Anne POSSOZ on fri 26 may 00
Hello,
Thecnical question.
Reading a book, I find mention of glaze acidity.
What is this? How can we compute the acidity of a glaze?
Which oxides are acid, basic and amphoth=E8res (?)?
Having the recipe and the oxide compusition, I should be able
to compute it myself. But how should I do that?
That book mention an acidity of 5.3.
I checked for acidity and basicity in all basic pottery books that I have
but there is no definition.
Even my favorite web site http://digitalfire.com/education/glossary/index=
.html
has no mention of what it is.
I know that it is a technical question but I'd love to understand precise=
ly
what it means and be able to determine myself the acidity or basicity
of glazes that I prepare.
I am sure that, on the long term, I will benefit of that information too.
Usually, in chemestry, acid or basic refers to hydrogen affinity.
But there is no hydrogen in a glaze, is it?
Thanks if anybody has a good answer or reference for that matter.
Anne
--=20
Anne Possoz Service Informatique Central Tel : (41/21) 693.22.49
Ecole Polytechnique Federale de Lausanne, 1015 Lausanne (Switzerland)
Smart on fri 26 may 00
Dear Anne,
If you speak French a little, you can visit my site and discover how to
calculate glaze acidity and determine what sort of glaze you can hope after
firing (mat, brilliant, etc=2E=2E=2E)=2E You will find that in the section "=
Ce qu'il
faut savoir" (What you have to know) and then "Calculer l'aspect d'une
gla=E7ure"(How to calculate a glaze aspect)=2E
Cordialement, Smart
Smart=2EConseil
Le site Fran=E7ais d=E9di=E9 aux passionn=E9s de c=E9ramique
The French site dedicated to ceramics lovers
smart2000@wanadoo=2Efr
http://perso=2Ewanadoo=2Efr/smart2000/
----- Original Message -----
From: Anne POSSOZ
To:
Sent: Friday, May 26, 2000 2:56 PM
Subject: acidity of a glaze: definition
Hello,
Thecnical question=2E
Reading a book, I find mention of glaze acidity=2E
What is this? How can we compute the acidity of a glaze?
Which oxides are acid, basic and amphoth=E8res (?)?
Having the recipe and the oxide compusition, I should be able
to compute it myself=2E But how should I do that?
That book mention an acidity of 5=2E3=2E
I checked for acidity and basicity in all basic pottery books that I have
but there is no definition=2E
Even my favorite web site
http://digitalfire=2Ecom/education/glossary/index=2Ehtml
has no mention of what it is=2E
I know that it is a technical question but I'd love to understand precisely
what it means and be able to determine myself the acidity or basicity
of glazes that I prepare=2E
I am sure that, on the long term, I will benefit of that information too=2E
Usually, in chemestry, acid or basic refers to hydrogen affinity=2E
But there is no hydrogen in a glaze, is it?
Thanks if anybody has a good answer or reference for that matter=2E
Anne
--
Anne Possoz Service Informatique Central Tel : (41/21) 693=2E22=2E49
Ecole Polytechnique Federale de Lausanne, 1015 Lausanne (Switzerland)
____________________________________________________________________________
__
Send postings to clayart@lsv=2Eceramics=2Eorg
You may look at the archives for the list or change your subscription
settings from http://www=2Eceramics=2Eorg/clayart/
Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink=2Ecom=2E
Tom Buck on sun 28 may 00
Anne P:
I have an Opinion, yet to be confirmed as Fact:
The elements listed on the far left of the Periodic Table (as now drawn up
amd widely circulated)....these are all elements that form Bases when
their oxides are combined with water, eg, Na2O + H2O = 2NaOH (lye). So
do lime CaO and magnesia MgO.
The elements at the right, the non-metals mostly, form Acids when
their oxides combine with water, eg, CO2 + H2O = H2CO3 (carbonic acid).
