
what I think will happen. I generally tell them it sounds like an
interesting experiment and encourage them to try it and find out.
They are much more excited to learn it themselves than for me to tell
them what will happen. I often tell them they have to make a nice
thick bisque dish for their experiment.

Recently, one of our students made a two level stand out of clay and
bisque fired it. On the top level it had a rack to put dish of
glass. This dish had a hole in the middle (like a shallow funnel).
The top dish contained glass so the melting glass ran down onto a pot
placed sideways on three points in a dish below. The pot below got
the extra dose of glazing from the glass flowing over it during the
firing. Small amounts of coloring oxides can be added to the glass.

There are some safety issues, so I encourage students to check with
me before trying something new. If you are not sure if something is
safe, check with a science teacher or a local potter about the fumes
that will come out of the kiln when you fire something. Ordinary
smoke is okay if your kiln is well vented. Otherwise, expect a call
from the fire department when your smoke alarm goes off. Table salt
is not okay for classroom firing. Salt contains soda which is a
great glaze ingredient, and potters have long used salt by throwing
it in their kilns when the kiln is 2,300 F. Potters realize that
salt also gives off toxic chlorine gas, so they take precautions.
Potters also know that the soda vapor tends to melt the kiln surfaces
as well as the pots. Salt kilns are dedicated to this use and they
are fueled with wood, gas, or oil - never with electric elements.

I am writing a lesson about stuff that melts. It combines science
and art. When it is posted, I will send another note.


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