Ron Roy on sat 8 mar 03
Hi Rob,
My best answer is to read our book - Mastering Cone 6 Glazes - the issues
is well explained and much of it is applicable to other temperatures. A
good section on testing glazes and having them tested.
Just because the Barium is fritted does not have any effect on the quality
of the final glaze - that is a function of the balance of oxides, melt and
amounts of both silica and alumina.
Any oxides "over supplied" means they will not be able to stay in solution
as the glaze cools. That is one way to make a glaze unstable. Even if the
glaze contains no toxic material the glaze will probably deteriorate in use
- i.e., - change colour and/or become etched - read less shiny.
For instance - If you have a stable glaze and you add 10% red iron - and
the glaze cannot hold that iron in suspension - the iron will crystallize
out on the surface of the glaze - get into my coffee - and worsen my
problem.
It makes sense to me that liner glazes (means any glaze that comes in
contact with food) should be relatively stable. It does not matter what you
do on the outside so much - as long as you are not concerned with the glaze
changing over time - at least you have fulfilled your obligation implicit
with making functional ware - if you have also made sure the glaze will not
shiver off or crack the ware.
There are some potters who charge so much for their tea pots they may think
they will hardly ever be used - perhaps - but still - there is no
guarentee. Perhaps a lable saying "only for occasional use" would be
appropriate.
Thanks for giving me the opportunity to express myself on this subject yet
again but in a different way. I have copied to ClayArt - hope you don't
mind.
RR
>I read with interest your comments in regard to to Clayarter's request about
>glazes that are "food safe" but, not necessarily good. You then commented on
>a particular condition that makes you suseptable to iron.
>
>Given the above, what is a safe glaze? I fire to c-8 reduction in a propane
>fired updraught. Mostly shino, celadon, also some red. I recently read a CM
>article in re: Tom Coleman and glazes he uses. Some are quite toxic by
>todays standards, yet one that interested me very much was one called,
>"Patina Green." The Barium used was way off the chart, yet it was on a t-pot
>(exterior I presume). If this glaze was reworked ulilizing Ferro Frit 3289
>(barium fritted) would the glaze then be a stable, usable glaze by your
>professional standards? If not (and I am not a community conscience here)
>why even display such a glaze on a functional item?
>
>Like every potter, part time or full time I want to put the best product I
>can make out there.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ivor and Olive Lewis on wed 21 feb 07
Dear Friends,
I have been having a look at some of the ideas we use to explain the =
nature of our glazes. There is an illustration on page 11 of "Ceramic =
Science for the Potter" by W.G. Lawrence and R. R. West. The same =
illustration is in "Introduction to Ceramics" by Kingery et al, p 98.
Given that this information is derived from interpretation of observed =
X-Ray data there are some questions that are not answered in either =
book.
The diagram shows metallic ions of Sodium buried in open cavities in =
broken distorted strings or arrays of interconnected tetrahedral =
silicate ions that form after a silica crystal lattice has been brought =
to the molten state.
The question that needs answering if we wish to consider what makes a =
glaze stable or causes it to be unstable concerns the forces that hold =
these occluded metallic ions in place. Which chemical forces are they? =
What is their magnitude? Are they uniform in strength? Or are they =
dissimilar for differing elements ?
If we cannot address this and explain what happens, then it could be =
that any, or all of the fluxing elements would be susceptible to attack =
and leaching by weak acids. In the end, perhaps some glazes are just =
more durable than others.=20
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Ron Roy on thu 22 feb 07
Hi Ivor,
Seems to me that if more SiO2 were added you would have a more durable
glaze (a more complete structure in this case) - which fits in with our
findings that - to make durable glazes you need a certain amount of SiO2
among other things.
RR
>Dear Friends,
>I have been having a look at some of the ideas we use to explain the
>nature of our glazes. There is an illustration on page 11 of "Ceramic
>Science for the Potter" by W.G. Lawrence and R. R. West. The same
>illustration is in "Introduction to Ceramics" by Kingery et al, p 98.
>Given that this information is derived from interpretation of observed
>X-Ray data there are some questions that are not answered in either book.