The elements inbetween, mostly "transition metals", tend to
exhibit "amphoteric" beviour, ie, they can go either way, depending what
else is closeby.
Now, take this notion to molten glass. It is chiefly a network
made up of silicon oxide molecules linked together in chains. But at very
high temperature, the silica molecules are free to roam around, and in a
sense behave as a solvent of other molecules. Alumina (Al2O3) dissolves in
this solvent, and the combination is "acidic" and if the molten glaze
contains lesser amounts of basic molecules (the oxides of the alkalies and
the alkaline earth elements in particular), the glaze will be classifiable
as an acidic glaze, and copper oxide will impart a green colour to the
glaze on cooling (oxidation conditions only). If, however, there are
sufficient molecules of the basic elements in the mix, the glaze will
be said to be basic and in it copper oxide will exhibit a blue colour (eg,
soda blue of raku).
Can one predict whether a glaze will be acidic or basic? Within
narrow limits, prediction may be possible...one would need the Seger
Formula plus other data ("ionic" radii of the mobile oxides?).
Perhaps those up on geophysics and geochemistry can make this
notion clearer.
til later. Peace. Tom B.
Tom Buck ) tel: 905-389-2339 (westend Lake Ontario,
province of Ontario, Canada). mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
Harry on wed 31 may 00
Tom, you make my brain hurt.
That is just about the most interesting idea I have heard in a long
time. I wish I knew something about chemistry so I could do some real
investigating on that. I may yet pit my feeble gray matter against this
question.
Keep the hardballs coming.
Harry
-----Original Message-----
From: Tom Buck
To: CLAYART@LSV.CERAMICS.ORG
Date: Tuesday, May 30, 2000 7:42 PM
Subject: Re: acidity of a glaze: definition
>Anne P:
> I have an Opinion, yet to be confirmed as Fact:
>The elements listed on the far left of the Periodic Table (as now drawn up
>amd widely circulated)....these are all elements that form Bases when
>their oxides are combined with water, eg, Na2O + H2O = 2NaOH (lye). So
>do lime CaO and magnesia MgO.
> The elements at the right, the non-metals mostly, form Acids when
>their oxides combine with water, eg, CO2 + H2O = H2CO3 (carbonic acid).
> The elements inbetween, mostly "transition metals", tend to
>exhibit "amphoteric" beviour, ie, they can go either way, depending what
>else is closeby.
> Now, take this notion to molten glass. It is chiefly a network
>made up of silicon oxide molecules linked together in chains. But at very
>high temperature, the silica molecules are free to roam around, and in a
>sense behave as a solvent of other molecules. Alumina (Al2O3) dissolves in
>this solvent, and the combination is "acidic" and if the molten glaze
>contains lesser amounts of basic molecules (the oxides of the alkalies and
>the alkaline earth elements in particular), the glaze will be classifiable
>as an acidic glaze, and copper oxide will impart a green colour to the
>glaze on cooling (oxidation conditions only). If, however, there are
>sufficient molecules of the basic elements in the mix, the glaze will
>be said to be basic and in it copper oxide will exhibit a blue colour (eg,
>soda blue of raku).
> Can one predict whether a glaze will be acidic or basic? Within
>narrow limits, prediction may be possible...one would need the Seger
>Formula plus other data ("ionic" radii of the mobile oxides?).
> Perhaps those up on geophysics and geochemistry can make this
>notion clearer.
> til later. Peace. Tom B.
>
>Tom Buck ) tel: 905-389-2339 (westend Lake Ontario,
>province of Ontario, Canada). mailing address: 373 East 43rd Street,
> Hamilton ON L8T 3E1 Canada
>
>___________________________________________________________________________
___
>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.
>
>
thansen@DIGITALFIRE.COM on wed 31 may 00
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I great place to learn about this is in Ian Curries 'Stoneware Glazes' book.
