M Richens on mon 7 apr 97
In article <2.2.32.19970329182946.006f74e4@mail.lglobal.com>, Gavin
Stairs writes
>Sorry to go on like this. I hope this is useful to somebody. It certainly
>is to me. Helps me to get my own mental image in tune.
>
>Gavin Stairs Hi Dannon
>http://isis.physics.utoronto.ca/
Hi Gavin,
It's not going on. I find it is a less questionable method than talking
out loud to yourself..
The Real problem is in going back far enough into the structure of
glasses, glazes and enamels.
THE work on glass structure was by Zachariasen (J Am Ceram Soc 54 3841
1932) who set out both the structures of crystalline (Well ordered with
every silicon connected to four oxygen) and vitreous silica ( Disordered
to random although same connectivity between Si and O). One difference
between the crystalline and vitreous forms is that the crystalline form
has a fixed melting point and the glassy form softens over a range as
the bond strengths vary with the network structure.
The work by Warren was also important and stuff by Mellor and Seger
Silica is not the only glass former but they are all characterised by
forming either a tetrahedral relationship with oxygen (like Silicon) or
a Triangular (like Boron). The ratio of valency(charge on ion) to ionic
radius is greater than seven ( Si 4 and .41 gives 9.8) This means a high
bond strength and finally the oxygens link to no more than two glass
former though they can link to other cations such as K or Ca.
These modifiers tend to be larger ions and fit into the structure by
combining with the oxygen and breaking a -Si-O-Si-O- bond to, say,
K-O-Si-O and breaking the chain. Depending on its size it will distort
the structure changing the properties such as softening point, hardness
and chemical resistance.
[Fluorine is different as it substitutes for oxygen]
eg Lithium.. rule of thumb. I take this as being about twice as active
in fluxing as Sodium and used commercially this means that to IMPROVE
the chemical properties of a glass you can replace 2 Na2O with 1 Li2O..
The general level of fluxes is reduced therefore the glass formers go up
therefore a better glass and better resistance. This is the upside...
The downside is that if you have too much Li2O and a good supply of
Alumina present you can start getting opal glasses with the expansion
coefficient dropping (Look at opal Pyrex). This is where I think the low
expansion coefficient for Li comes from.
Another possible problem is that Li being so active it can sometimes
destabilize ceramic colours.
Going back to your point about higher cone fire giving better chemical
resistance.
A lot of this is down to the amount of glass formers present. Generally
speaking the higher your Si content is above 50% then the better the
resistance (and incidentally the higher the temperature needed to fire
properly)
A well formulated and _Correctly_ fired glass should behave properly wrt
attack. After all there are fully fritted porcelain enamels (showing my
background again) which are highly acid resistant or alkali resistant
(_BUT_ not often both.. Horses for courses) which fire at 840 C over 5-7
minutes .not even cone 012.
On your final point. Yes it is still amazing that it works but that is
half the fun.
Regards
Max
--
Max Richens max@richens.demon.co.uk +44 (0) 1925756241
Enamel Consultant - Ceramist - Analyst programmer
Software for Batch Formulation and Millroom control.
On-On.. Cheshire Hash House Harriers
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