I was assuming that the tumbling was done in water. It sounds like everyone else is assuming it's done dry. (at least I'm assuming there wouldn't be much frictional heating with water).
Which is it! 

On Sat, Jan 29, 2022, 16:45 paul gerhold <gerholdclay@gmail.com> wrote:
I would look at what chemicals are used to color the bisque. Maybe some decompose at low enough temp that just the friction heating would cause decomposition to occur.


Sent from my iPad

> On Jan 29, 2022, at 12:40 PM, Robert Harris <robertgharris@gmail.com> wrote:
> Pretty much the only likely chemical reactions are ones that give off CO2.
> Something like a bicarbonate reaction with acid  Not saying that's what it
> is, just more likely. See if the gas puts out a match. You could see if
> adding a bit of vinegar speeds up the evolution of the gas.
> Only other possibilities, both fairly farfetched would be hydrogen (match
> wil light it - stand back) or hydrogen sulfide (which would be really
> obvious).
> Robert
>> On Fri, Jan 28, 2022, 08:44 Bryan Johnson <bryj@cheqnet.net> wrote:
>> When I tumble polish bisqued colored porcelain jewelry I need to "burp" the
>> barrels after an hour or so due to the gas given off by the porcelain. What
>> is the gas?
>> It doesn't happen after high fire.
>> Bryan
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