--
====================================================
T o n y H a n s e n thansen@digitalfire.com
http://digitalfire.com Calculation/Database Software for Ceramic Industry
http://ceramicsearch.com Search engine for the Ceramic Industry
--
--==__==__A8420XZ382__==__==--
Hank Murrow on thu 1 jun 00
>I great place to learn about this is in Ian Curries 'Stoneware Glazes' book.
>
And a wonderful additional way would be to take in a Currie Workshop during
his North American tour in Sept/Oct. Check with Ruth Ballou
to learn locations and times. Good Luck, Hank in
Eugene
Hank Murrow on fri 2 jun 00
SNIP...
> Can one predict whether a glaze will be acidic or basic? Within
>narrow limits, prediction may be possible...one would need the Seger
>Formula plus other data ("ionic" radii of the mobile oxides?).
> Perhaps those up on geophysics and geochemistry can make this
>notion clearer.
> til later. Peace. Tom B.
Dear Tom and Anne; I have come to believe that IONIC POTENTIAL
(valency/ionic radius) is a more likely predictor of behavior of oxides in
a glaze, as it marries relative size AND strength of charge. For a
discussion of this point, see Cardew, Pineer Pottery", page 302 in the
appendix. For some time I've been treating Phosphorus as a glass former,
with stunning glazes the result. Can't understand why the glaze software
doesn't recognize this. For a deeper treatment of Ionic Potential, Linus
Pauling figured out these simple arithmetic ratios, & I bet his book "The
Nature of the Chemical Bond" is still the most direct & simple treatment of
Ionic Potential around.
I got a lot of imagery out of the opening chapters of Brian Mason's
book "Principles of Geochemistry", 1st ed.(Wiley & Sons, N.Y., 1952) I
didn't like his revised edition-- better for geol. students, maybe, but not
for me.
W.A.Weyl wrote a book, collected 1930s series of articles from
Ceram. Soc., called "Colored Glasses" (Dawsons of Pall Mall, 1959) which I
found *very* useful for the static "network former/modifier theory"
treatment.
I hope these are a help to you as they have been to me, Hank in Eugene
Michael Banks on sun 4 jun 00
Hank,
I agree that Linus Pauling's table of Ionic Potentials (a measure of
electronegativity calculated in 1960) gives a good insight into glaze oxide
behaviour for non-transition elements. Another good predictor is the table
of Electrode Potential numbers (Eo) -a measure of the energy in volts of
electron transfer). Eo correlates very well with Goldschmidts Rules, a
system predicting the distribution of chemical elements in melts derived
from empirical work by V.M. Goldschmidt -a pioneering geochemist early in
the 20th century.
However, ionic potential, electrode potential and Goldschmidts Rules are not
good predictors of the behaviour of the transition metals, particularly
their solubility's in glaze or igneous melts (which I regard as pretty
similar anyway). Transition metal oxide behaviour is better predicted by
Crystal Field Theory (developed during the late 1960's), and aluminosilicate
mineral behaviour (e.g. feldspar) by the Quasicrystalline Model of igneous
melts (developed in later decades).
Both crystal field theory and the quasicrystalline model post-dated Cardew's
writings, or were otherwise unknown to him. Unfortunately I don't know of
any more modern books which address the pertinent connections between modern
melt chemistry concepts and ceramic glazes. Writers with Renaissance-man
knowledge such as Cardew, don't seem to exist any more.
Yes, the failure to address phosphorus (and the strong network modifier,
fluorine) is a bit of a loophole in available glaze calc software. I
suspect that these elements and fluxes such as Mn, Co, Cu, Ti etc are too
disruptive of the Seger formulae format.
Michael Banks,
Nelson,
NZ
----- Original Message -----
Hank Murrow wrote:
> SNIP...
