Snail mused about temp increase. I see bubbles even before starting the tumbler. These could be just air displacement.
They only get slightly above room temperature.
JM  I do a low bisque, around 010. The pieces are quite smooth after 8 to 24 hours with no additional grIit. Clay with grog probably wouldn't work.
After high fire they are tumbled again. The polishing after bisque eliminates the need for grit. It might speed up the polishing to use fine, or other finishing compound, but not needed. I like not having to deal with / risk getting SiO in my clay reclaim.  I like to change the water occasionally. 
Bryan Johnson

On Sat, Jan 29, 2022 at 5:47 PM Snail Scott <> wrote:

> On Jan 28, 2022, at 8:26 AM, Bryan Johnson <> 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?

Could the interior air have warmed that much from friction; enough to create a noticeably larger air volume?

Can you aim a temperature gun at it, and compare the exterior temps at the start and finish?