Tom Buck on sat 14 oct 00
Chris:
You raise some interesting questions on my post. I respond below
(as best I am able to).
Tom Buck ) tel: 905-389-2339
(westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
On Sat, 14 Oct 2000, Chris Schafale wrote:
> Hi Tom,
>
> Thanks for taking the time to reply so extensively. I do understand
> the basic background, and the concept of limit tables. My
> question was really about the idea that alumina above 0.3 or so
> was too much for a cone 6 glaze. The Green & Cooper limits that
> I've been using show alumina up to 0.65 as within limits, and it so
> was a surprise to me to see your comment. In your post, I was
> especially interested in this part, where you discuss alumina above
> 0.5 (by the way, what about the 0.3-0.5 range?):
Green/Cooper's limit tables always assume Boric Oxide is in the recipe,
and acts as a flux oxide, able to "solubilize" alumina. So they set the
Al2O3 range much higher than others who downplay B2O3 in C6/C10 glazes.
The 0.3-0.5 Al2O3 range is the make/break point for C6 glazes. Glaze
chemistry is quite complex, so many activities going on side by side, so
one cannot make absolute predictions on what will happen when one mixes a
glaze higher in Al2O3 than specified in, let's say, Insight's table
(non-lead). If one abides rigorously to the flux oxide makeup in the
Table, 0.6 CaO, 0.2 MgO, 0.2 KNaO, then 0.35 Al2O3 and 3.5 SiO2 will on
most bodies give good glass, glossy. If one drops the MgO, adds more KNaO
(and the glaze doesn't craze thereby), the Al2O3 could work at 0.4 and the
Silica close to 4.0. Again, lowering the CaO, raising the KNaO, and
introducing Zinc Oxide (oxidation only), will allow more Al2O3 and SiO2 in
the mix (relative to the flux oxide content). To reach 0.5 Al2O3 amd 5.0
SiO2, one must use some B2O3 in the mix, keeping the amount below 0.5
moles.
Predictability suffers a loss when P2O5, TiO2, BaO, and SrO
are introduced into mix.
> a glaze that can be then be rendered mature by adding significant
> amounts of Calcium Oxide (whiting)...it seems two glasses then
> exist on the surface of the pot, the expected flux oxides alumino-
> silicate glass and a calcium aluminate type glass. But the
> CaO.Al2O3 mix has to be kept sufficiently low (as part of the overall
> recipe) to prevent severe discontinuities from occurring.
> So, in summary, if we take the alumina too low, we lose the glaze
> off the pot. If we take the alumina too high, the glaze surface starts
> to break up and will be unacceptable for foodware, the bread and
> butter product for many studio potters.
>
> Questions:
>
> What do you mean by "severe discontinuities" and "glaze surface
> starts to break up"? What would this look like in practice, and how
> would it affect food use?
High amounts of alumina make the glaze melt more viscous, and one could
reach the point where, the melt occurs on some surfaces areas and not on
others, leaving the pot partly "unglazed" ... a crawled glaze is an
example.
If the claybody were fully mature, ie, 2% porosity or less, and
the glaze sections had sealed surfaces, then the pot would not be a danger
if used in food service. But this is not an eyeball situation; one would
have to have the pot tested.
> Does this also happen when you use excess boron to melt a high
> alumina glaze?
If you stay with the accepted Limit for B2O3, then chances are good
you will get a smooth surface.
> If you have a glaze that appears melted and glossy, can it still have
> too much alumina?
By definiton, NO. The amount of Al2O3 that is suitable is that amount that
makes good glass that stays on the pot.
> Floating Blue, for instance, seems to have way too much alumina
> and silica for a cone 5-6 glaze, but it seems to melt just fine. The
> analysis for the recipe I use is:
>
> Floating Blue
> =============
> NEPHELINE SYENITE... 48.00 44.86%
> GERSTLEY BORATE..... 27.00 25.23%
> EPK KAOLIN.......... 5.00 4.67%
> SILICA.............. 20.00 18.69%
> cobalt carbonate.... 1.00 0.93%
> IRON OXIDE RED...... 2.00 1.87%
> RUTILE.............. 4.00 3.74%
> ========
> 107.00
>
> CaO 0.44* 5.45%
> MgO 0.00* 0.02%
> K2O 0.11* 2.36%
> Na2O 0.44* 5.97%
> TiO2 0.23 4.00%
> ZrO2 0.00 0.04%
> Al2O3 0.69 15.29%
> B2O3 0.47 7.16%
> P2O5 0.00 0.01%
> SiO2 4.39 57.67%
> Fe2O3 0.06 2.05%
>
> Cost/lb 0.57
> Si:Al 6.40
> SiB:Al 7.09
> Expan 7.93
If you set B2O3 as part of the Flux Unity, you will see that the Al2O3 and
SiO2 come close to what is considered ok, and hence will make a
predictable melt. Note also how dominating are the most active fluxes
at C6, namely, CaO, KNaO. The role of B2O3 is to offset the propensity to
craze or crawl, perhaps in some cases to blister (Li2O is fingered when
this happens).
> I'm very interested in the idea of "two glasses...on the pot" because
> this is what another high-boron glaze I use looks like when I fire it
> too high -- it seems to separate into two components, one which
> runs and one which doesn't. Is this what you are talking about?
>
Yes, that is partly what is involved. But trying to predict the two-phase
effect is beyond what is generally known. Such glazes are discoverd by
experimentation (try and see).
Good pots BFN. Peace. Tom B.
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