earlk on tue 17 jan 06
Since most cone 6 glazes contain boron fluxes
in one form or another I was wondering how the
inclusion of boron might affect the hardness
( scratch resistance ) of the fired glaze.
Anybody done any tests on this, or seen anything in
the literature?
Thanks...
earlk...
bothell, wa, usa
John Hesselberth on tue 17 jan 06
On Jan 17, 2006, at 12:01 PM, earlk wrote:
> Since most cone 6 glazes contain boron fluxes
> in one form or another I was wondering how the
> inclusion of boron might affect the hardness
> ( scratch resistance ) of the fired glaze.
>
> Anybody done any tests on this, or seen anything in
> the literature?
Hi Earl,
Eppler and Eppler report that "up to 12% B2O3 improves the durability
of alkali silicate glasses." They go on to say that more drastically
lowers the acid resistance. Typical B2O3 levels in cone 6 glazes is
about 6%, although one like Waterfall Brown gets up to 14%. They also
say "All leadless glazes are generally resistant to scratching."
Taylor and Bull say that where scratch resistance is important, as in
floor tiles, it is achieved by incorporating coarse particles of high
hardness materials such as alumina or zircon.
There may be some other brief mentions in the literature, but I think
an important point is that every glaze I have tested for scratch
resistance is between 6 (feldspar) and 7 (quartz) on the Mohs scale
and that is significantly harder than metal utensils--regular
stainless table ware is easily scratched with a 5--my test kit
doesn't go any lower. The bigger problem with glazes is metal
marking--the glaze abrades the metal--not scratch resistance except
maybe on floor tiles. What people often think is scratching is really
metal marking.
Regards,
John
Tom Buck on tue 17 jan 06
Earl: your question seems simple, sadly it isn't.
the surface hardness of a borosilcate glass (with Al2O3) depends
on several factors.
Lab glassware, eg, is made with a low level of B2O3 in the
mix (almost zero or zero Al2O3), and hence is almost quartz-like in
hardness.
When B2O3 and Al2O3-SiO2 are combined in a glaze mix there will be
a range of B2O3 content. High B2O3 levels as in raku glazes will tend to
be a bit on the soft side, ie, scratchable. As the B2O3 level drops for a
higher Cone, the Al2O3-SiO2 will dominate the surface, and yield a hard
surface, unless the B2O3 is higher than seems needed. then the glaze will
be scratchable.
What is involved in a C6 glaze that contains B2O3 is whether the
B2O3 glass will dissolve in the Al2O3-SiO2 glass when both are in the
molten state. If the B2O3 is low, then a solution will be formed, and when
the mix cools, the glass will have Al2O3-SiO2 properties on its surface.
If the B2O3 goes above a certain level (a guess: over 8 mole percent),
then there will be two immiscible liquids in the melt, and there will be a
reasonable chance that some B2O3 will be on the surface of the cooled
glaze and hence make the glaze more open to scratching.
It has been observed that glaze fit becomes uncertain when the
B2O3 content goes above 12 mole percent. a likely cause is that two
separate glasses, quite distinct from each other, will be formed, and
present an irregular surface on the pot.
BTW, B2O3 has a boiling point of 1500 oC (2730OF) (or thereabouts)
so once the glaze mix is molten there will be some B2O3 that will leave
the scene, and this could be significant at 1305OC (2380oF) the
temperature that is reached in a Cone 10 firing. In general, one should
avoid B2O3 in a C10 glaze.
Perhaps others on Clayart will have data on B2O3 levels in
alumino-silicate glazes.
til later. peace Tom B.
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
Ivor and Olive Lewis on thu 19 jan 06
Dear Friends,
Tom has given us a good elementary insight into the nature of glazes =
which rely on Boron compounds and frits for their ability to melt.
One fact he omitted but did imply is that Boron oxide is well known as a =
Glass Former. Except under special circumstances it is never found as a =
crystalline substance. In the solid state it is usually a glass. =
Materials of use to the potter, in addition to fritted formulations and =
the Calcium Borate minerals are Sodium Tetra-Borate, as an anhydrous =
compound or as one of several hydrated compounds, Boric Acid, Meta-Boric =
Acid and Boron Oxide which is a Hygroscopic Glass under common =
circumstances. Lithium Borate and Potassium Metaborate are known but if =
they are similar in behaviour to Sodium Tetraborate they will be soluble =
in Water.
I think evidence that supports Tom's assertion that there may be some =
volatilisation may come from the presence of a green tinge in gas which =
burns as it exits the kiln stack. But this is conjecture and my =
spectroscope is out of action. Sodium Metaborate has a similar boiling =
point to Boric Oxide. I do not think the Sodium compound would decompose =
knowing its Heat of Formation though I do like the notion of Phase =
Separation when the percentage of Boron in a melt is high. This effect =
could be responsible for that "Floating Blue" effect.
I also agree with Tom the compounds of boron have no place in Cone 10 =
glazes, except of course, if a potter intends making "Floating Blue"
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Daniel Semler on thu 19 jan 06
Hi Earl, All,
A paper was produced many years ago now (1930s) entitled "The Influence of
Chemical Composition on the Physical Properties of Glazes". It is by F.
P. Hall
and I wish I could remember the journal. I have a photocopy from my
library but
the journal name is not on it. Hang on, my notes .... here 'tis :
Journal of the American Ceramics Society
Hall F. P. 1930. Influence of the chemical composition on the physical
properties of glazes. 13:182
This is a lengthy paper describing a lot of research and examining various
physical properties for glazes of differing composition. B2O3 is
considered and
tensile strength, Young's modulus and CTE are investigated.
Hall must have been a patient and methodical person. There is a lot here.
Worth looking out. It includes a number of charts plotting component
percentages versus the value of the physical property.
Thanx
D
Daniel Semler on thu 19 jan 06
Hi again,
OK so I'm a twit. I checked the paper below and wonderful as it is, it does
not, I believe, address hardness. Apologies. Of course still worth a
read. I'll
see if I have anything else that may.
> Journal of the American Ceramics Society
>
> Hall F. P. 1930. Influence of the chemical composition on the physical
> properties of glazes. 13:182
>
Thanx
D
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