Slow-Firing Raku to Cone 6

[Callum Donovan-Grujicich]

I would like to make a large (4.5ft tall) sculpture out of cone 6 porcelain, but I have a problem. I cannot find anywhere that would fire such a large sculpture in one piece. I have seen artists wrap large sculptures in fibre blankets or custom raku-style kilns to fire them. Is a slow-firing and slow-cooling version of this method possible (using propane as fuel) with fibre blankets and up to cone 6 bisque for porcelain? The clay would be relatively thin (about 1cm thick).

Thank you, 

Callum

[Sorcery]

If you willed the other sculpture to move, I reckon you can will this up too!

Sorce

[Piedmont Pottery]

You might want to look at some of the works by Nina Hole, who used to fire quite large sculptural pieces in fiber kilns built around the pieces.  You might also want to consider using a lower maturing temperature porcelain, if you're set on porcelain.

[Callum Donovan-Grujicich]

I was thinking of maybe using a large steel tank insulated with ceramic fibre blanket.  The whole kiln would be raised like a large top hat kiln. I don’t know many details yet as I still have a lot of things to research. From what I have heard, kilns insulated with ceramic fibre blankets heat up and cool down very quickly. I wouldn’t want to use soft bricks because they are very expensive. My concern is that the porcelain would crack from thermal shock. Is there a way to fire this type of kiln slowly? Or is it actually okay to bisque fire thin porcelain quickly?

[Bill Kielb]

39 minutes ago, Callum Donovan-Grujicich said:

I was thinking of maybe using a large steel tank insulated with ceramic fibre blanket.  The whole kiln would be raised like a large top hat kiln. I don’t know many details yet as I still have a lot of things to research. From what I have heard, kilns insulated with ceramic fibre blankets heat up and cool down very quickly. I wouldn’t want to use soft bricks because they are very expensive. My concern is that the porcelain would crack from thermal shock. Is there a way to fire this type of kiln slowly? Or is it actually okay to bisque fire thin porcelain quickly?

A bisque fire sinters the materiał and usually tops out in the range of cone 04. If you are once firing to cone six(6) or maturity you Still need to go slow through the bisque range for the same reasons to go slow while bisque firing. Drive off chemically bound water, burn out organics etc....
Fiber kilns have very low mass, therefore thermal inertia therefore hard to go slow. Can be done with lots of patience and gas adjustments along the way. Problem for most folks is increasing gas 10% raises the amount of heat by more than 20% so nothing is intuitive while doing this.

going slow is likely mandatory BTW to try and minimize cracking.

there is a question that needs to be asked, what is slow to you?

[Callum Donovan-Grujicich]

4 hours ago, Bill Kielb said:

A bisque fire sinters the materiał and usually tops out in the range of cone 04. If you are once firing to cone six(6) or maturity you Still need to go slow through the bisque range for the same reasons to go slow while bisque firing. Drive off chemically bound water, burn out organics etc....
Fiber kilns have very low mass, therefore thermal inertia therefore hard to go slow. Can be done with lots of patience and gas adjustments along the way. Problem for most folks is increasing gas 10% raises the amount of heat by more than 20% so nothing is intuitive while doing this.

going slow is likely mandatory BTW to try and minimize cracking.

there is a question that needs to be asked, what is slow to you?

When I say slow, I just mean slow for a ceramic fibre kiln. So something like six to eight hours of the heat being on.

[Bill Kielb]

16 minutes ago, Callum Donovan-Grujicich said:

When I say slow, I just mean slow for a ceramic fibre kiln. So something like six to eight hours of the heat being on.

That would be fast for a bisque. Bisque firings average 10-12 hours and probably do not exceed 200 degrees per hour to cone 04! I ask the question because firing speed is not something I find folks teach so usually when I ask just about anyone they have never been given a basis for what might be fast or slow.

Bisque firings are slowest when significant changes are occurring with the wares, but in the end probably don’t speed up beyond 200 degrees per hour. Since this will be a single fire to cone six, you probably want to treat it as a bisque firing profile going to cone 6 if I understand correctly.

For large stuff, hand built, maybe thick my take would be not to exceed 200 degrees per hour to be considered slow. Even slower if I was concerned about any cracking.

Take a look at the slow bisque below, it is a well established automatic schedule used in the Bartlett controllers which have been used in a majority of kilns in the US for years now. Hopefully it provides a perspective on slow for that occasional critical piece you need to fire. It ends in 15 hours and never exceeds 200 degrees per hour. The medium slow ends in just under eleven but never exceeds 300 degrees per hour.

Hard to fire a fiber kiln that slow, takes a bunch of patience, but can be done.