This is to address several items of concern raised by what I though was a quick and simple post about the absence of molecules and the nature of lithium in glassy materials. What follows is not for technophobes or those who want recipe advice. Rather, we're going to dig into the guts of this stuff a little harder than is usually seen here on Clayart, but all efforts will be made to keep the level of the content within the bounds of the local reality here. Mainly to encourag interest in these matters, which do matter, among the broader public (and it's become large) reading this missive.
Paul wrote:
"Last year at NCECA, Tom Buck went into great detail explaining the difference between those units and "molecular equivalents", saying those ceramic formula units are not molecular equivalents. So are they "moles"? I always assumed that was just a contraction of "molecule".So what IS that unit if it's not a molecule and not a molecular equivalent? I'd like the real chemists here to explain it to me."
A mole is the weight, in grams, of 6.23 x 10^23 atoms or molecules of any substance -- elements or compounds as the case might be.
6.23 x 10^23 atoms of oxygen will weigh 16 grams.
6.23 x 10^23 molecules of CaO will weigh 56 grams.
6.23 x 10^23 molecules of feldspar will weigh (something like) 556 grams
And so on.
A molecule is an chemically bonded aggregation of atoms which, naturally, has weight determined by the contribution of the elements involved. 6.23 x 10^23 units of that aggregation determines the "molecular weight" of the substance.
The Seger "formula" is a convention that compares the sum of the molecular weights of the alkalies and alkaline earths (R2O and RO) with those of the other "oxides" in the glaze. The reasons why anyone would want to do this is another discussion and one probably already in Clayart's archives. If not let me know and we'll put together something to use. Anyway, the Seger (or "empirical") representation dissects the substances present in the glaze first of all into component "oxides" and then places these into three categories: alkalies, amphoterics and acids. What it shows is the ratio between the sum of the alkalies (R2O and RO), which is always equal to 1, and the other "oxides" in the glaze. It is the conversion from representation to reality that causes confusion until one gets used to it.
Personally, I'm geared to using mole% of the "oxides" in evaluating the glass/glaze instead of the Seger formula. It's really a matter of what you're used to and the personal shortcuts one develops over time. When I did glazes all of the time I did use the Seger formula and was comfortable with it - convoluted though it be.
Someone who studies crystallography wrote:
"Glass does consist of molecules. It is however not crystalline. It is the crystal state that consists of ordered arrangements of molecules, not atoms,and is called the crystal lattice. Glass is a super cooled liquid and is usually a mixture. That does not stop it from having molecules."
No, dammit, there are NO molecules in glass. If there were it wouldn't be glass.
I'm very adamant on this point, as the failure to understand this denies very fundamental qualities of state of matter that we know as glass. Yes, I said state of matter – solid, liquid, gas, plasma AND glass. Glass is neither solid nor liquid, but a unique arrangement of elements unto itself. It is not useful to confound it with anything else – especially molecular substances.
Let's take the dictionary sort of definition of molecule as follows: a molecule is "the smallest particle of an element or compound capable of retaining the chemical identity of the substance…" In the crystalline solid state there is a rigid arrangement of the atoms involved and this defines its chemical identity. In the glassy state this rigid order does not exist or is at least highly ambiguous. Now, with this as a basis, compare the properties of crystalline silica (SiO2/flint/sand/quartz) with those of vitreous silica/fused quartz/silica glass. In either state we have only atoms of Silicon (Si) and Oxygen (O) – in the former there is complete order. In the latter, order is poorly defined.
You'd be real hard put to find the 573 ºC alpha-beta "inversion" in glassy silica. Silica glass has a thermal expansion that is the same in any direction, and one which is very much lower than that of any crystalline forms of silica. Quartz has thermal expansion that differs depending on which axis of the crystal it is measured. Similarly, comparisons of other properties, such as index of refraction, specific heat, specific volume or elastic properties will show that there are substantial differences between the crystalline silica and glassy silica.
Molecules, as I stated before, are defined by very rigid atomic arrangements that do not exist in glass. This is why X-ray studies (likely familiar to our friend the crystallographer) do not show the neat pinpoints (or peaks) locating the atoms involved in glasses, as they would in a crystal, but instead show hazy blotches.
Further, one cannot compare the differences between crystalline and glassy silica with those one would see between ice (crystalline) and water (a liquid, not glass). Silica glass has no "freezing point", as characterized by the absence of any heat of fusion, and its properties of specific heat, etc., are dependent on how it is cooled, whereas the properties of water in any form are indifferent to such considerations. Liquid water and glass are different things. Water has molecules. Glass does not.
See Chemistry of Glass, Vogel, ACerS, 1985. Nowhere within it will one find any reference to "glass molecules". Alternatively, see Fundamentals of Inorganic Glasses, Varshneya, Academic Press, 1994 for a similar dearth of references to "glass molecules"—but much splendid (and practical) discussion of the glassy state.
Tom Buck writes privately:
"…are you saying that Lithium exists as ions in the liquidus? not asLi2O molecules? please elaborate on how and when one should use the term "ion" in glaze technology."
I'm not clear as to how the term liquidus is being used here. In the realm of physical chemistry liquidus is taken to mean a line on a phase diagram showing the temperature above which a mixture of substances becomes liquid (at a specified pressure – usually atmospheric). For example, taking mixtures of Na2O and SiO2, the liquidus at 50/50 (mol%) is 1089 ºC and at ~62/38 is 846 ºC forming a eutectic or low point. Above these temperatures the mixture is liquid and below it the mixture is crystalline. Plotting these temperatures against the composition of the mixture describes the liquidus for that mixture.
Glasses (including the glassy material in glazes or traditional ceramic bodies) are almost wholly ionic in character. That is, the atoms involved are engaged as little magnets, and (for the most part) do not share electrons (no covalency). As such it is always proper to refer to the elements in a glaze as ions.
It might be better to articulate the sense of the term "oxide" as used in glaze technology a little more clearly. Only in certain circumstances do we find oxides as such in ceramic glazes – for example in really "dry" matte glazes rich in CaO or BaO. Usually the combinations in glaze are more complex (CaSiO3 or BaSiO3). In glass one cannot find such stuff per se. We resort to describing the glaze (or glass) in terms of the "oxides" present as a convenience and from this infer the possible outcome of fusing the mixture (or of the fused mixture).
I don't know if that answered the question(s), but hope so and I'm sure I'll hear about it otherwise. I must say that this has been fun after being on the bench for a while now.
KPP -- going to lunch
