Lili Krakowski on sun 14 may 06
Malcolm! Ivor-- I was not talking feet.
May presented her friend's problem which was that when said friend glazed
her earthenware pots with lead-containing tin-opacified--i.e.
Majolica--glazes her pots did not leak. No customer complaints. Friend
changed to lead-free tin-opacified glazes and customers started to complain
the pots leaked.
Sharpening a goosequill I drew three conclusions: that the same clay body
was used; that the application of the glaze was the same --namely stilts or
no stilts; wiped feet, etc.--same consistency glaze; and that the firing was
the same.
Deerstalker firmly on my head, and magnifying glass in hand, I then
concluded that there was one logical answer; known to science as the Butter
and Cream Cheese Conclusion--the lead-containing glazes being the Butter
part, the lead-free glazes the cream Cheese Part. (When I talk Skience I
give myself goosebumps! Must put on my Inverness cape)
Ok. When you put butter on hot toast a lot melts into the toast making the
toast soggy. (Observation, not complaint!) When you put cream cheese on hot
toast there is only a tidge of melting into the toast. A teensy bit of the
top layer of toast will be softened, but the rest remains crisp.
Now what I imagine, envisage, about May's friend's problem is this: that she
is using a body that remains too porous at c.04. That when she used lead
glazes somehow the glaze disassociated itself in some way and that lead
separated from the glaze, seeped into the body and gave it a degree of
vitrification it was not "entitled" to...Enough lead remaining in the glaze,
however, to form a proper glaze. My theory goes on. The leadless glaze
does NOT disassociate--if that is the proper term--and like the cream
cheese, only penetrated the clay on which it sits a tiny bit...thereby NOT
adding anything to the vitrification process of the clay, which remains too
porous.
This is a THEORY only, and only a lab, I expect could test it. However I am
convinced the problem is with the clay body, that May's friend needs to
check that out....If May's friend would do an absorption test on 1. the
unglazed clay 2. a fragment of a piece glazed with the lead containing
glaze 3. a fragment of a piece glazed with the new lead free glaze, I think
we all could learn a lot.
Lili Krakowski
Ivor and Olive Lewis on mon 15 may 06
Dear Lili,
Thank you for your insights.
Butter and Cheese. I am not sure that this is a good analogy but it does =
make sense if you remember that Butter is a two phase colloid (water =
droplets dispersed in a low melting point fat) while Cream Cheese is =
also a two component mixture (fat and casein)possibly as Solid Solution =
but not a colloid.
IF Lead in the original recipe was in the form of Lead oxide or Lead =
carbonate then it might wick into the porous clay when it melted (mp =
PbO=3D888 deg C; Pb3O4 =3D 830 deg C. Lead carbonate decomposes below =
these temperatures). Now if, for safety, it was one of the Lead =
Silicates that was being used, both Lead Metasilicate and Lead =
Orthosilicate have melting points in the 750 deg C region. I do not =
think these compounds would dissociate or decompose The thermodynamic =
numbers for Pb3O4 support this. But I do think that a small proportion =
of liquid formed when the lead compound first melted nearest the clay =
would wick in via capillary force where it would dissolve any free =
silica and clay at the surface of the pot. That would give you the =
effect you have imagined.
There is not enough information about the leadless glaze but if it =
contained a frit the initial melts would be at a much higher =
temperature.
Best regards,
Ivor
May Luk on mon 15 may 06
Hello Lili & Ivor & Malcolm & friends;
Thanks for contemplating the 'leaky' problem and all
the solutions suggested. I didn't think of the clay at
all. It was an interesting thought. I know for a fact,
she has not tested the clay for absorption. With that
said, we accept that terra cotta is porous. It is
understood that the glaze provides a coating for the
clay. BTW. I had fired this terra cotta to cone 8 a
few times. It has a wide firing range. The vases are
not fired on stilts and the foot ring bottoms are not
glazed.
From our speculation, the tin glaze is pin holing
without the lead base (low sol), because it's a much
stiffer glaze. We think the water is coming from the
pinholes. However, I am going to suggest to my friend
to do an absorption test and see what can make out of
it. Nothing ventured, nothing gained. :-)
Thanks to Ivor for the ‘first principle’ It is much
appreciated. We are carrying with the testing, slowly,
but surely.
Last time I did an absorption test, it was making too
much noise in my open plan kitchen. Then I gave up.
Many thanks
May
LondON, UK
William & Susan Schran User on mon 15 may 06
On 5/15/06 3:26 PM, "May Luk" wrote:
> With that
> said, we accept that terra cotta is porous. It is
> understood that the glaze provides a coating for the
> clay. BTW. I had fired this terra cotta to cone 8
Are you certain you're writing about the proper clay or cone temperature?
Terra Cotta is traditionally a low fire earthenware clay and would most
likely melt at cone 8.
Do you mean cone 08?
Getting the glaze to fit to the clay often requires multiple tests to find
the proper recipe that will not craze on the clay being used.
A higher bisque firing will often help with glaze fit also.
-- William "Bill" Schran
Fredericksburg, Virginia
wschran@cox.net
wschran@nvcc.edu
May Luk on tue 16 may 06
Hello Bill;
No, it wasn't a typo. I did fire the terra-cotta to
cone 8. I know that it is not called terra-cotta after
that treatment. :-) What happened was, I found some
terra-cotta bowls that I had bisque long beore that
time and I had switched to working in stoneware then.
I just fired it and see what happened. The bowls
became very 'tight' and the clay colour turned into a
burnt umber colour rather than the warm reddish tone.
The bowls also gotten smaller. Also the glaze really
reacted with the clay, like stoneware.
So that was my experience with this brown clay, AKA
earthenware (Valentine Clay) in a lower temperature.
Regards
May
Taylor Hendrix on tue 16 may 06
Hey May,
Paul McCoy, ceramic professor at U of Baylor, regularly pushes his
studio earthenware to med high cones (5-8) even in soda. Saw some
student work with spectacular bloating....or was that spectacular
student work with bloating?
Oh well,
t in r tx
On 5/16/06, May Luk wrote:
> Hello Bill;
>
> No, it wasn't a typo. I did fire the terra-cotta to
> cone 8. I know that it is not called terra-cotta after
> that treatment. :-) What happened was, I found some
> terra-cotta bowls that I had bisque long beore that
> time and I had switched to working in stoneware then.
> I just fired it and see what happened. The bowls
> became very 'tight' and the clay colour turned into a
> burnt umber colour rather than the warm reddish tone.
> The bowls also gotten smaller. Also the glaze really
> reacted with the clay, like stoneware.
>
> So that was my experience with this brown clay, AKA
> earthenware (Valentine Clay) in a lower temperature.
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