Alex Solla on tue 11 dec 07
Good morning all.
Richard asked: whether or not you have had them tested for barium release, and whether the test results encouraged or discouraged you from using the glaze.
If you have not had them tested, why not?
Interesting question.
When I first was experimenting with high calcium glazes (fake ash) I was testing every glaze recipe I came across. One such was a "Polished Marble" recipe from Val Cushing. Amazing glaze. Great color response (from the barium), gentle rivulets. Looked like stone! Loved it.
So I decided to have it tested at the wet lab at Alfred U. When I got my results back I was really surprised. LOTS of barium release. I figured that couldnt be right. So I sent sample #2... which weeks later I found was fired slightly hotter (cone 10 touching)... results from sample #2 were great. Now I was in a quandary. Made sample #3, cone 9 touching.
Results were again, NOT foodsafe. If leaching barium could be affected by even minor variations in kiln temperature, there was no way I could guarantee each pot would be safe. So I quit using it on the insides of forms.
In the end, it definitely discouraged me from using the glaze. It also prompted me to begin using glaze calc software and to really try to understand all the different issues happening while my glazes melt and cool. Eleven years later I still have my glazes tested, have switched from barium as my "color responsive" flux to strontium... and it all seems to have been for the best.
Hope this answers Richard's question.
cheers,
Alex Solla
Cold Springs Studio Pottery
4088 Cold Springs Road
Trumansburg, NY 14886
607-387-4042 voice/fax
http://oohmyheck.blogspot.com/
www.coldspringsstudio.com
---------------------------------
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Dan Semler on tue 11 dec 07
Hi Alex,
The temperature dependence has been noted. Janet DeBoos has some
examples of glazes leaching high quantities of barium at cone 10, that
leach a 10th as much at cone 11. Its interesting to see how sensitive
this glaze you used was to that. So I have to ask, did the glaze
itself look much different between the firings ?
Thx
D
Ivor and Olive Lewis on wed 12 dec 07
Dear Alex Solla,=20
Your information shows that there is a need to study the Basic Chemistry =
of Barium Carbonate. Without fundamental knowledge of its behaviour in a =
silicate melt as temperature rises and its reactivity of its products of =
decomposition during cooling we are guessing about durability. Without =
testing, those who would wish to use this material might unfairly be =
prevented from doing so.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
<glaze. Great color response (from the barium), gentle rivulets. Looked =
like stone! Loved it.
So I decided to have it tested at the wet lab at Alfred U. When I got my =
results back I was really surprised. LOTS of barium release. I figured =
that couldn't be right. So I sent sample #2... which weeks later I found =
was fired slightly hotter (cone 10 touching)... results from sample #2 =
were great. Now I was in a quandary. Made sample #3, cone 9 touching.
Results were again, NOT foodsafe. If leaching barium could be affected =
by even minor variations in kiln temperature, there was no way I could =
guarantee each pot would be safe. >>
Ivor and Olive Lewis on wed 12 dec 07
Dear Dan Semler,=20
Did Janet De Boos explain this phenomenon <<...Janet DeBoos has some =
examples of glazes leaching high quantities of barium at cone 10, that =
leach a 10th as much at cone 11...>>
Given the decomposition temperature of Barium Carbonate, this would be =
understandable and, to some degree, predictable.
Best regards,
Ivor
John Hesselberth on wed 12 dec 07
On Dec 12, 2007, at 1:39 AM, Ivor and Olive Lewis wrote:
> Given the decomposition temperature of Barium Carbonate, this would
> be understandable and, to some degree, predictable.
Hi Ivor,
While this could be due to better decomposition of barium carbonate,
I think it is equally or more likely that it is due to more complete
melting of the glazes and better formation of the silica soup. Lots
of cone 10 glazes are marginally melted unless they incorporate a bit
of boron or zinc. At cone 6 I have shown repeatedly that marginal
melting results in lots of leaching of whatever is in the glaze.
I guess my point is that cone 10 evolved historically because that is
where available ground up rocks seemed to give a decent looking
glaze--not because any work was done to show that glaze stability was
maximized at that temperature. I would expect lots of cone 10 glazes
would leach less stuff if they were taken to cone 11 or 12.
