Dave Finkelnburg on sun 19 dec 04
Dear Ivor,
As you know, I've only followed Clayart sporadically lately, including this discussion about sagging glass. Sagging glass -- panes getting thick due to glass flowing over time -- is an "urban legend."
Technically speaking, glass can be considered a supercooled liquid which can exhibit room temperature creep over time, geologic time that is. Humans haven't been on this planet long enough to test whether glass will flow at room temperature.
Zachariasen never claimed to have everything right. He just proposed a model. His model, though, seems to me to be a fairly good one. He missed when he underestimated how many corners of a silicon-oxygen tetrahedron are connected in a silicate glass structure. He did score a bullseye with his claim that boron wants to form a glass. To this day no one has succeeded, to my knowledge, in crystallizing boron from a glass melt. Zarchariasen is frequently misquoted, so it is best to read his original paper. It's not long, or complicated. The reference is: W. H. Zachariasen [J. Chem. Soc. 54 (1932) 3841]
A good discussion of glass structure is found in, "Introduction to Glass Science and Technology," by J.E. Shelby, The Royal Society of Chemistry, Cambridge, England, 1997. A new, updated version is currently in press and should be out in early 2005.
XRD, NMR and other sophisticated techniques now available to glass scientists have shown clearly the model of amorphous silica as having short range order (silicon-oxygen tetrahedra) but no significant long range order is correct. To me that is quite analogous to a supercooled liquid.
For those who wonder why this is important to potters, I am convinced the study of glass will help us better understand glazes, not to mention the interconnecting glassy phase of porcelains.
Good potting,
Dave Finkelnburg
From: Ivor and Olive Lewis
Subject: Re: Can glass move/sag over time?
So far there has been no evidence presented in this discussion. NO
before and after values. NO records of time elapsed. NO date of
manufacture. ALL hearsay.
Get hold of a copy of Charles Bray's Book. Fair enough, he only give a
brief summary of manufacturing methods but it is interesting to read.
We forget that artisans have been grinding glass for a couple of
millennia, that Plate Glass, ground to have parallel faces was used
for mirrors and in furniture. When was Versailles built?
No one has taken up the idea that I gave about "Creep". My old boss,
Dr Jim Cairney had about forty Creep Machines in the basement lab to
test experimental turbine blade steel samples. I do have values for
the Creep of glass but they are at elevated temperatures.
I doubt the assumption that glass is a supercooled fluid. Density
variation between crystalline and fused quartz do not, in my opinion,
support the Zachariasen molecular Model of a generalised Glass
Structure.
Said my piece on this one.
Best regards,
Ivor.
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Ivor and Olive Lewis on tue 21 dec 04
Dear Dave,
Thank you for your summary.
I agree with you, the Zachariasen Model is a useful tool which can be
used to explain some of the properties of glass. ( Will try to get a
copy of that paper). But there are other models. These seem to be
ignored in literature freely available to the potting arts community.
I agree with you, we know little about the nature of the vitreous
phase which we use in several ways.
As I understand the nature of materials, a "Super Cooled Fluid"
retains it fluidity when it is undercooled. . Under the influence of
external factors a super cooled fluid will spontaneously crystallise.
This is a metastable state. However, the degree of undercooling may
have an influence which I have not considered.
As I understand it, a material with short range order but long range
disorder is better referred to as "Amorphous".
In the case of Silicon Dioxide the short range order is vested in the
tetrahedral SiO4^+ units, which in quartz are arranged to fit an
infinite lattice. Pure Fused Silica differs by having its tetrahedral
units oriented in a random manner but, never-the-less, attached to
each other to make a structure which is ridged. What is the "Glass
Transition Temperature" of Silica glass? .
What I find to be inadequately explained is the function of the
material oxides called fluxes, especially a lack of differentiation
between those that have low melting points and those which are
refractory. I cannot believe they float randomly in the silicate soup.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Dave Finkelnburg on tue 21 dec 04
Dear Ivor,
I don't mean to run on about this subject...it is a bit out of the mainstream for the list...so I'll try to make this brief. You are right--things don't float randomly in the soup!
Vitreous silica has a Tg above 1,000 degrees C. That compares with the Tg of most soda-lime aluminosilicates (window glass) of about 550-650C.
Silica tetrahedra in fused silica are not arranged entirely at random. Rather, they are joined at the corners with Si-O-Si bond angles ranging from ~120 to ~160 degrees and averaging ~144-degrees. The size, charge and electron density of cations in the spaces within that network, of course, create the property variations we observe.
Many glasses are formed with difficulty and crystal precipitation is avoided only by quenching at thousands of degrees per second (the glass is poured, molten, into a cold liquid). There are certain composition regions, though, where molten glass has such high viscosity that it crystallizes with difficulty and sufficient working time is available to cool and anneal the glass without thermal shock. Virtually all window glass is formed from a small composition region of the alumina, silica, alkali-alkaline earth (mostly soda, calcia) phase diagram for this reason. Still, a little spit, a little dust, a little sodium silicate will all nucleate crystals in such glass and if it is held at about 1,000C for a couple of hours crystals will grow on the surface of most such glass.
From a glass science perspective as I understand it, much study is devoted to determining just where in the network structure flux and network ions reside, and from that trying to work out how they affect glass properties. In silicates, alkali and alkaline earth ions are found in the empty spaces between silica tetrahedra. The size of such cations affects their mobility and consequently the electrical properties of a glass, as well as its corrosion resistance. Alumina replaces silica at low concentrations as the cation in some network tetrahedra but is not charge-balanced. It takes another cation, say a sodium adjacent to the tetrahedron, or a calcium between two alumina tetrahedra, to maintain charge balance. Such cations are fairly tightly bound by the network. Phosphorous and boron form more complex networks than silica. The glasses we work with as glazes are, in general, so complex as to defy effective structural study, limiting us to infer what is happening by
looking at simpler systems.
I don't think it's safe or productive to try to generalize further. Why transition elements (glaze colorants) do what they do to cause light absorption or transmission is a subject of great interest. What binds, or fails to bind, these elements in a glass network is equally critical. There's quite a bit of information available in the literature answering such questions, but it takes focused study as well as time, energy, and access to the journals to find it.
Good potting!
Dave Finkelnburg
From: Ivor and Olive Lewis
I agree with you, the Zachariasen Model is a useful tool which can be
used to explain some of the properties of glass. ( Will try to get a
copy of that paper). But there are other models. These seem to be
ignored in literature freely available to the potting arts community.
I agree with you, we know little about the nature of the vitreous
phase which we use in several ways.
As I understand the nature of materials, a "Super Cooled Fluid"
retains it fluidity when it is undercooled. . Under the influence of
external factors a super cooled fluid will spontaneously crystallise.
This is a metastable state. However, the degree of undercooling may
have an influence which I have not considered.
As I understand it, a material with short range order but long range
disorder is better referred to as "Amorphous".
In the case of Silicon Dioxide the short range order is vested in the
tetrahedral SiO4^+ units, which in quartz are arranged to fit an
infinite lattice. Pure Fused Silica differs by having its tetrahedral
units oriented in a random manner but, never-the-less, attached to
each other to make a structure which is ridged. What is the "Glass
Transition Temperature" of Silica glass? .
What I find to be inadequately explained is the function of the
material oxides called fluxes, especially a lack of differentiation
between those that have low melting points and those which are
refractory. I cannot believe they float randomly in the silicate soup.
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