Karl Platt on thu 18 oct 01
Iandol wrote:
>>But I am concerned about Molecules of Glass! Please Paul, what are these?
I'm hopeful that this is merely a smarta$$ question.
Before this goes a lot further I'd like to make the point (clearly) that
glass, whether as a substance unto itself or as the matrix of a ceramic
glaze, is absolutely not comprised of molecules. That this is so is one of
the distinguishing features of the stuff called glass. Molecules are defined
by very specific relationships between the atoms making them up. The
distances by which the atoms are separated and the angles between them are
very rigidly defined in molecules. These sorts of absolute spatial
relationships do not exist in glass, where the situation of the atoms
involved is more ambiguous.
Going back to Lithium… Lithium is little used in either glazes or glass
unless some specific results are desired. Low expansion properties or color
development (copper blue) are two good examples. It also tends to make
glazes/glasses harder than their soda/potash cousins. The limited use of
lithium is for a number of reasons. Not the least of which is cost.
As noted in a previous post, the atomic weight of lithium is very low and a
gram of lithium oxide will contain many more atoms of lithium than a gram of
any other element you'd use in ceramics. This points out the usefulness of
considering glaze composition on a molar basis – i.e. the basis of how many
atoms are in play and not merely their weight.
The effects lithium has are related to its smallness as an ion, and to its
relatively high charge as an ion. Being small it tends to make more compact
structures, hence lower expansion, higher density and hardness. Being highly
charged it can act disruptively by interceding weaker bonds.
Lithium as an agent to produce low expansion bodies will be successful only
within narrow limits of composition – it does not act alone to evolve low
expansion in bodies. The limits are so narrow that your insurance carrier
won't be willing to take the risk of flameware. This matter should be
addressed somewhere in Clayart's archives.
In glassmaking lithium was fashionable as a melting aid until it was seen
that it had the collateral effect of rotting refractories quite severely.
This is also related to its small size, which allows it to migrate rather
freely, but also to its high charge, which together tend to "attack" the
refractories more aggressively than the other alkalies. Similar effects are
seen at glaze/body interfaces on glazes ceramic wares.
Karl P. Platt
kplatt@glass.com
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Nevin Murtha on thu 18 oct 01
Just to clarify some chemistry and physics.
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.
I spent too many hours in crystallography and optical mineralogy class to
forget that one.
-----Original Message-----
From: Karl Platt
To: CLAYART@LSV.CERAMICS.ORG
Date: Thursday, October 18, 2001 12:42 PM
Subject: Non existent molecules and lithium
Iandol wrote:
>>But I am concerned about Molecules of Glass! Please Paul, what are these?
I'm hopeful that this is merely a smarta$$ question.
Before this goes a lot further I'd like to make the point (clearly) that
glass, whether as a substance unto itself or as the matrix of a ceramic
glaze, is absolutely not comprised of molecules. That this is so is one of
the distinguishing features of the stuff called glass. Molecules are defined
by very specific relationships between the atoms making them up. The
distances by which the atoms are separated and the angles between them are
very rigidly defined in molecules. These sorts of absolute spatial
relationships do not exist in glass, where the situation of the atoms
involved is more ambiguous.
Going back to Lithium… Lithium is little used in either glazes or glass
unless some specific results are desired. Low expansion properties or color
development (copper blue) are two good examples. It also tends to make
glazes/glasses harder than their soda/potash cousins. The limited use of
lithium is for a number of reasons. Not the least of which is cost.
As noted in a previous post, the atomic weight of lithium is very low and a
gram of lithium oxide will contain many more atoms of lithium than a gram of
any other element you'd use in ceramics. This points out the usefulness of
considering glaze composition on a molar basis – i.e. the basis of how many
atoms are in play and not merely their weight.
The effects lithium has are related to its smallness as an ion, and to its
relatively high charge as an ion. Being small it tends to make more compact
structures, hence lower expansion, higher density and hardness. Being highly
charged it can act disruptively by interceding weaker bonds.
Lithium as an agent to produce low expansion bodies will be successful only
within narrow limits of composition – it does not act alone to evolve low
expansion in bodies. The limits are so narrow that your insurance carrier
won't be willing to take the risk of flameware. This matter should be
addressed somewhere in Clayart's archives.
In glassmaking lithium was fashionable as a melting aid until it was seen
that it had the collateral effect of rotting refractories quite severely.
This is also related to its small size, which allows it to migrate rather
freely, but also to its high charge, which together tend to "attack" the
refractories more aggressively than the other alkalies. Similar effects are
seen at glaze/body interfaces on glazes ceramic wares.
Karl P. Platt
kplatt@glass.com
_________________________________________________________________
Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp
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Paul Lewing on thu 18 oct 01
on 10/18/01 3:48 PM, Karl Platt at vidreiro@HOTMAIL.COM wrote:
>>> But I am concerned about Molecules of Glass! Please Paul, what are these?
> Before this goes a lot further I'd like to make the point (clearly) that
> glass, whether as a substance unto itself or as the matrix of a ceramic
> glaze, is absolutely not comprised of molecules.
OK, before this goes any farther, let's bring this back to what the
original question was. I was referring to that unit that is expressed by a
Seger flux unity analysis. I always assumed that that represented a
typical, or average, irreducible molecule (chunk, piece, unit, whatever) of
fired glaze, which is essentially a glass.
So, Karl and, I guess, Ivor are saying those are not molecules.
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. I'm just a compulsive
glaze tester.
Paul Lewing, off to Pennsylvania in the morning.
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