Mike Bailey on tue 22 jun 99
Hi, to all, but especially the glaze testers,
first a confession. When I was organising a glaze course a few years ago
we took 100 gms of most of the usual minerals and fired them to 1000 deg
C. The idea was that we'd discuss the loss of mass - clay losing
chemical water etc. whiting losing 44% CO2 etc. I expected the barium
carb to lose weight too (that's the confession - I hadn't tested this
beforehand) but 100 gms of Barium carb. came out weighing 100gms -
suspicion fell on the tester - they hadn't weighed it out correctly. Re
did experiment - same result.
I've since found out that the CO2 doesn't evolve from the Barium Carb
until about 1,450 deg Centigrade! And yet we do all the calculations in
our computers as if the Barium Carb. does give off 22% CO2 and only
brings 78% BaO to the glaze.
So, hence my question. Does anyone know if Barium carb actually does
release CO2 when it's in a glaze ?
Cheers,
Mike,
--
Mike Bailey. Bath. U.K.
Tom Buck on wed 23 jun 99
Mike Bailey:
Does BaCO3 decompose in a glaze mix at full temperature? The
chemical literature is vague on this.
There are three crystalline forms of BaCO3, alpha, beta, and
gamma, with alpha being the stable one, the other two undergoing a shift
at somewhat below 1000 oC, gamma goes to beta, and beta goes to alpha.
The alpha BaCO3 has been observed to melt at 1740 oC at standard air
pressure.
My guess is that because molten BaCO3 is not observed to boil but
decomposes, then BaCO3 will break down in a glaze mix given other strong
fluxes being present so that the glaze will form a liquid at standard
stoneware temperatures. I would think a glaze liquid might dissolve
BaCO3 intact if the BaCO3 were present in small amounts, say 5% or less.
But if BaCO3 were present in amounts of 20-30%, then there well might be
two forms of Barium present in the dry matt glaze, BaO in the glass
matrix, and BaCO3 present as a filler (like a spinel or an opacifier).
Has anyone done any glaze testing to see the form of Barium in a
fired glaze? Perhaps Prof. Hale should investigate this for us.
Tom Buck )
tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
On Tue, 22 Jun 1999, Mike Bailey wrote:
> ----------------------------Original message----------------------------
> Hi, to all, but especially the glaze testers,
>
> first a confession. When I was organising a glaze course a few years ago
> we took 100 gms of most of the usual minerals and fired them to 1000 deg
> C. The idea was that we'd discuss the loss of mass - clay losing
> chemical water etc. whiting losing 44% CO2 etc. I expected the barium
> carb to lose weight too (that's the confession - I hadn't tested this
> beforehand) but 100 gms of Barium carb. came out weighing 100gms -
> suspicion fell on the tester - they hadn't weighed it out correctly. Re
> did experiment - same result.
>
> I've since found out that the CO2 doesn't evolve from the Barium Carb
> until about 1,450 deg Centigrade! And yet we do all the calculations in
> our computers as if the Barium Carb. does give off 22% CO2 and only
> brings 78% BaO to the glaze.
>
> So, hence my question. Does anyone know if Barium carb actually does
> release CO2 when it's in a glaze ?
>
> Cheers,
>
> Mike,
>
> --
> Mike Bailey. Bath. U.K.
>
Tim Dippold on wed 23 jun 99
>From: Mike Bailey
>Reply-To: Ceramic Arts Discussion List
>To: CLAYART@LSV.UKY.EDU
>Subject: Does barium carb give off CO2 ?
>Date: Tue, 22 Jun 1999 08:45:57 EDT
>
>----------------------------Original message----------------------------
>Hi, to all, but especially the glaze testers,
>
>first a confession. When I was organising a glaze course a few years ago
>we took 100 gms of most of the usual minerals and fired them to 1000 deg
>C. The idea was that we'd discuss the loss of mass - clay losing
>chemical water etc. whiting losing 44% CO2 etc. I expected the barium
>carb to lose weight too (that's the confession - I hadn't tested this
>beforehand) but 100 gms of Barium carb. came out weighing 100gms -
>suspicion fell on the tester - they hadn't weighed it out correctly. Re
>did experiment - same result.
>
>I've since found out that the CO2 doesn't evolve from the Barium Carb
>until about 1,450 deg Centigrade! And yet we do all the calculations in
>our computers as if the Barium Carb. does give off 22% CO2 and only
>brings 78% BaO to the glaze.
>
>So, hence my question. Does anyone know if Barium carb actually does
>release CO2 when it's in a glaze ?
>
>Cheers,
>
>Mike,
>
>--
>Mike Bailey. Bath. U.K.
>
>
I would think it wiould give off CO (carbon monoxide) Because It becomes CO2
when it gets another oxygen molecule from the atmosphere or any other source
of o2. CO is very unstable and doesnt last very long any way.
Hope this helps!
Tim Dippold
Albuquerque New Mexico USA
_______________________________________________________________
Get Free Email and Do More On The Web. Visit http://www.msn.com
Craig Martell on thu 24 jun 99
Hello Mike:
According to Frank Hamer it does. I remembered reading that BaCO3 is
difficult to decompose unless there is some reduction involved so I
consulted my library. Anyway, Hamer says that the reduction is necessary to
breakdown the BaCO3 at about 900 C. He says that heat alone won't do it and
if this isn't done you get more of an opacifier and anti flux from the
barium and not the form that gives the nice matts.
The equation is: BaCO3 plus CO, at 900 degrees C, yields BaCO2 plus
CO2(carbon dioxide). He says that BaCO2 is quite unstable and will then
decompose into the desired form of baria.
I think that Hamer says BaCO3 will finally decompose without the aid of
reduction but it takes quite a bit of time and the firing needs to be
extended in order to complete the reaction. I would assume that a lot of
this is influenced by other oxides in the glaze composition. One could use
barium frits which contain BaO instead of the carbonate when firing in
oxidation thereby eliminating the need for a protracted firing. I use Ferro
Frit # cc-257 which has about 33% BaO even though I fire in reduction. Just
don't like fretting about the raw barium carbonate in the shop. I don't use
barium glazes on my domestic pots just outsides of jars and vases that don't
contact food.
regards, Craig Martell in Oregon
Ron Roy on thu 24 jun 99
See the Hamer Dictionary - Barium Carb. - disassociates at 900C in
reduction. Apparently there does not need to be much reduction to start
the breakdown process and so this can be a factor in even oxidation firing.
If there is no reduction the higher disassociation temperature of 1450C is
lowered over longer firings in the presence of active fusions.
RR
>----------------------------Original message----------------------------
>Hi, to all, but especially the glaze testers,
>
>first a confession. When I was organising a glaze course a few years ago
>we took 100 gms of most of the usual minerals and fired them to 1000 deg
>C. The idea was that we'd discuss the loss of mass - clay losing
>chemical water etc. whiting losing 44% CO2 etc. I expected the barium
>carb to lose weight too (that's the confession - I hadn't tested this
>beforehand) but 100 gms of Barium carb. came out weighing 100gms -
>suspicion fell on the tester - they hadn't weighed it out correctly. Re
>did experiment - same result.
>
>I've since found out that the CO2 doesn't evolve from the Barium Carb
>until about 1,450 deg Centigrade! And yet we do all the calculations in
>our computers as if the Barium Carb. does give off 22% CO2 and only
>brings 78% BaO to the glaze.
>
>So, hence my question. Does anyone know if Barium carb actually does
>release CO2 when it's in a glaze ?
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
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