Regards,
John
John Hesselberth
www.frogpondpottery.com
"Man is a tool-using animal....without tools he is nothing, with
tools he is all" .... Thomas Carlyle
Ivor and Olive Lewis on thu 13 dec 07
Dear John Hesselberth,
I have no quarrel with your logic or the fact that fusible fluxes are =
sometimes used to assist cone ten glazes to mature. But if this has to =
be done because the batch will not fully vitrify at a designated cone =
value then I would not consider it to be a well constructed glaze.
However, what has not been considered so far in the current discussion =
is the fact that the element Barium does form a rare, stable, almost =
infusible, insoluble, crystalline Alumino Silicate mineral of the =
Felspar family called Celsian that is unaffected by acids.
So much to learn....the mind ignites !!
Best regards,
Ivor
John Hesselberth on thu 13 dec 07
On Dec 13, 2007, at 12:11 AM, Ivor and Olive Lewis wrote:
> But if this has to be done because the batch will not fully
> vitrify at a designated cone value then I would not consider it to
> be a well constructed glaze.
Exactly Ivor. But many, many glazes that are in regular use at all
temperatures are not well constructed. So before one can even begin
to answer a question like yours, one must assure that the glaze is
well melted at both cones. Otherwise the experimental results may be
completely obscured by the more complete silica melt at the higher
temperature. A quick look at the compositions Janet DeBoos used
suggests to me that quite a few were not fully melted. For example
the Seger values for silica range from 1.6 to over 6. Alumina levels
are are high as 1.6. In fact in every sample but 3 (out of 30) the
leaching level was at least twice as high at cone 10 as it was at
cone 11. In my work at cone 6 I found that leaching leveled off when
(I believe) the sample got fully melted.
Yes there is much to learn and, unfortunately, most of the academics
have moved one to more trendy aspects of ceramics, e.g. their use in
space, electronics, and medicine.
Regards,
John
John Hesselberth
www.frogpondpottery.com
"Man is a tool-using animal....without tools he is nothing, with
tools he is all" .... Thomas Carlyle
Richard Aerni on fri 14 dec 07
On Fri, 14 Dec 2007 11:16:19 +1030, Ivor and Olive Lewis
wrote:
>Dear John Hesselberth,
>Have to admit I have not studied Janet De Boos' article, "Living
Dangerously" (Ceramics Technical No 3, pp 66 - 77) in depth but I would
consider her work to be valid since she is a Graduate Chemist as well as
being a well qualified ceramic artist.
>Perhaps this is the kind of information which people should consider before
they dive into the various recipe collections in search of a solution to an
aesthetic problem.
Ivor,
I have been in contact with Ceramics Technical for a reprint of Janet
DeBoos's article in Volume 3 of the magazine, but they say there are no
copies available, the issue is gone. They have forwarded my correspondance
to Janet and I hope she will agree to let me post the article on clayart, if
I can get a copy.
I guess I would disagree with Lili on the point she made, (paraphrasing)
that everything that can be said about barium, has been said. I feel as
though much that needs to be explored, or said, hasn't been. While I
understand her concern that most potters are newbies, and so all toxic
substances should be preached against, I guess I am at heart a proponent of
education and wise usage. If I believed otherwise, I would not be
participating on clayart. I view it as a tool for my own education as much
as it is a tool for newbies.
Best wishes,
Richard Aerni
Rochester, NY
Edouard Bastarache Inc. on fri 14 dec 07
"I guess I am at heart a proponent of
education and wise usage.
Richard Aerni"
Right on...
Gis la revido,
(A la revoyure)
Edouard Bastarache
Spertesperantisto
Sorel-Tracy
Quebec
http://www.sorel-tracy.qc.ca/~edouardb/Welcome.html
http://perso.orange.fr/smart2000/livres.htm
http://www.pshcanada.com/Toxicology.htm
http://www.ceramique.com/librairie/
http://www.flickr.com/photos/30058682@N00/
http://myblogsmesblogs.blogspot.com/
Ivor and Olive Lewis on fri 14 dec 07
Dear John Hesselberth,
Have to admit I have not studied Janet De Boos' article, "Living =
Dangerously" (Ceramics Technical No 3, pp 66 - 77) in depth but I would =
consider her work to be valid since she is a Graduate Chemist as well as =
being a well qualified ceramic artist.
