Tom Buck on sat 21 jan 06
Ivor et al:
Because you asked for this data, Ivor, I post it to Clayart; I had
written this earlier for Carol.
here is Ionic Potential, etc. peace Tom
Tom Buck ) -- primary address.
"alias" or secondary address.
tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street, Hamilton ON L8T 3E1 Canada
---------- Forwarded message ----------
Date: Thu, 19 Jan 2006 16:44:29 -0500 (EST)
From: Tom Buck
To: carol@knighten.org
Subject: Boron's Ionic Potential
carol:
there is a characteristic of an atom called ionic radius, which is
a measure of the atom's ability to bind to itself and other atoms.
many ceramic technolgists consider the number derived by taking the
valency of the atom and dividing it by the ionic radius to come up with
P=ionic potential as a key guide to the atom's ability to form GLASS
by combining with oxygen atoms and other atoms.
Boron exhibits a valence of 3+ when it combines with oxygen, and
with an ionic radius of 0.20, the element's glass-forming potential is
15.0
Silicon exhibits a valence of 4+ when it combines with oxygen and
it has an ionic potential of 0.39 hence its P = 10.25
to give you an idea of the glass-forming potential of other common atoms
we use:
Aluminum P= 5.3; tin, 5.4; titanium, 5.8; chromium, 4.7;
phosphorus, 14.3.
So, boron is top glass-maker, phosphorus (if present) is 2nd; and
silicon is third (and the main ingredient of mid-fire and highfire
glazes).
A key article of commerce is Boron-3 Oxide (aka B2O3) and this
compound is sold as 1: Glass or "fused" form; 2) powder, technical, or
powder high-purity 99.99+%. as potters we seldom have access to B2O3 as
such...we get it in in frit form, ie, combined with other oxides (silicon,
aluminum, metals). Or as a natural mineral, ulexite, borax, colemanite
(Turkish), gerstley borate (soon gone but subbed with a version based on
ulexite).
all the above to explain to you that Boron Oxide, when present,
will always show up as a glass.
a glaze on a pot's surface is essentially a glass with other
oxides dissolved or dispersed in it.
if the glaze recipe contains some borate material (one of the
above), a boron-based glass will melt early on probably Cone 012/010 and
in turn have an effect on the melting of alumino-silicate glass. A base
clear that contains B2O3 and one that fits the claybody will be a mix of
the two glasses (boria/silica). If silica dominates, ie, is most of the
glass in the "Liquidus" (name used by techies), then how much B2O3 glass
is also present will affect the nature of the cooled glass surface (ie,
glaze). If the B2O3 glass is on the small side, it will probably
dissolve/disperse readily in the silica melt, and thereby not noticeably
cause any effect on the cooled surface, If the B2O3 is "middling", it
may/will cause small blemishes/defects in the clear glaze giving it a very
slightly "frosty" look, and leading to knife marks if the pot is a plate.
and finally, if the B2O3 content is too much for the silica glass to
"handle", there is a strong likelihood that the two glasses will keep
separate and lead to a mottled cooled glaze, and glaze fit may not happen.
There is some data in the literature on this mix of B2O3 and SiO2.
Check with Am Cer Soc archives.
stay healthy peace Tom
Tom Buck ) -- primary address.
"alias" or secondary address.
tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street, Hamilton ON L8T 3E1 Canada
On Tue, 17 Jan 2006 carol@knighten.org wrote:
> Tom -
>
> I've been trying to get a handle on what the phases present in a ^6 glaze are.
> At melt point would be a start. Your comment about the two immiscible
> liquides in the melt if the B2O3 is high got my attention. Can you give me a
> pointer on the B2O3 in alumino-silicate glass?
>
> Thanks,
> Carol
>
> --
> ============================
> Carol Marians
> (503) 626-4965
> carol@knighten.org
> http://carol.knighten.org
> ============================
>
>
>
Ivor and Olive Lewis on sun 22 jan 06
Dear Tom Buck,=20
Thanks for including me in the loop on this one. What Co-ordination =
number are you attributing to the Boron Ion in making this determination =
and what units are being used for the Ionic Radius. I ask the latter =
because the value in my book of tables exceeds yours by a factor of X60.
Kingery et al have a good discussion on the nature of the glass forming =
oxides. From memory they support what you say about B2O3.
Best regards,
Ivor
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