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refractory materials question.

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 (4m=B2, 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=B0C over three days)? I cannot say at what temperature the cracking started, but I would say no higher than 500-600=B0C 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.

Reply to
ali
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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

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.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.

Reply to
David Billington

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.

Reply to
Terry Harper

If they were in fact a pure silica brick they would in fact be most prone to thermal shock as you've indicated. Silica block are used in the building of glass furnaces for above the glass line liner block. They're usually iron colored prior to being fired for the first time. When fully fired they turn white. I've used them on a number of furnaces and have always taken approximately 1 week to bring them up to temperature because of the thermal problems with both the liner and the superstructure. The temperature your stating in your query indicates the cracking started at about where the quartz inversion begins and would in fact be very no extremely problematic if they were pure silica brick, which I have a feeling they are but cannot be sure since I cannot see them. If they have a brand name on them that might help you in identifying them. Henry

Terry Harper

Reply to
Henry Halem

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