Karl P. Platt on fri 10 oct 97
Gotta say it again --
TiO2 is *not* a glassformer under normal circumstances --- not at all.
The Seger representation of glaze composition is *not* broken-up into
non-glass formers, Sort-of glass formers and glass-formers. It represents
oxides as being alkaline, amphoteric (neutral) and acid -- chemically
speaking. Were it otherwise B2O3 would be included in the rightmost column
as its presence in a glaze serves mainly to form glass -- and quite acidic
glass at that. The acidity is revealed by the development of Ti blues.
Actually, if one looks into the history of it there was some polemic about
the location of B2O3 in the formula owing to this. SnO2 would also be
placed in the acids column, but it has nothing to do with forming glass,
either. It can enter (to an extremely limited degree) a silica based
glass, but alone it is an extreme feat to produce a glas of this
substance. SO2 (while essentially never used in glazes) would also be
placed in the acids column
Alkalies-intermediates-Acids
Ron Roy on sun 12 oct 97
Karl is right in his own way - when I answered this question about TiO2 I
was well aware of how TiO2 acts in glazes at our temperatures - lots of
documentation in my library.
In this case the point is - it should not be grouped with the fluxes - nor
with the amphoterics.
B2O3 is another matter - because it acts as a flux and a glass former it is
useful to place it in different positions in the Seger formula. It's
function in glazes is much more ambiguous than TiO2. These two oxides are
so different in their role in glass making that no-one should think of them
as similar.
I have a concern here. I know there are many potters that are just
beginning to get into glaze analysis and I admit to oversimplifying.
Dragging the many abnormalities of glaze chemistry into the conversation at
a basic level must only be confusing to most of them. Surely it is
forgivable to leave out some of the gory details in the hope of promoting
better understanding of the basic questions.
>----------------------------Original message----------------------------
>
>Gotta say it again --
>
>TiO2 is *not* a glassformer under normal circumstances --- not at all.
>
>The Seger representation of glaze composition is *not* broken-up into
>non-glass formers, Sort-of glass formers and glass-formers. It represents
>oxides as being alkaline, amphoteric (neutral) and acid -- chemically
>speaking. Were it otherwise B2O3 would be included in the rightmost column
>as its presence in a glaze serves mainly to form glass -- and quite acidic
>glass at that. The acidity is revealed by the development of Ti blues.
>Actually, if one looks into the history of it there was some polemic about
>the location of B2O3 in the formula owing to this. SnO2 would also be
>placed in the acids column, but it has nothing to do with forming glass,
>either. It can enter (to an extremely limited degree) a silica based
>glass, but alone it is an extreme feat to produce a glas of this
>substance. SO2 (while essentially never used in glazes) would also be
>placed in the acids column
>
>Alkalies-intermediates-Acids
Ron Roy
93 Pegasus Trail
Scarborough,Canada
M1G 3N8
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/people/ronroy.htm
Carol Ratliff.clayart.CLAYART.MAILING LIST on mon 13 oct 97
Ron,
Boy do I have to agree. I read that post and then printed it out so I could
bring it into my ceramics teacher and ask him to explain in english what was
being said. I would be overwhelmed if I was a young student considering
going into ceramic engineering or chemistry. I appreciate the basic
explanations.
carol ratliff,
san diego
In a message dated 97-10-12 09:26:24 EDT, you write:
<< Surely it is
forgivable to leave out some of the gory details in the hope of promoting
better understanding of the basic questions. >>
Gavin Stairs on thu 16 oct 97
At 09:25 AM 12/10/97 EDT, Ron Roy wrote:
....
>In this case the point is - it (TiO2) should not be grouped with the
fluxes - >nor with the amphoterics.
....
May I butt in? It seems to me that the hoary and sainted Seger system
breaks down with materials like opacifiers and matting agents which do not
participate in the glass formation process, and remain more or less inert.
They are essentially what are called fillers in composite materials. The
glass forms around unaltered, unmelted lumps (which may be as fine as dust)
of the filler material. It is this composite nature which gives the useful
glaze characteristic, such as opacity, through light scattering, or
mattness, through refractory lumps of the right scale, or inhomogeneity of
melt temperature.