>The diagram shows metallic ions of Sodium buried in open cavities in
>broken distorted strings or arrays of interconnected tetrahedral silicate
>ions that form after a silica crystal lattice has been brought to the
>molten state.
>The question that needs answering if we wish to consider what makes a
>glaze stable or causes it to be unstable concerns the forces that hold
>these occluded metallic ions in place. Which chemical forces are they?
>What is their magnitude? Are they uniform in strength? Or are they
>dissimilar for differing elements ?
>If we cannot address this and explain what happens, then it could be that
>any, or all of the fluxing elements would be susceptible to attack and
>leaching by weak acids. In the end, perhaps some glazes are just more
>durable than others.
>
>Best regards,
>Ivor Lewis.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Dave Finkelnburg on sat 24 feb 07
Ivor,
Hope you've had some relief from the heat!
I certainly don't know the answer to your question.
Perhaps it would be useful if I focus on what I know,
rather than what I do not know. In this case, I know
certain ions are held much more tightly by a silicate
glass than other ions.
For example, alkaline earth elements (Mg, Ca, Ba,
Sr) are held much more tightly than the alkali
elements (Li, Na, K). What do these elements have in
common? We know the alkaline earths are all divalent
(have +2 charge) while the alkalis are only monovalent
(1 + charge). This evidence would point to the charge
of the cation as the binding agent for these elements.
X-ray diffraction showed many decades ago that the
silicon atom in a silicate glass is always surrounded
by 4 oxygen atoms. The Si-O bonding is quite strong,
hence the strength properties of glass. The nature of
the Si-O bond is apparently much different than that
of the Cu-O bond or whatever it is in the glass that
is holding the Cu in the glass network, however weakly
it is holding it. I don't mean to imply, by the way,
that Cu substitutes in the same place in the glass
network as Si, because it certainly can't. Copper and
other transition metals seem to be outside the
SiO4(2-) tetrahedra.
Something else we do know, is the aluminum atom can
substitute directly for Si in the glass network, but
that leaves a charge imbalance. Some glass
researchers argue that charge balance holds a limited
number of cations in the alumino-silicate glass
network.
I find it fascinating that copper is rather weakly
bound in the glass, but other transition
metals--cobalt, chrome, iron for example--do not leach
nearly so readily in the presence of an acid. What do
you think is the bonding force at work in the case of
copper?
Good potting,
Dave Finkelnburg
From: Ivor and Olive Lewis
I have no quarrels with your appraisal
<have a more durable glaze (a more complete structure
in this case) - which fits in with our findings that -
to make durable glazes you need a certain amount of
SiO2 among other things.>>
But we are still left with the question, "What holds
the alkali element ions in place ?" The choice has to
come from the limited range allowed by Chemistry:
Electrovalent, Covalent, Van der Walls, Hydrogen bond
or Metallic bond.
Without this knowledge I can see no reason why the
metallic ions with a valency of 2 are not removed from
a glaze as freely as copper ions with a valency of 2.
They are, as we know, very responsive to acids in
other situation. Why should Magnesium, Calcium,
Strontium and Barium be better secured in a silicate
glass than Copper ?
____________________________________________________________________________________
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Ivor and Olive Lewis on sat 24 feb 07
Dear Ron Roy,
I have no quarrels with your appraisal
<glaze (a more complete structure in this case) - which fits in with our =
findings that - to make durable glazes you need a certain amount of SiO2 =
among other things.>>
But we are still left with the question, "What holds the alkali element =
ions in place ?" The choice has to come from the limited range allowed =
by Chemistry: Electrovalent, Covalent, Van der Walls, Hydrogen bond or =
Metallic bond.=20
Without this knowledge I can see no reason why the metallic ions with a =
valency of 2 are not removed from a glaze as freely as copper ions with =
a valency of 2. They are, as we know, very responsive to acids in other =
situation. Why should Magnesium, Calcium, Strontium and Barium be better =
secured in a silicate glass than Copper ?
Thanks for coming in on this.