> Dear Tom and Anne; I have come to believe that IONIC POTENTIAL
> (valency/ionic radius) is a more likely predictor of behavior of oxides in
> a glaze, as it marries relative size AND strength of charge. For a
> discussion of this point, see Cardew, Pineer Pottery", page 302 in the
> appendix. For some time I've been treating Phosphorus as a glass former,
> with stunning glazes the result. Can't understand why the glaze software
> doesn't recognize this. For a deeper treatment of Ionic Potential, Linus
> Pauling figured out these simple arithmetic ratios, & I bet his book "The
> Nature of the Chemical Bond" is still the most direct & simple treatment
of
> Ionic Potential around.
> CHOP
iandol on sun 4 jun 00
I have missed something? Why is qualification of a glaze as acidic or =
basic important? I am at a loss to understand why there is a need to =
define a glaze as being acid or basic except that in use, for example =
storing chemical solutions, this knowledge may prevent corrosion of the =
glaze by the contents of the container.
Does this interpretation, acidic/basic, of glaze quality mean that a =
surface which results from soda or salt glazing is basic because it is =
rich in alkali? Therefore it must not be used to store acidic solutions.
Or does it mean that some colorants may act as indicators, like a litmus =
test? Can the problem be solved stoichiometricly using the Seger =
information? I like Hank=92s idea of Ionic Potential so perhaps =
Coordination Number is also important in obtaining stable glazes.
Ivor Lewis. In South Australia.
Anne POSSOZ on mon 5 jun 00
Thanks to all the persons who gave different view on that subject.
After all that reading, I start to get the feeling that this acidity
question was a view from the last century and that today, while
understanding more, the view is more like that acidity cannot be
computed precisely to give really a clue for a glaze.
At first order, it seems to be a ratio of Silica+B2O3 divided by
fluxes Alumina. There are some factors of 3 (others?) somewhere but I
could not get the right places for them.
Nevertheless, using a Seger formula, it is closely related to
the SiO2/Al2O3 ratio (and SiO2+B2O3/Al2O3) that should be kept
in a reasonable range depending of the firing temperature and
the type of glaze wanted.
The original article where I red about this was from the last
century copper red glaze at Sevres (1888).
It says that better red are obtained for more acid glazes.
That means less alimuna and more silica. Some boric oxide too.
I was just curious how to reproduce a quoted number of 5.3 acidity.
Having the glaze mentionned in oxides (%), I though that I should
be able to convert it in moles and get, at least roughly,
the quoted number. But I failed. I miss some factor of 3
here and there. Is that important?
Is B2O3 more or less acid than SiO2? Probably not equally. But by how much?
Funny. In the article:
http://art.sdsu.edu/ceramicsweb/articles/tech_handouts/titania_in_glaze.html
there are contradictory sentences:
- glaze/glass composition having a notable
amount of B2O3 and a relatively low alkali content (or high acidity)
- Excess B2O3, again, reduces the acidity of the melt (...)
Finally, I'll go back with the view that acidity is related to the
electronegativity. This is certainly not an easy matter as there
are so many tentatives to define electronegativity:
http://www.webelements.com/webelements/elements/text/eneg/Cu.html
Tom and Michael have interesting comments. Although we should not forget
that all these oxides (what a soup) have a behavior very difficult
to predict at these high temperatures.
The wonderfull webelement has also all kind of interesting information
like "Standard Reduction Potentials". Untill now, I was unable
to figure out what I could do with these numbers like:
http://www.webelements.com/webelements/elements/text/redn/Cu.html
but I guess that those numbers could also be used.
At some stage, I feel sorry to talk so much about physic/chemestry
when this is clayart and not claytech. But I find it astonishing that
untill now there is still no agreement of the reason of copper
red glazes: copper metal or cuprous oxide (Cu2O)?
Color is so fascinating... And how much I admire chinese who made
it first without all what we know today.
Anne
P.S. Thanks also to all who discussed privately on that matter.
--
Anne Possoz Service Informatique Central Tel : (41/21) 693.22.49
Ecole Polytechnique Federale de Lausanne, 1015 Lausanne (Switzerland)
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