[Clayart] Offgassing bisque

Robert Harris robertgharris at gmail.com
Sun Jan 30 18:49:46 UTC 2022


The thing about water is that has one of the highest specific heat
capacities of common substances.

It takes over 4000  Joules to heat 1kg by 1 degree centigrade (at room
temp).
Compare iron (450J), granite (790J)  and Nitrogen Gas (736J).

What this means is that you need 5 times more friction to heat 1g of water
than 1g of air. In addition while 1 g of water is 1 cubic centimeter, 1
gram of air is 800cc!

So if you have a tumbler that holds around a pint and you half fill it with
water, you need about 2000 times the amount of friction to raise the
temperature by the same amount as if it was all air.

And to give some numbers to pressure increases, to increase the pressure by
1% (is this detectable!) you would need to heat the gas up by close to 3
centigrade (about 5F). To get a 10% pressure increase you would need to
heat by almost 50F!

So, while frictional heating in tumblers does happen, the amount it would
have to warm to get a noticeable pressure increase (enough that burping is
required) seems unlikely.

I think Bryan's original assumption that there is some sort of chemical
reaction going on is much more likely.

Robert



On Sun, Jan 30, 2022, 03:54 David Woof <woofpots at hotmail.com> wrote:

> Hi Robert, All,
> I was just going to post and offer a related suggestion when I opened
> Robert's post: my contribution is that if there is any friction caused heat
> build-up then the heat expanded air molecules, if trapped under a tight
> cover seal, would hiss or woosh out on opening the canister.
>
> It wouldn't take an extreme heat rise to create a small but measurable
> positive pressure.
>
> Robert my dinner is waiting and I can't take time to wrap my head around
> the lubricating effect caused by the water vs possible friction heat from
> the porcelain objects and the polishing medium in water..
>
> Perhaps you or someone would run this thru to a logical conclusion if you
> think this has merit.
>
> Now Woof aller manger vite!!!
> ..............................................................................................................................................
>
> Apologies, but my Muse is also hinting" j'ai encore une "in the studio!!!
>  What she wants she gets!!!
>
> @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
> ________________________________
> From: Clayart <clayart-bounces at lists.clayartworld.com> on behalf of
> Robert Harris <robertgharris at gmail.com>
> Sent: Saturday, January 29, 2022 7:17 PM
> To: Clayart international pottery discussion forum <
> clayart at lists.clayartworld.com>
> Subject: Re: [Clayart] Offgassing bisque
>
> 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 at 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.
> >
> > Paul
> >
> > Sent from my iPad
> >
> > > On Jan 29, 2022, at 12:40 PM, Robert Harris <robertgharris at 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 at 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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