iandol on fri 2 nov 01
Dear Earl Brunner,
Thank you for responding. Sorry to be late with a reply but I lost =
Digest 203.
No, it is not a trick question and I have a firm belief that the =
Eutectics found in Ceramic Phase Equilibrium Diagrams do not initiate =
the melting processes we take advantage of when we fire either clay or =
glaze, or for the glazes I have seen published in the books or on =
Clayart.
Regarding Barium Carbonate, I will put that to the torch and see what =
happens. If it is going to melt at 900 Celsius the observation of =
fluidity should be easy to see on the charcoal block. But I doubt this =
will happen as I have a reliable reference which gives 1360 Celsius as =
the decomposition point
A low melting point for Barium carbonate is not consonant with other =
members of that group. The other Alkali Earth Carbonates decompose =
releasing carbon dioxide, leaving a refractory oxide as a residue.
The melting points of the Felspars are interesting. I think it was =
Singer and Singer who concluded from their studies that Orthoclase =
retained its external shape when heated as high as 1400 Celsius but the =
X ray analysis showed the crystalline structure was destroyed, therefore =
it had become molten. Mineral fusion test suggests 1300 Celsius but =
Albite fuses at 1200 Celsius and Anorthite is 1300 Celsius. However, if =
we consider the degree to which Boron is poured into all sorts of glazes =
I can clearly understand why fusion occurs, since Colemanite melts as =
low as 700 Celsius.
A point which I believe to be important is this. Some people who write =
about glazes as though they were glaze chemists ignore accepted chemical =
concepts. I have yet to read a book or a dictionary which discusses the =
things we do and the knowledge we need using terms such as Solvent, =
Solute, Solution, Saturation and Sintering when they discuss the melting =
and cooling of a glaze recipe.
EUTECTIC: Can be used:-
1 To mean something which will melt quite easily.
2 Meaning loosing physical integrity and bending under stress. This =
seems to be borrowed and contracted from the term "Deformation Eutectic" =
coined somewhere between 1900 and 1910. (read Singer and Singer for =
information on this)
3 In a precise sense, it is an invariant point in a system where the =
addition of heat will cause an increase in the proportion of liquid to =
solid phases and the extraction of heat will increase of the proportion =
of solid to liquid phases without there being any change in temperature. =
I discuss this in an article currently under review by my Editor.
4 The proportions substances in a mixture having a melting point lower =
than all the other possible combinations of those substances. (Note; =
this is about mass and makes no mention of the melting points of the =
contributing materials in the recipe).
I am firmly convinced (and will not be swayed from this opinion) that =
the first phase of maturation of a glaze is sintering of the components, =
with liquid phase sintering dominating the process, followed by melting =
of the component with the lowest fusion point (In the order of Raw =
Borate compounds, Raw Lead compounds, Frits, Soda Felspars, Potash =
Felspars [and if you can get them the Calcium Felspars] . The liquids so =
formed become solvents for the remaining refractory materials, =
increasingly so as temperature rises. Intimate mixing of the original =
powdered materials assists in creating a homogeneous solution which will =
cool rapidly to form a glassy or vitreous substance.
You may quote me if you wish. And I hope you will appreciate my earlier =
concern.
Best regards,
Ivor Lewis. Redhill, South Australia.
Gail Dapogny on fri 2 nov 01
Dear Ivor,
I have been carefully reading your message, and trying valiantly to
undestand, but I am totally minus a background that is of any help. I'm
just determined to take it all in, and am also interested in safety issues
especially with barium, and want to pursue these. I suspect that many on
this list are in the same boat. So I'm sticking my neck out (in terms of
potential for looking dumb!)
Can you give us the implications spelled out pretty simply of your various
conclusions?
Thanks very much,
------Gail
>Dear Earl Brunner,
>Thank you for responding. Sorry to be late with a reply but I lost Digest 203.
>No, it is not a trick question and I have a firm belief that the Eutectics
>found in Ceramic Phase Equilibrium Diagrams do not initiate the melting
>processes we take advantage of when we fire either clay or glaze, or for
>the glazes I have seen published in the books or on Clayart.
>Regarding Barium Carbonate, I will put that to the torch and see what
>happens. If it is going to melt at 900 Celsius the observation of fluidity
>should be easy to see on the charcoal block. But I doubt this will happen
>as I have a reliable reference which gives 1360 Celsius as the
>decomposition point
>A low melting point for Barium carbonate is not consonant with other
>members of that group. The other Alkali Earth Carbonates decompose
>releasing carbon dioxide, leaving a refractory oxide as a residue.
>
>The melting points of the Felspars are interesting. I think it was Singer
>and Singer who concluded from their studies that Orthoclase retained its
>external shape when heated as high as 1400 Celsius but the X ray analysis
>showed the crystalline structure was destroyed, therefore it had become
>molten. Mineral fusion test suggests 1300 Celsius but Albite fuses at 1200
>Celsius and Anorthite is 1300 Celsius. However, if we consider the degree
>to which Boron is poured into all sorts of glazes I can clearly understand
>why fusion occurs, since Colemanite melts as low as 700 Celsius.