Enjoy your weekend,
Best regards,
Ivor
Chris Groat on sun 25 feb 07
I don't know anything for sure regarding the stability of Copper in a
glaze; however, one thing did occur to me as I was reading through these
posts. One person mentioned that group I elements were weakly bound in a
glaze. Group I elements have an oxidation state of 1. It was also
mentoined that Group II elements, and several transition metals are fairly
stable in most glazes. Most transition metals and all Group II elements
have oxidation states of 2. Copper is the only transition metal that we
use regularly as a coloring oxide that can have an oxidation state of 1.
As I mentioned earlier, the Group I metals have an oxidation state of 1
and they are weakly bound. Perhaps an oxidation state of +1 is what causes
the instability. The lower charge could result in weaker electrostatic
forces between atoms and molecules. Perhaps the 'lucky' copper atoms that
end up with an oxidation state of +2 remain in the glaze, and those that
end up with +1 are easily leached. Just a thought...
Chris
Dan Semler on sun 25 feb 07
Hi Dave, Ivor,
I haven't studied this at all really but a thought occurred while
reading this, so I thought I'd toss it out there and see if its of use
or just dross.
Is the problem that Cu is weakly bound in the silicate or is it
that its especially attracted to the acid leaching solution, while the
others are not ?
thanx
D
claystevslat on mon 26 feb 07
Dan -- One of the curiosities of copper is that
it hardly seems to bind to silica at all. IIRC, there has
been some work done for the semiconductor industry on a
copper silicate that's created using an aerogel film of
SiO2, but it's something specific to thin-film reactions,
and probably not relevant to pottery glazes. I cannot
remember who did the work.
You can get lost for months following ideas in chemistry.
It doesn't always get you better glazes.
-- Steve S
--- In clayart@yahoogroups.com, Dan Semler wrote:
>
> Hi Dave, Ivor,
>
> I haven't studied this at all really but a thought occurred
while
> reading this, so I thought I'd toss it out there and see if its of
use
> or just dross.
>
> Is the problem that Cu is weakly bound in the silicate or is it
> that its especially attracted to the acid leaching solution, while
the
> others are not ?
Ivor and Olive Lewis on tue 27 feb 07
Dear Dan Semler
You suggest <is it that its especially attracted to the acid leaching solution, while =
the others are not ?>>
I thought of that as a possibility, but the thing against it would be =
that Copper Ions would have to bond with oxygen atoms. For this to =
happen you have to increase the oxygen content of the glaze and as I =
said to Dave, this will cause the melting point to fall. or it would =
reduce the viscosity of the melt
Copper Ions would have difficulty bonding directly onto Silicon atoms =
since each species would repel the other. Both are positive as ions.
Perhaps the problem might be solved by reducing the M2O fluxing =
ingredients in favour of increasing the MO fluxes, since this would push =
the oxygen ratio up and give us more NBO's.
But the question is still there. What is the nature of the Chemical =
Bond?
Thanks for putting forward some ideas.
Best regards,
Ivor
Ivor and Olive Lewis on tue 27 feb 07
Dear Steve S,
Greenwood and Earnshaw report the existence of Copper Silicide, Cu5Si, =
stating that it has a metallic structure and is electrically conductive.
But this throws no light on why Copper (2) Oxide is so reluctant to =
enter into combination with Silicon dioxide.
Best regards,
Ivor
Ivor and Olive Lewis on tue 27 feb 07
Dear Chris Groat,
I'm not sure if I am thinking straight about this one. But if Copper is =
reduced to an oxidation state of plus 1 then it is in the cuprous =
condition and the colour of the glaze would be red but if it were =
reduced to an OS of plus 2 then you get the green blue colours of the =
cupric condition. Examples shown in MC6G are of the green variety. If =
two elections are being donated then it might be possible to think that =
it is being held more firmly. But if it can be leached, that seems to =
show that it is free to be removed. So the question hangs, what is the =
binding force that operates?
Thanks for making a contribution,
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Chris Groat on tue 27 feb 07
Ivor, thanks for clearing that up. I do know for a fact that Copper (I) is
generally less stable than Copper (II). That's why I made the suggestion.
