Bill Walker on fri 11 apr 97
I was rather surprised that I was able to find the information
on the purple barium glaze that turned pink during the leaching
test.
The glaze contained 10% Barium Carb. Other ingredients were
F-4 feldspar, CaCO3, ZnO, Gerstley Borate, GF-113 Frit, EPK
and "Pansy" Purple Stain (I don't know the source). The
molecular formula of the glaze was:
K2O 0.050 Al2O3 0.300 SiO2 1.538
Na2O 0.124 B2O3 0.237
CaO 0.557
BaO 0.122
ZnO 0.148
It was fired on a red body to cone 04 in an electric kiln.
The chemist reported that the glaze released 1300 ug/ml Barium
(which is 1300 ppm) and noted "Unsafe !!!" (I had previously
said that I thought it was reoprted to be off the scale.)
Bill Walker
Alfred NY USA
Gavin Stairs on sat 12 apr 97
At 11:32 AM 11/04/97 EDT, Bill Walker wrote:
>I was rather surprised that I was able to find the information
>on the purple barium glaze that turned pink during the leaching
>test.
Thanks for looking it up. This turns and anecdote into something like data.
>The glaze contained 10% Barium Carb.
An astonishing amount, really.
>... The molecular formula of the glaze was:
>
> K2O 0.050 Al2O3 0.300 SiO2 1.538
> Na2O 0.124 B2O3 0.237
> CaO 0.557
> BaO 0.122
> ZnO 0.148
>
>It was fired on a red body to cone 04 in an electric kiln.
>
>The chemist reported that the glaze released 1300 ug/ml Barium
>(which is 1300 ppm) and noted "Unsafe !!!" (I had previously
>said that I thought it was reoprted to be off the scale.)
My goodness, this thing is about 1/3 soluble materials. 1300ug/ml is
0.13%. This is an enormous amount. I would be interested to know the
dissolved matter in the sample. Probably about 1% total.
This is not a glaze at all. It is merely a coating of soluble salts mixed
with some silica. Even the alumina is very low. The glaze color probably
changed because the whole glaze coat had been dissolved into a honeycomb of
microfissures through which the Ba, among other things, could leach out at
an enormous rate. Think: The cup/mug probably had about 15g of glaze on
the inside. Of this maybe 1g was Ba. The cup held maybe 250g of solvent.
So if all the Ba went into solution, the concentration would be 0.4%. What
was reported was fully 1/3 of that. So about 1/3 of the measured fraction
of the glaze went into solution! Phenomenal. Small wonder it changed color.
After I'm done being amazed at this, I'll probably have a go at trying to
understand what is wrong here. Probably simply an immature, low fire glaze.
It's interesting that you can start out with four reasonably insoluble
materials, plus one somewhat soluble one, and end up with something that
literally comes apart in weak acid. I wonder what is in that stain?
There's a lesson or two in this.
Gavin
=================================
Gavin Stairs
http://isis.physics.utoronto.ca/
Gavin Stairs on mon 14 apr 97
Slight addition to my previous post.
Bill Walker wrote me off list to tell me that this glaze is pretty much
within limits for a glaze of this cone. I agree. So what went wrong?
1) Quite possibly it was misfired. It was presumably not fully mature.
2) Perhaps it was inaccurately made up. Maybe too much Al2O3, or not
enough SiO2? Or perhaps the stain addition unbalanced it. The stain is
Cr, Sn, Co, Ca and Si, according to my Mason stain list. If there was a
lot of stain in the glaze, it may have led to an imbalance that simply left
too little glass former.
3) Perhaps there were complications from the Gerstley Borate (solution
separation, or batch variation), or one of the other source materials.
If the problem was simply an error in formulation or technique, then this
is merely a cautionary tale. Moral: we must always take care with our
techniques, so that we get what we expect.
If the problem is in the glaze formula itself, then we have work to do in
understanding glaze formation as expressed in our limit formulae and
standard methods.
Gavin
=================================
Gavin Stairs
http://isis.physics.utoronto.ca/
Ron Roy on tue 15 apr 97
>Bill Walker wrote me off list to tell me that this glaze is pretty much
>within limits for a glaze of this cone. I agree. So what went wrong?
I was curious about this so decided to check against the limits I use.
I reformulated using different materials - the recipe follows.
9.6 - Neph Sy
12.5 - Whiting
9.9 - Barium Carb.
29.5 - frit 3134
4.9 - Zinc ox.
27.8 - EPK
5.8 - silica.
When I check against the limits provide with Insight for a cone 04 glaze I
get an under supply of MgO. KNaO, B2O3 and SiO2.
When I check with B2O3 included in unity with fluxes and use the limits
supplied by David green and Cooper for that situation I get an over supply
of CaO and an under supply of SiO2.
In both cases there is an indication of a shortage of SiO2 - perhaps it is
time to review what limits we are using. If anyone cares to send me theirs
I will try to make some sense of this. Does anyone else care to work on
this?
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
ronroy@astral.magic.ca
Gavin Stairs on wed 16 apr 97
At 12:11 PM 15/04/97 EDT, Ron Roy wrote:
>----------------------------Original message----------------------------
>
>>Bill Walker wrote me off list to tell me that this glaze is pretty much
>>within limits for a glaze of this cone. I agree. So what went wrong?
I have to confess to a bit of sloppiness. What Bill wrote was:
I checked my glaze notes and all of the oxides in that glaze are within the
recommended limits for cone 04 glazes, except CaO was a bit high.
>I was curious about this so decided to check against the limits I use.
....
>In both cases there is an indication of a shortage of SiO2 - perhaps it is
>time to review what limits we are using. If anyone cares to send me theirs
>I will try to make some sense of this. Does anyone else care to work on
>this?
When I last looked at this, my beta version of Insight had expired, so I
looked at Robert J. Wilt's Glazechem. There I found a number of limits for
cone 04 glazes, with different limits for Glossy Clear, Satin and Matt. In
these limits, the glaze in question was flirting with the edges, but as a
satin or matte, it was pretty much compliant. Now, looking at the limits
in Insight, I get a somewhat different picture.
Clearly, the limits that are in use are not all the same. I'd certainly
like to get to the bottom of this, but I fear it will be no small feat.
Gavin
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