Friday, November 27, 2009

Blown to Pieces

We have had an ongoing question, Why does a refired pot (or multi-refired pot), occasionally, explode in the kiln sending shards over the entire kiln and thus ruining all the other pots around it?

(Now, I am not talking about a bisque explosion which occurs around 200 - 400 F and is prevented by a long warm up which drives out the water.)

The possibilities are that the pot has absorbed some water during reglazing or just from a highly humid environment and since most stoneware pottery has at least a 1 -3 % absorption, the trapped water heating up causes the explosion.

Another possibility, the pot has been fired several times and going through quartz inversion around 1063F, too fast will cause the pot to explode. This can be with pots that have thick walls, or uneven walls. Or even including the possibility that during multiple refirings, cristobalite is created (above 2100F), and that is part of the cause, as it goes through cristobalite inversion around 500 F.
To test this I took two multifired stoneware cups and dunked one in water and just stuck the other one in front of the burner and heated it to red heat (1500 F). I then tried the same thing with the wet stoneware cup. Neither exploded.
So I tried a multifired porcelain cup and a wet multifired porcelain cup and heated them to red heat in about 10 minutes. Neither explode but the porcelain did crack into multiple fractures after cooled.

The conclusion is inconclusive.

The problem with this phenomenon is that it is intermittent. It happens once in a while and not every time. The exasperating thing is that when it happens it can ruin a whole kiln load.

Does anyone have a better design to test this? I would like to be able to make it happen to prove one way or another.

John Britt


John Bauman said...

My guess is that it is the first one (trapped moisture). I guess this because the only times I've had it happen on a refire is with pots that have either sat around for a long time (thereby having enough time to absorb moisture from the atmosphere), or were reglazed.

This is further confirmed by the probability that, as "difficult" as it is for a pot -- at only 3% rate -- to take on moisture, it is equally difficult for that pot to rid itself of that moisture quickly. Porosity works both ways.

That's why you can put a relatively wet bisqued-and-glazed pot into a firing and take it up pretty fast without fear of explosion, but cannot do the same with an already high-fired piece. In the case of the bisque piece, it's not how much moisture the piece contains, rather, it's how quickly and easily the moisture can escape from the still porous walls.

John Britt said...

Sounds like everyone thinks it is water. I just want to prove that it is water, as it occassionally happens to potters with memorable results yet no one can say exactly the temperature or when it will occur and not occur.

I often refire pots and hope it won't happen but would like to know when and how to prevent it.

I would think that if water were the cause (and it could be) it would hapen between 200 and 500F (depending on the mass of the pot) but most reports are between 500 and 1200 F. It only happens intermittently and sometimes with pots that have been used and other time with plain unused refires. The cause of this is presumably high humidity. (Which would mean that it would happen more in Georgia and less in Arizona?)

Anyway, I will try some tests. I imagine it is with stonewares that are underfired? or higher prosity but not sure. It may have to be just a specific porosity range that won't let the water easily evaporate during heating.

If you know of any scientific papers on this I would love to read them. (There must be something for industry but can't find anything.)