On Sun, 6 Jan 2008 01:17:59 -0800 (PST), 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.

First of all, it is normal when heating a glass furnace up for the
first time to have a long drying-out period. This is mainly aimed at
the various mortars used in the structure. Probably at least 24 hours
at 110C would be normal. After that maybe 3degC per hour, depending on
the heating method.

Next, if the blocks had already been used in the glass furnace, they
could well have hot faces into which volatile materials had passed,
and so a surface layer of composition different from that of the rest
of the blocks. It is possible that the effect that you see is such a
layer spalling off.

A further reason could be the absence of allowance for thermal
expansion in your structure, which could put the hot faces into
compression and again cause a layer to spall off.

Although silica undergoes phase changes as the temperature rises, I
don't think that this accounts for the effect that you report.
--
Terry Harper
URL: http://www.btinternet.com/~terry.harper/