The divisions among the normal glaze constituents predict glaze behavior
because they all participate in the formation of glass in particular ways.
The glass formers provide the basic glass matrix. The amphoterics modify
the glass melting temperature, viscosity and other properties to make a
glaze which sticks to the pot and has a chance to expand/contract at the
right rate. The fluxes depress the melting point, help to initiate
melting, and so forth. I am really glossing over the complexities here in
order to get to the point, which is that very refractory constituents never
get into the glass at normal firing temperatures. They therefore never act
as glass formers, amphoterics or fluxes. As to the chemistry of the glaze,
they are inert.
These inert materials do influence certain properties, such as thermal
expansion, colour, opacity and texture. They may also affect the
durability of the glaze. Perhaps the classification scheme requires
another column: inerts. These constituents would then be considered when
calculating the physical properties of the glaze, but not when calculating
the chemistry.
I have not done the calculation to see if this proposal works. It would be
interesting to see if the chemistry of a Ti glaze, for example, works to
predict its melting point and balance without the Ti in the calculation,
and then to predict its coefficient of expansion with the Ti included. Ti
may not be the best test case, since I guess some Ti actually gets
dissolved in the glaze. Maybe the Ti has to be split between amphoterics
and inerts.
Gavin
stairs@echo-on.net
http://isis.physics.utoronto.ca/
416 530 0419 (home) 416 978 2735 (work)
Toronto, Ontario, Canada
Karl P. Platt on fri 17 oct 97
A physicist....cool. This post deservs a long reply which I don't have
time to do at the moment, but a few brief points are worth making:
> The glass formers provide the basic glass matrix. The amphoterics modify
> the glass melting temperature, viscosity and other properties to make a
> glaze which sticks to the pot and has a chance to expand/contract at the
> right rate.
>
The alkalies and alkaline earths may also be described as the
amphoterica are above.
> I am really glossing over the complexities here in
> order to get to the point, which is that very refractory constituents never
> get into the glass at normal firing temperatures
>
For some folks cone 13 is normal
> Perhaps the classification scheme requires
> another column: inerts.
>
Nothing in the glaze does nothing or any thing in tht glaze does
something -- I'll argue that this category cannot exist
> It would be
> interesting to see if the chemistry of a Ti glaze, for example, works to
> predict its melting point and balance without the Ti in the calculation,
> and then to predict its coefficient of expansion with the Ti included.
>
This raises (again) the relative merit of predictive calculations.
> Maybe the Ti has to be split between amphoterics
> and inerts.
>
The solubility of TiO2 in silicate melts is real low and it has profound
influences on color.
Again I want to make the point that the Seger representation places the
"oxides" in terms of alkalies, amphoterics and acids. The relative
properties of each element added to the mix are (or have been )
determined in a number of ways ranging from spectral analysis of the
colors produced by the 3d transition elements (the common oxides used to
color glasses/glazes) in different melts to determining the oxygen
activity in various melts of oxides -- complicated and not wholly
precise.
Well, at the moment I need to go fix a kiln -- no small task here. I
went to order elements yesterday, gave the guy the specs and he replied
by asking me how much the elements weighed.... ohmygod........
Abracos,
KPP
David Hewitt on mon 20 oct 97
In message , Gavin Stairs writes
>----------------------------Original message----------------------------
snip
> They therefore never act
>as glass formers, amphoterics or fluxes. As to the chemistry of the glaze,
>they are inert.
>
>These inert materials do influence certain properties, such as thermal
>expansion, colour, opacity and texture. They may also affect the
>durability of the glaze. Perhaps the classification scheme requires
>another column: inerts. These constituents would then be considered when
>calculating the physical properties of the glaze, but not when calculating
>the chemistry.
If you look at a phase diagram of Al2O3 and TiO2 you see that this
produces a eutectic at 80% Al2O3 / 20% TiO2 with a temperature of 1705C.
(MP Al2O3 = 2050C and TiO2 = 1830C). While this bit of information
doesn't get you very far in knowing what the addition of TiO2 will have
in a glaze recipe, I think that it does indicate that it is not right to
classify it as inert.
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP6 1DQ, UK. Tel:- +44 (0) 1633 420647
URL http://digitalfire.com/education/people/hewitt.htm
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