DEVITRIFICATION: what is it, how does it happen, and how can it be prevented?
Part I: The structure of glass
A glass is by definition a liquid at all temperatures. Compositionally a
typical glass consists of the 'glass (or network) formers' (SiO2 and
B2O3), fluxes (also called alkaline metals, K2O and Na2O), and the
network (or matrix) modifying alkaline earths (CaO, BaO, PbO, ZnO).
Other oxides capable of glass network formation include GeO2, P2O5,
As2O5, As2O3, Sb2O3, and to a limited degree V2O5, ZrO2 and Bi2O3. The
oxides of Ti, Zn, Pb, Al, Th and Be can be included in varying
concentrations, but will not on their own, yield a glass. These, and
other oxides that will not form a glass (including Sc, La, Y, Sn, Ga, In,
Mg, Li, Sr, Cd, Rb, Hg, and Cs) are used as network modifiers.
Within the glassy matrix there are bridging oxygen ions and non-bridging
oxygen ions (which ionically bond to Li+, Na+, K+ or Ca++ in a typical
glass). Technically, glass is a metal oxide polymer with SiO2 being the
principal monomer of all commercial glasses. In three dimensions, this
polymer can be depicted as a random 'cobweb' of silicon ions each bonded
to either three or four oxygen atoms. Progressing along any branch of
the polymer chain we see Si-O-Si-O-Si-O-Si-O with an occasional matrix
modifier ion (as Pb, lead) substituting for a Si. Branching from each Si
will be another similar chain of O-Si-O-Si-etc. Occasionally, a chain
will terminate at a non-bridging oxygen, which has a strong negative
charge. This oxygen will be protruding into a microscopic cavity in the
glassy matrix within which resides a counterbalancing, positively charged
alkaline metal flux ion (as Na, sodium).
A pure SiO2 glass is called fused quartz. It can be manufactured from
rock quartz crystals, and has no flux nor matrix modifiers within it.
Because of the uninterrupted strength of the Si-O bonds it yields a very
high temperature softening ('hard') glass. Occasional interruptions of
the Si-O matrix with modifying ions (with weaker metal-O bonds) will
lower the viscosity profile of ('soften') the glass. The more alkaline
earth oxides added to the glass, the more the viscosity profile is depressed.
This is glass when everything 'goes right'. When glass devitrifies, it
doesn't necessarily revert to its former, solid, crystalline
constituents. In many instances a devitrified glass will not be
chemically altered, but rather it will crystallize into its ceramic analogue.
Copyright 1996, Michael Olsen
Unauthorized replication for profit is prohibited
(yes, I am writing a book)