iandol on wed 20 nov 02
Hello friends,
Just read an interesting comment relating to the behaviour of Barium =
Oxide in glazes.
"Used as a flux and to aid the formation of a matty structure. If the =
BaO content exceeds about 0.3 equivalents, barium takes the role of a =
refractory rather than a flux, increasing melting temperature. When used =
in small amounts, it improves gloss and mechanical strength, and aids =
resistance to organic acids.." R. Newcomb, "Ceramic Whitewares." 1947.
No references are given for the final comment.
Anyone any ideas as to the origins of this knowledge which seems to =
contradict current ideology?
Best regards,
Ivor Lewis
Lily Krakowski on wed 20 nov 02
Hello!
I mean to post something re: crazing from this same INVALUABLE booklet, but
I still am getting over the fatigue of a long drive over the weekend.
However: Ceramic Glazes,by Felix Singer, in the new edition by W.L.
German, and published by Borax Consolidated Limited says (p17) "Other
fluxing oxides such as the alkaline earth oxides, CaO and BaO, tend to lower
the firing temperatures but if the addition is overdone they can behave as
refractory materials." (The original booklet is older than this and was
around already in the late 40s. The scarcity of commas is his, not mine,
suggesting it was published in a less affluent time.)
Parmlee says that "the use of barium carbonate in the glaze batch is
generally limited t a few molcualr equivalents because it reacts slowly and
acts generally as a refractory, often producing dull or matt textures. If
fritted or fired at the higher temperatures it becomes a vigorous flux." (p
23)
on p 154 he says that substituting barium for lime decreasses the thermal
expansion but "...in larger additions..." raises annnealing temperature[s]
Since, says the intro. , Parmelee died in 1947 he wrote his book earlier.
iandol writes:
> Hello friends,
>
> Just read an interesting comment relating to the behaviour of Barium Oxide in glazes.
> "Used as a flux and to aid the formation of a matty structure. If the BaO content exceeds about 0.3 equivalents, barium takes the role of a refractory rather than a flux, increasing melting temperature. When used in small amounts, it improves gloss and mechanical strength, and aids resistance to organic acids.." R. Newcomb, "Ceramic Whitewares." 1947.
> No references are given for the final comment.
> Anyone any ideas as to the origins of this knowledge which seems to contradict current ideology?
> Best regards,
> Ivor Lewis
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Lili Krakowski
P.O. Box #1
Constableville, N.Y.
(315) 942-5916/ 397-2389
Be of good courage....
John Hesselberth on wed 20 nov 02
On Wednesday, November 20, 2002, at 12:47 AM, iandol wrote:
> When used in small amounts, it improves gloss and mechanical strength,
> and aids resistance to organic acids.." R. Newcomb, "Ceramic
> Whitewares." 1947.
> No references are given for the final comment.
> Anyone any ideas as to the origins of this knowledge which seems to
> contradict current ideology?
Hello Ivor,
I don't have a specific answer to your question, but please remember
that most statements from that era regarding acid resistance pertain
only to lead-based glazes. The overriding issue of a huge percentage of
the research done from, say, 1900 to 1950 was directed at keeping lead
from leaching. It was so overriding that it was not necessary to mention
it in every article or book. Even books published well after 1950 drew
on that work, failing to mention that it only applied to keeping lead in
a glaze.
In other words statements in the ceramic literature are often irrelevant
in today's world because of that simple fact. The work was clearly well
done at the time, but it was done in a different context than we live in
today.
Regards,
John
http://www.frogpondpottery.com
http://www.masteringglazes.com
Paul Lewing on wed 20 nov 02
on 11/20/02 6:38 AM, John Hesselberth at john@FROGPONDPOTTERY.COM wrote:
> I don't have a specific answer to your question, but please remember
> that most statements from that era regarding acid resistance pertain
> only to lead-based glazes. The overriding issue of a huge percentage of
> the research done from, say, 1900 to 1950 was directed at keeping lead
> from leaching. It was so overriding that it was not necessary to mention
> it in every article or book. Even books published well after 1950 drew
> on that work, failing to mention that it only applied to keeping lead in
> a glaze.