Perhaps this is the kind of information which people should consider =
before they dive into the various recipe collections in search of a =
solution to an aesthetic problem.=20
As I said, the fact that there is such a marked change in the proportion =
leached by heating samples an additional 25 deg C suggests to me that =
the chemical reaction I have proposed is activated. The exceptions in =
the table given by Janet De Boos do not invalidate that hypothesis. Her =
results might serve as a warning to those who favour working with lower =
kiln heats. It might also serve as a model for new students and recently =
graduated teachers and guide them towards a critical attitude when =
reading or consulting technical text books before acting on or =
disseminating information.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Ivor and Olive Lewis on sat 15 dec 07
Dear Richard,
The De Boos article was published 10 years ago.
In her discussion Janet makes the following statement.... " At this =
early stage of testing it is hard to draw statistically sound =
conclusions without appropriate repetition to eliminate personal errors =
and variations in mixing, application and firing".... I wonder if she =
has published an update?
Reasons for the remarkable reduction in leaching rate between samples =
giving high values fired to cone 10 and those fired to cone 11 which all =
give relatively low values are not given.
One factor giving cause for concern is the behaviour of Barium =
Carbonate. Where does Hamer's information that Barium Carbonate has to =
be reduced with Carbon Monoxide come from?
There are conflicts in numerical information given by F. Hamer, CRC =
Handbook, Kaye and Laby, and Greenwood and Earnshaw.
How can we be certain that Barium Carbonate as Hamer tells us, =
decomposes at 900 deg C when CRC tells us that it Melts at 1550 Deg C, =
Kaye and Laby say decomposition is at 850 deg C while Greenwood and =
Earnshaw (p 114) say that with a partial pressure for CO2 of one =
atmosphere Barium Carbonate will decompose at 1360 deg C.
I wonder if Barium rich glazes fired to 1360 or say cone 14 would leach =
any Barium ?.
Without reliable information we are in the dark. I suspect many of the =
ideas we use are equally unreliable.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Ron Roy on sat 15 dec 07
Hi Alex,
Thanks for this - very interesting - when I calculate that glaze there is
only 0.89 silica - John and I found that you need about 2.5 silica to start
getting stability at cone 6. So the fact that you did get some stability at
cone 11 is interesting.
Percent of barium oxide in the fired glazes is over 30%
V. C. Polished Marble.
FORMULA & ANALYSIS
------------------
*BaO........ .33 ......... 30.57%
*CaO........ .64 ......... 21.67%
*MgO........ .01 ......... 0.22%
*K2O........ .01 ........... 0.73%
*Na2O....... .00 ......... 0.04%
Fe2O3...... .00 ........ 0.25%
TIO2....... .00 ........... 0.01%
AL2O3...... .24 ........ 14.54%
SiO2....... .89 ........... 31.98%
RATIO 3.74
EXPAN 851.16
WEIGHT 166.89
Any chance you can post the test data?
RR
> Richard asked: whether or not you have had them tested for barium
>release, and whether the test results encouraged or discouraged you from
>using the glaze.
>If you have not had them tested, why not?
>
>
> Interesting question.
> When I first was experimenting with high calcium glazes (fake ash) I was
>testing every glaze recipe I came across. One such was a "Polished Marble"
>recipe from Val Cushing. Amazing glaze. Great color response (from the
>barium), gentle rivulets. Looked like stone! Loved it.
>
> So I decided to have it tested at the wet lab at Alfred U. When I got my
>results back I was really surprised. LOTS of barium release. I figured
>that couldnt be right. So I sent sample #2... which weeks later I found
>was fired slightly hotter (cone 10 touching)... results from sample #2
>were great. Now I was in a quandary. Made sample #3, cone 9 touching.
>
> Results were again, NOT foodsafe. If leaching barium could be affected
>by even minor variations in kiln temperature, there was no way I could
>guarantee each pot would be safe. So I quit using it on the insides of
>forms.
>
> In the end, it definitely discouraged me from using the glaze. It also
>prompted me to begin using glaze calc software and to really try to
>understand all the different issues happening while my glazes melt and
>cool. Eleven years later I still have my glazes tested, have switched from
>barium as my "color responsive" flux to strontium... and it all seems to
>have been for the best.
>
> Hope this answers Richard's question.
>
> cheers,
> Alex Solla
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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