BBarus on fri 19 oct 01
>Thank you to everyone for this interesting post - it is exactly the kind=20
>of information that non-techie brains love to get : understandable and not=
=20
>boring. I have really enjoyed this part of the discussion -
Now onto glass: has anyone ever fixed gold foil (heavy leaf) to porcelain,=
=20
fired to 018 and then melted clear glass - such as Bullseye fusible glass=20
on top of the gold? Will I get a mess, an interesting something or a=20
beautiful something? For what special problems and glitches should I=20
watch? Basically I'm trying to get some very durable gold=20
"smalti". Thanx for any advice
Barb
>This is to address several items of concern raised by what I though was a=
=20
>quick and simple post about the absence of molecules and the nature of=20
>lithium in glassy materials. What follows is not for technophobes or those=
=20
>who want recipe advice. Rather, we're going to dig into the guts of this=20
>stuff a little harder than is usually seen here on Clayart, but all=20
>efforts will be made to keep the level of the content within the bounds of=
=20
>the local reality here. Mainly to encourag interest in these matters,=20
>which do matter, among the broader public (and it's become large) reading=
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>this missive.
>
>Paul wrote:
>
>"Last year at NCECA, Tom Buck went into great detail explaining the=20
>difference between those units and "molecular equivalents", saying those=20
>ceramic formula units are not molecular equivalents. So are they "moles"?=
=20
>I always assumed that was just a contraction of "molecule".So what IS that=
=20
>unit if it's not a molecule and not a molecular equivalent? I'd like the=20
>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=
=20
>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,=
=20
>has weight determined by the contribution of the elements involved. 6.23 x=
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>10^23 units of that aggregation determines the "molecular weight" of the=20
>substance.
>
>The Seger "formula" is a convention that compares the sum of the molecular=
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>weights of the alkalies and alkaline earths (R2O and RO) with those of the=
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>other "oxides" in the glaze. The reasons why anyone would want to do this=
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>is another discussion and one probably already in Clayart's archives. If=20
>not let me know and we'll put together something to use. Anyway, the Seger=
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>(or "empirical") representation dissects the substances present in the=20
>glaze first of all into component "oxides" and then places these into=20
>three categories: alkalies, amphoterics and acids. What it shows is the=20
>ratio between the sum of the alkalies (R2O and RO), which is always equal=
=20
>to 1, and the other "oxides" in the glaze. It is the conversion from=20
>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=20
>glass/glaze instead of the Seger formula. It's really a matter of what=20
>you're used to and the personal shortcuts one develops over time. When I=20
>did glazes all of the time I did use the Seger formula and was comfortable=
=20
>with it - convoluted though it be.
>
>Someone who studies crystallography wrote:
>
>"Glass does consist of molecules. It is however not crystalline. It is the=
=20
>crystal state that consists of ordered arrangements of molecules, not=20
>atoms,and is called the crystal lattice. Glass is a super cooled liquid=20
>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=
=20
>glass.
>
>I'm very adamant on this point, as the failure to understand this denies=20
>very fundamental qualities of state of matter that we know as glass. Yes,=
=20
>I said state of matter solid, liquid, gas, plasma AND glass. Glass is=20
>neither solid nor liquid, but a unique arrangement of elements unto=20
>itself. It is not useful to confound it with anything else especially=20
>molecular substances.
>
>Let's take the dictionary sort of definition of molecule as follows: a=20
>molecule is "the smallest particle of an element or compound capable of=20
>retaining the chemical identity of the substance " In the crystalline=20
>solid state there is a rigid arrangement of the atoms involved and this=20
>defines its chemical identity. In the glassy state this rigid order does=20
>not exist or is at least highly ambiguous. Now, with this as a basis,=20
>compare the properties of crystalline silica (SiO2/flint/sand/quartz) with=
=20
>those of vitreous silica/fused quartz/silica glass. In either state we=20
>have only atoms of Silicon (Si) and Oxygen (O) in the former there is=20
>complete order. In the latter, order is poorly defined.
>
>You'd be real hard put to find the 573 =BAC alpha-beta "inversion" in=
glassy=20
>silica. Silica glass has a thermal expansion that is the same in any=20
>direction, and one which is very much lower than that of any crystalline=20
>forms of silica. Quartz has thermal expansion that differs depending on=20
>which axis of the crystal it is measured. Similarly, comparisons of other=
=20
>properties, such as index of refraction, specific heat, specific volume or=
=20
>elastic properties will show that there are substantial differences=20
>between the crystalline silica and glassy silica.
>
>Molecules, as I stated before, are defined by very rigid atomic=20
>arrangements that do not exist in glass. This is why X-ray studies (likely=
=20
>familiar to our friend the crystallographer) do not show the neat=20
>pinpoints (or peaks) locating the atoms involved in glasses, as they would=
=20
>in a crystal, but instead show hazy blotches.
>
>Further, one cannot compare the differences between crystalline and glassy=
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>silica with those one would see between ice (crystalline) and water (a=20
>liquid, not glass). Silica glass has no "freezing point", as characterized=
=20
>by the absence of any heat of fusion, and its properties of specific heat,=
=20
>etc., are dependent on how it is cooled, whereas the properties of water=20
>in any form are indifferent to such considerations. Liquid water and glass=
=20
>are different things. Water has molecules. Glass does not.
>
>See Chemistry of Glass, Vogel, ACerS, 1985. Nowhere within it will one=20
>find any reference to "glass molecules". Alternatively, see Fundamentals=20
>of Inorganic Glasses, Varshneya, Academic Press, 1994 for a similar dearth=
=20
>of references to "glass molecules" but much splendid (and practical)=20
>discussion of the glassy state.
>
>Tom Buck writes privately:
>
>" are you saying that Lithium exists as ions in the liquidus? not asLi2O=20
>molecules? please elaborate on how and when one should use the term "ion"=
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>in glaze technology."
>
>I'm not clear as to how the term liquidus is being used here. In the realm=
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>of physical chemistry liquidus is taken to mean a line on a phase diagram=
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>showing the temperature above which a mixture of substances becomes liquid=
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>(at a specified pressure usually atmospheric). For example, taking=20
>mixtures of Na2O and SiO2, the liquidus at 50/50 (mol%) is 1089 =BAC and at=
=20
>~62/38 is 846 =BAC forming a eutectic or low point. Above these=
temperatures=20
>the mixture is liquid and below it the mixture is crystalline. Plotting=20
>these temperatures against the composition of the mixture describes the=20
>liquidus for that mixture.
>
>Glasses (including the glassy material in glazes or traditional ceramic=20
>bodies) are almost wholly ionic in character. That is, the atoms involved=
=20
>are engaged as little magnets, and (for the most part) do not share=20
>electrons (no covalency). As such it is always proper to refer to the=20
>elements in a glaze as ions.
>
>It might be better to articulate the sense of the term "oxide" as used in=
=20
>glaze technology a little more clearly. Only in certain circumstances do=20
>we find oxides as such in ceramic glazes for example in really "dry" matte=
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>glazes rich in CaO or BaO. Usually the combinations in glaze are more=20
>complex (CaSiO3 or BaSiO3). In glass one cannot find such stuff per se. We=
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>resort to describing the glaze (or glass) in terms of the "oxides" present=
=20
>as a convenience and from this infer the possible outcome of fusing the=20
>mixture (or of the fused mixture).
>
>I don't know if that answered the question(s), but hope so and I'm sure=20
>I'll hear about it otherwise. I must say that this has been fun after=20
>being on the bench for a while now.
>
>KPP -- going to lunch
>
>
>
>
>----------
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