firing on vermiculite board vs. mullite boaRd

[Havi]

Hello Everybody,
I wonder if there is any difference in firing on top of Verm. board vs. Mull. board.
Heat distribution, heat absorption,
susceptibility to thermal shocking....

Anything?

Thanks,

Havi

[Bert Weiss]

Vermiculite boards come in a variety of densities and thicknesses and are not smooth on top. Mullite shelves are smooth and dense. Both can warp, V-board is probably more prone to warping. Both tend to stick to glass. V-board is probably more prone to sticking. The more mass you place in your kiln, the slower it heats up and the slower it cools off.

I have never used V-boards as shelves. I routinely use HD fiber boards as shelves. They are much less prone to sticking, as well as lower density than V-boards.

[Havi]

Many thanks, Bert for your response.
But you have not answered my question.....
I own both kinds of shelves, and I know exactly how their surfaces look and how to treat them.
Perhaps someone else who reads this might be able to comment more to what I am interested.
Suppose both shelves are the same thickness - which one will get warm quicker the Ver. OR the mullite?
Which will keep the heat more the Verm. or the mullite? - This might be interesting if one thinks about thermal shock. Which of these boards is more susceptible to create thermal shock?

And if the vermiculite is twice as thick as the mullite, will there be a difference between the heating and/or cooling of the shelf? - again, might cause or help create a thermal shock

Thanks in advance,

Havi

[Don Burt]

Those questions have a lot of variables. You might obtain more opinions if you present a scenario that describes the components in detail between two test cases and ask which one is most likely to cause breakage of the glass piece if you miscalculate the schedule in a certain way. In order to break the glass, given that the glass is in a form that can successfully be fired somehow, you need to use an inappropriate schedule. So you need to pick a mistake, then ask how your two favorite flavors of the two kiln shelves (note Bert's comment 'come in a variety of densities') might increase the likelihood of breakage. Do you want to break it on the way up or down, by the way?

So lets say we have a dog bone shape of about 8" long x 4" at the end, and 2" in the middle. 1/8" BE white opal with an identical 1/8" black opal on top. Prefused and annealed as properly as such a thing could be. Have another, but placed with the black on the bottom. Two hourglass shapes if you will. Overhead elements only. Fire it at full power until they break. Which breaks first on mullite? Which breaks first on the piece of vermiculite that you are most fond of? Then do it again but go slow to 600C and them open the kiln lid and watch which one breaks first as the temperature crashes.

I have no idea what the answer is, or what I would do with that information if I knew it. With my luck the shelves would break.

[Havi]

Thanks Don,
I'll be more concrete....
I have a piece that i fired and it broke on me. It broke once,
It broke twice.....

I suspect that this was bad schedule, bad annealing for sure [I see the edges]
I want to try a third time, and make an effort to do it right.

Being that I have both shelves, I was wondering if it would be better to fire on a 1" thick vermiculite board, OR on 1/2" mullite board?
The first 2 firings were on mullite.

Which one is likely to do the job better?
Many thanks,

Havi

[Bert Weiss]

Mullite is certainly more dense than any vermiculite board. So, mullite will heat and cool slower.

[Havi]

Thanks, Bert.


H.