ali wrote:
Hello. I am only loosely connected with glass in that I am a potter
working with glazes. However I am wondering if anyone could help me
out with some information.

Last year I built a kiln (4m², wood fired). The bricks (more big
blocks actually) that I built it with came from a glass works. I being
a fairly young potter, was following the advice of a more experienced
potter in the area, who convinced me that these blocks were fine for
building a pottery kiln as they were certainly refractory, having come
from the glass industry. The guy I bought them from had them in his
yard for a number of years since the glass works closed down and he
reclaimed them. As you can understand he had no knowledge of the
quality of the blocks.

To cut a long story short, I built my kiln, packed it and preheated it
for over a week before bringing up the temperature (slowly) to begin
the firing. To my horror I saw the bricks at the front of the kiln
starting to crack and splinter... I stopped the firing. The potter who
advised me on buying the bricks is convinced that the bricks still had
some humidity in them and this was the reason. I do not think so. they
were under shelter for a good six months, and the preheating was in my
opinion long enough. Having spoken with people who have some more
technical knowledge, I now believe that the reason is more to do with
the composition of the bricks themselves. From what I understand, in
the glass industry the refractory materials are often almost pure
silica. If this were the case with my big blocks, then would I be
right in thinking that they would react in this way to a relatively
fast rise in temperature (my intention was to fire the kiln to 1300°C
over three days)? I cannot say at what temperature the cracking
started, but I would say no higher than 500-600°C at the very most.
If anyone has any information to offer on the subject then I would be
most grateful.

Alistair,
St Amand en Puisaye,
France.

I think most high temp bricks have a high quantity of alumina in them as
well as varying degrees of silica. What would be helpful is information
about the colour, texture, density, maybe size in order to help identify
the type of brick. You might look for some info at
http://www.thermalceramics.com/ also you may find info at
http://www.ims-insulation.com/default.asp .

My limited experience is with helping a little building a couple of
glass furnaces and the larger, 1.5m x 1.5m x 1.5m IIRC, was ramped up to
a working temp of between 1100C to 1300C over a similar period the first
time without the issues you had. I have also made item in castable with
thicknesses of about 25mm and dried them for about 6 hours then ramped
upto 1000C - 1200C in 3-4 hours without issue, a larger item with
thickness upto about 60mm was dried and fired overnight upto about 500C
before being put into service as as a gathering port
again without issue.

The small furnace did suffer a pot crack after the initial heat and
that seems to have most likely been caused because the pot
was left outside for a couple of years and was subjected to rain and
frost, although it looked undamaged it may have suffered. Is it possible
the bricks you got had been subjected to frost.