>A point which I believe to be important is this. Some people who write
>about glazes as though they were glaze chemists ignore accepted chemical
>concepts. I have yet to read a book or a dictionary which discusses the
>things we do and the knowledge we need using terms such as Solvent,
>Solute, Solution, Saturation and Sintering when they discuss the melting
>and cooling of a glaze recipe.
>
>EUTECTIC: Can be used:-
>1 To mean something which will melt quite easily.
>2 Meaning loosing physical integrity and bending under stress. This seems
>to be borrowed and contracted from the term "Deformation Eutectic" coined
>somewhere between 1900 and 1910. (read Singer and Singer for information
>on this)
>3 In a precise sense, it is an invariant point in a system where the
>addition of heat will cause an increase in the proportion of liquid to
>solid phases and the extraction of heat will increase of the proportion of
>solid to liquid phases without there being any change in temperature. I
>discuss this in an article currently under review by my Editor.
>4 The proportions substances in a mixture having a melting point lower
>than all the other possible combinations of those substances. (Note; this
>is about mass and makes no mention of the melting points of the
>contributing materials in the recipe).
>I am firmly convinced (and will not be swayed from this opinion) that the
>first phase of maturation of a glaze is sintering of the components, with
>liquid phase sintering dominating the process, followed by melting of the
>component with the lowest fusion point (In the order of Raw Borate
>compounds, Raw Lead compounds, Frits, Soda Felspars, Potash Felspars [and
>if you can get them the Calcium Felspars] . The liquids so formed become
>solvents for the remaining refractory materials, increasingly so as
>temperature rises. Intimate mixing of the original powdered materials
>assists in creating a homogeneous solution which will cool rapidly to form
>a glassy or vitreous substance.
>You may quote me if you wish. And I hope you will appreciate my earlier
>concern.
>Best regards,
>Ivor Lewis. Redhill, South Australia.
Gail Dapogny
1154 Olden Road
Ann Arbor, MI 48103-3005
(734) 665-9816
gdapogny@umich.edu
Edouard Bastarache on fri 2 nov 01
Hello all,
It would be interesting to know how people can be sure
that barium measured in a glaze leachate is =AB free =BB,
and not linked to other chemicals present in the glaze?
If linked, it would probably not be so soluble anymore.
The toxicity of barium compounds depends on their
solubility, with the more soluble forms being more
toxic than the relatively insoluble forms.
Here are a few exemples of soluble barium compounds in
the industrial setting :
barium nitrate, sulfide, chloride, acetate, hydroxide and
carbonate.
Insoluble compounds, as the sulfate used in radiology, are
inefficient sources of Ba2+ ions to cause the intoxication.
We know there is a =AB barium scare =BB in North America, oil companies
sell industrial =AB barium free =BB oils and greases but, still offer lea=
ded
ones. So, =AB go figger =BB.
Last year I checked with toxicologists working for the Quebec Govt. and
there is no such thing as a single case of barium intoxication in the
toxicological literature from eating or drinking from wares covered by
barium glazes.
Here is some toxicological information drawn from Quebec =AB OSHA =BB's
toxicology database concerning barium carbonate since it is in this form
that it is added to glazes; the access is free but there is a problem for
most
of you, it is in french :
The URL is :
http://www.reptox.csst.qc.ca/
"Barium carbonate:
1-This product is absorbed by inhalation and ingestion.
It does not skin absorb.
2-Acute effects:
Possible irritation of: skin, eyes, respiratory tract, gastro-intestinal
tract.
If ingested or inhaled in a large enough dose: excessive salivation ,
nausea, vomiting, diarrhea, stimulation of all muscles followed by
flacid palsy, cardiac arythmias (tachycardy and ventricular fibrillation)=
,
hypertension, hypokaliemia(low plasma potassium), death by cardiac or
respiratory insufficiency.
3-Chronic effect:
If inhaled: baritosis (a begning asymptomatic pneumoconiosis).
4-Carcinogenesis, mutagenesis and pregnancy:
No known effects have been described experimentally or otherwise.
5- Lethal dose 50(LD50 ):
Mouse(Oral) : 200 mg/kg
Rat (Oral) : 418 mg/kg"
which makes it a very toxic material by ingestion.
The absorption by ingestion of soluble barium compounds may
be fatal in doses lower than1 gram.
Later,
Edouard Bastarache
Irreductible Quebecois
Indomitable Quebeker
Sorel-Tracy
Quebec
edouardb@sorel-tracy.qc.ca
http://sorel-tracy.qc.ca/~edouardb/
http://www.absolutearts.com/portfolios/e/edouardb/
References:
1-Sax's Dangerous Properties of Industrial Materials, Lewis C., last
edition.
2-Chemical Hazards of the Workplace, Proctor & Hughes, last edition.
3-Occupational Medicine, Zenz C., last edition.
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