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.