Karl P. Platt on sat 27 oct 01
Ok, I want to dispense with this and go into ZnO and reduction instead.
It's actually a far more interesting topic.
Nevin Murtha wrote:
>I return your challenge to cite a single source that states their are
no molecules in glass.
I'm happy to learn that you've studied crystalline materials in the
context of geology. This is certainly a valuable pursuit. Please refer
to the first post on this topic wherein said references are made.
Additionally see Angell & Wong's work on glass structure - and then
follow its references - for detailed treatment of this topic.
For a comprehensive discussion on bonding in glass see H. Rawson
Proceedings of the IV International Congress on Glass, pp 62-66. <--
esoteric.
Although rather than cite esoteric and/or ponderous professional
reference books, anyone interested in a well rounded and highly
accessible discussion of the glassy state is referred to Elements of
Materials Science and Engineering, 4th Ed., Van Vlack, Addison-Wesley,
1980. This is a widely used introductory level textbook that should be
easy for anyone to get a hold of.
Banks wrote:
"Point two is factually correct, silicon-oxygen bonds (which predominate
in > most silicates, glasses etc) are modelled to be at least 50%
covalent (using> Pauling's electronegativity concept) and much stronger
than an ionic bond (c/f. Hurlbut C.S. & Klein C, Manual of Mineralogy,
19th Ed, 1977, John> Wiley & Sons).... ...so that a crystal can be
"regarded" (in an informal sense) as a very large molecule,in much the
same way that polymerised substances and macromolecules are inorganic
chemistry."
This is dissembling obfuscation.
First I must wonder why you guys keep referring to mineralogy textbooks
in a discussion about the vitreous state. Murtha cites Dana and now it's
Hurlbut/Klien. Both of which, by the way, I have and refer to in
analyzing "stones" (undesirable crystalline inclusions) in glass melts.
And neither of which have a lot to say about the glassy state. After
all, they're books about crystals.
One can have either informal "regard" or REGARD for the subject matter
at hand, and in the present case it's not crystals, nor crystalline
minerals, but glassy material. I am not interested in informality, but
specificity. Specificity in the interest of broader understanding as to
the dynamics of the interactions which can be controlled to give the
most repeatable, most useful or most beautiful results.
Let's first of all agree that we're talking about glass, not crystals,
and there's no evidence anywhere in the glass/ceramic literature to
affirm the view that covalency exists as a significant factor in
silicate glasses ( as they'd be encountered in potting). Covalency
pertains to Si when it is in 6-fold co-ordination and this is not a
common thing to observe in silicate glasses. This is not to say that
there aren't molecular (covalently bonded) glasses or that covalency
hasn't been observed in silicate glasses. On the contrary, there are
many molecular glasses - o-terphenyl or sorbitol being common examples -
and covalency has been shown to appear, in slight ways, in silicate
glasses. There are also many fused salts (covalent things) which will
more or less easily cool from molten into a glass. Yet for the purposes
of understanding with some precision the behavior of ceramic glazes,
clay bodies or glasses, there is nothing to be gained by confounding the
well established ionic-in-nature behavior of these materials with
anything else.
John Hesselberth makes a very cogent point that should not be ignored.
I'll keep it in mind.
Now, on to ZnO.....
Zinc oxide is a little bit of an oddball compared with most anything
else one might put into glaze, and its presence does assert strong
influences on glazes which are reduced. Someone posted their observation
that a small pile of ZnO taken along for the ride in a firing completely
vanished. What this is owing to is the ease with which ZnO is reduced to
the metal and how volatile the metal is.
ZnO is used in glazes for a number of reasons and benefits. Where ZnO
replaces CaO we find that the glaze will melt more readily. Indeed, the
whole class of "Bristol" glazes, raw glazes maturing between cones 6-9,
are predicated on its use. It also gives the glaze improved chemical
durability, and can act to improve "fit" by making the glaze film more
elastic.
ZnO also has the effect of strongly modifying a number of colors. For
example, it will tend to turn chrome brown or cobalt greenish. ZnO will
also poison Cr-Sn pinks. Last, but certainly not least, ZnO is a key
addition to "crystalline" glazes which develop large feathery crystals
of zinc silicate in the form of willemite.
My experience with ZnO as a raw material is that it's sort of funky,
which is to say that its properties, in particular particle size
distribution, historically lacks uniformity. Either lot-to-lot or
between brands. This poses a couple difficulties in glaze making. These
occur in terms of balancing the slip or the degree to which it fuses
into the molten glaze or in the amount of opacity it might give. Some
brands/lots of ZnO will tend to stay in suspension in the slip better
than others. Although ZnO is pretty light and tends to stay suspended
fairly well in general, although if there's much coarser material it can
(and will) settle.
I had a real bad experience with its influence on opacity early on. We
were manufacturing glazed tile, and I was the glaze guy. There was a lot
change in the middle of a run which caused a dramatic increase in
opacity. This obscured the "specks" put into the glaze - you've seen
these in a lot of commercial installations. The specks serve a couple
purposes, not the least of which is to obscure pinholes in the interest
of lowered scrap rated. Well this new lot of ZnO opacified so hard the
specks vanished. This ruined a lot of product for which I was scolded.
The difficulties given by such variability can be especially
complicating in the studio context where thorough mixing of glazes (into
slip) is actually pretty rare.
ZnO, when added to a glaze, has the tendency to cause droplets of Zn
rich glassy material to form within it - sort of like oil in water. The
extent to which this occurs - in respect to both the number and size of
the droplets - depends on the composition of the glaze, and how the
glaze was cooled. This glassy material differs in its index of
refraction from the surrounding glaze and this gives rise to opacity.
Similarly, in cases where there's enough Zn in the glaze, and conditions
are right, crystalline zinc silicate can evolve and lend opacity for the
same reasons Zn rich glassy material will. The tendency of Zn to
separate itself from the surrounding material in its extreme form gives
crystalline glazes.
As to reduction, we can observe that if we had placed samples of CaO
and/or MgO into the kiln with the ZnO sample they would have survived
the reducing firing unfazed. This suggests that the bonds holding Zn to
O are weaker than those between Ca or Mg and O. This is also related to
the easier (lower temperature) fusing one sees when replacing CaO or MgO
with ZnO in a glaze. MgO, it is noted, is a very useful thing for making
bright glazes at higher temperatures -- but that's another story.
It should be noted that any "oxide" in a glaze will tend toward its
least oxidized state with increasing temperature. In the case of ZnO,
the temperatures at which it comes undone in a reducing environment are
relatively low. What happens is that when ZnO comes apart we are left
with Oxygen, which will either go away or stay in the glaze, depending
on whether conditions are right for its acceptance elsewhere, and
elemental Zn which vaporizes quite readily. A reducing environment
(actually one rich in electrons) only serves to hasten this process.
How this comes to bear on the final amount of Zn left in the finished
glaze depends greatly on whether reduction started before the ZnO melted
in with every thing else, and how hard it was. When Zn is combined in
the glaze, either as glassy or crystalline material, it is more
difficult, but not impossible to reduce owing to its environment. To the
extent that any elemental Zn remains in the glaze when the fire is cut
it will scavenge oxygen.
ZnO is not essential to development of either Cu red or Ti blue glazes.
Rather than hashing it over again I'll refer the interested to several
long essays I submitted to Clayart on these topics several years ago.
These can be found by searching w/ Google and/or the archives held @
Ceramics.org.
OK, I have to go back to reality now and hope this was helpful to someone.
KPP - contemplating a Mexican lunch
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