Maybe it his nothing to do with it, but I was thinking that if the
oxidation were incomplete, or if the glaze contained something that
prevented complete oxidation, some of the molecules would be Copper (I),
and therefore less stable. So perhaps both forms are bound in coordination
complexes, but if there is even a small fraction of the Copper (I) form in
the glaze, perhaps it would be enough to cause problems with leaching.
Chris
Ivor and Olive Lewis on thu 1 mar 07
Dear Chris Groat,=20
Agreed. But it seems that the oxidation state must remain at +2 though =
the coordination number can be 2,3,4 or 6. Eight electrons need to be =
donated to get a stable complex and preserve the electrovalency of +2 so =
it is probable four ligands are needed to maintain that state. Likely =
donors are non bridging oxygen atoms in a disrupted (well fluxed) =
silicon dioxide lattice. If the electrovalency were reduced to +1 would =
the colour change from green to red?
Best regards,
Ivor
Ivor and Olive Lewis on thu 1 mar 07
Dear Paul Haigh,
Yes, I can accept that bigger molecules and ions have greater difficulty =
in crawling out of the interior of a glaze when beckoned by water or a =
weak organic acid and that the cupric Ion is small. But that does not =
address the question "What keeps them in place anyway"
And in the case of Copper it does matter a great deal. If it is the =
Cupric state we are talking about, and it is because we are looking for =
a green colouration, then, if it just sits there it will be colourless. =
So my money is on a coordinate covalent bond with a coordination number =
of 4.
Thanks for joining the discussion.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Paul Haigh (pablo) on fri 2 mar 07
Ivor-
Now let me start out by saying that my degrees are not in inorganic chem, but in analytical chem. I was turned off when I was handed a copy of Wolfsburg and subjected to Prof. Wood's lullabye of a class at 8 am. I could go on about how I will be handed another copy of that book when I am greeted in Hell...
Anyway, a coordinate covalent bond is a somewhat artificial designation as a kind of bond- the term only means that one species in the bond contributes both electrons, and it exists out of tradition rather than significance (as do many chemistry terms... something that plagued me right through grad school). If the copper is just sitting there, it most definitely will have a color. This is not a "color center" arrangement where some other force needs to knock it into another oxidation state- it's already an ion. The color is determined by interaction of photons with those outer electrons, and their behavior is effected by the oxidation state, the ions that copper is bound to, and and any hypsochromic or bathochromic effect of the matrix.
"But that does not address the question "What keeps them in place anyway" "
I respectfully disagree. If a copper bearing molecule is bound in a glass matrix, then it can be kept there by an analogous force to what keeps a prisoner in a cage. It just cannot get out. This gets toward the idea of glaze/glass stability. Too much other weird junk, and not enough of what forms the actual glass- you will have a unstable matrix that will degrade for similar reasons that any material with compromised purity may degrade.
I specifically addressed Na and K in my post simply because they are soluble (as are all alkalis)- and their existence in a solid glaze would be PROOF that the glass matrix was providing stability and keeping the included components from dissolving. I'm going from memory, but I think that scan that was discussed was an alkali at high temp- so I dunno what happened upon cooling to that particular ion.
Fun stuff
Paul Haigh
Londonderry, NH
Ivor and Olive Lewis on wed 7 mar 07
Dear Chris Groat,
Scientific ideas have always been a state of flux. New ways of thinking =
and working reveal the increasing complexity of Nature. The idea of =
retention and displacement of materials from complex compounds is not =
new. Many mineral species function in this way and the concept has been =
exploited in the design of Ion Exchange Resins
It may well be that a portion of the Copper used in some glaze recipes =
is restrained by physical means. This proposition then raises a new set =
of questions. But the analogy given by Paul Haigh needs detail that =
would link it to the example of copper being leached from a glaze. Have =
you, in your reading, learned of any mechanism that might entrap copper =
in an alkali aluminium silicate glass ?
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Chris Groat on wed 7 mar 07
Have you, in your reading, learned of any mechanism that might entrap
copper in an alkali aluminium silicate glass?
Ivor, No. But I would certainly like to learn more as it would help you on
your quest to figure out why copper is so easily leached from glazes. I do
know that Copper II oxide can be easily dissolved in organic acids, but
that in no way proves that there is free Copper II oxide floating around
in a glaze.
Chris
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