In other words statements in the ceramic literature are often irrelevant
> in today's world because of that simple fact. The work was clearly well
> done at the time, but it was done in a different context than we live in
> today.
John, could you comment on the differences, if any, between keeping lead
from leaching and keeping other elements from leaching, assuming all other
things like firing temperature and atmosphere are same? What would make
statements about lead leaching irrelevant to the leaching of other things
like barium?
Thanks, Paul Lewing
John Hesselberth on wed 20 nov 02
Hi Paul,
I have no data on this. I've never tested for lead leaching and I'm not
aware that any of the early workers tested for anything but lead. It
may be similar; it may be quite different. As far as I know it is just
a big hole in the knowledge base and it's not likely to get filled.
Regards,
John
On Wednesday, November 20, 2002, at 04:06 PM, Paul Lewing wrote:
> John, could you comment on the differences, if any, between keeping lead
> from leaching and keeping other elements from leaching, assuming all
> other
> things like firing temperature and atmosphere are same? What would make
> statements about lead leaching irrelevant to the leaching of other
> things
> like barium?
> Thanks, Paul Lewing
http://www.frogpondpottery.com
http://www.masteringglazes.com
iandol on thu 21 nov 02
Dear Lily Krakowski,=20
Yes, I have read the Singer?Borax Consolidated information and Parmellee =
as well as Parmellee and Harman.
At the moment the kiln is a tad warmer than I like for cracking the seal =
so you will have to wait until tomorrow for my observations about the =
solubility of Barium Carbonate in a clear Felspathic glaze, in the range =
2%-40% of glaze volume.
When people mention the word "Flux", I like to ask "Now, just what does =
that mean?"
Is it that it helps things to melt at a lower temperature?
Is it that it means it makes things flow more freely when you get to the =
maturing temperature of your clay?
Or does it mean something else?
Thanks for your contribution.
By the way, Wollastonite is a natural mineral of the compound Calcium =
Silicate. I think it is metaphorically speaking, the best thing since =
the invention of sliced bread. Wish I could find the Barium Equivalent.
Best regards,
Ivor Lewis.
Ron Roy on fri 22 nov 02
Barium is not a refractory in high fired glazes - same for Calcium,
Strontium Magnesium and Zinc - but it may appear so to those who do not
understand the way these fluxes work when oversupplied.
When these fluxes are present in quantities that will not remain dissolved
in a glaze as it cools - they will all form crystals on the surface -
because they cannot remain in solution. They give the appearance of not
being melted because they are not shiny.
We have all see such glazes - properly melted - some even showing signs of
moving due to gravity - but still looking as if they are not melted -
Alumina mattes for instance. All this means is that - for whatever reason -
the surface of the glaze has recrystallized.
It is also true that if one flux grossly dominates all others - melting is
impaired to some degree - because - a balance of fluxes gives better
melting.
I just read the descriptions of both Barium and Strontium in Taylor and
Bull - Strontium is even better at improving gloss and hardness in a glaze
and has the added advantage - because you use less of it - of lowering
expansion ( to help cure crazing.)
>Just read an interesting comment relating to the behavior of Barium Oxide
>in glazes.
>"Used as a flux and to aid the formation of a matty structure. If the BaO
>content exceeds about 0.3 equivalents, barium takes the role of a
>refractory rather than a flux, increasing melting temperature. When used
>in small amounts, it improves gloss and mechanical strength, and aids
>resistance to organic acids.." R. Newcomb, "Ceramic Whitewares." 1947.
>No references are given for the final comment.
>Anyone any ideas as to the origins of this knowledge which seems to
>contradict current ideology?
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
| |
|
