Maggie McMahon on sat 30 nov 96
Dear Joseph Herbert - I read you last post with great interest. I think
your point of being scared when reading other posts is well taken. Potters
are self regulated. The ways in which people come to point of producing
utilitarian wares are varied. I shudder when I think of the East Village
cooperative studio where I worked in 1970 - the studio mantra was if it
came from the earth it had to be okay. So we happily worked in an
unventilated studio with the kiln blasting away and sold pots believing
they were safe. Even if one has the benefit of an excellent learning
situation which didn't come for me until nearly a decade later - there is
no guarantee that lessons will be remembered. I don't have the power to
enforce safety issues once a student is out of the classroom. More
important, students don't know what they are not being taught! ClayArt is
especially important as a forum for the discussion of and source of
information for safety issues - if people feel like they can question
without out fear of sarcasm, ridicule, or condemnation. The entire list
benefits from specialized expertise. No one person knows it all - although
there may be an exception or two, let's hope that they continue to share
with mere mortals like myself! Ignorance rather than malice is the
problem. But I concur with what you must be thinking - ignorance is no
excuse. Which brings me to MY assumptions about the safety issues
concerning crazing. I do question equating the dangers of crazing with
shivering. For instance, is there any glaze to earthenware fit which will
not craze? Other than "corelle" - which is glass rather than clay , I
can't remember not observing crazing on any transparent/translucent glaze
over a light body - over time with repeated use. (My assumption is that
although crazing can be minimized, it always takes place and is visible
only in certain instances). For me there is crazing and CRAZING. A glaze
which pings upon cooling and results in what appears to be crazing but can
actually be felt across the surface is CRAZING (a fancy name for the glaze
cracking over the body because of poor glaze fit) - definitely a candidate
for reformulating or for non utilitarian use. But crazing: the tiny
network of little cracks that appear on the inside of cups and mugs is
unavoidable with repeated use over time. I welcome information if my
(many) assumptions are incorrect. Maggie McMahon-UT/Chattanooga
Martin Rice on sun 29 sep 02
Hi, All:
I wrote the day before yesterday about my first "real" firing. I decided
last night to put the new things I had made into the dishwasher to see how
they fared (though I didn't include my three favorite pieces from this
firing until I knew more).
This morning I opened the dishwasher and found crazing on all the pieces.
You had to look closely, but it was there on all of them. None of the pieces
were crazed when I took them out of the kiln or when, a day later, I put
them in the dishwasher.
Over the months I've been reading Clayart, there have been many discussions
about crazing, usually in relation to very specific circumstances. If you
don't mind reading all this, I'd like to give you the specifics of what I've
done. Perhaps you can then give me some general ideas about what this
happens with what I'm doing.
The claybody:
70 ball clay
10 feldspar
10 calcium carbonate
10 talc
This is a bisque ^04 claybody.
For the bisque firing, I candled on low with the door (top loader) propped
open a few inches overnight. The firing with peep holes open went: 2 hours
on low, 2 hours on medium, and 3 hours on high until the Orton cone slumped.
I didn't take the bisqued ware out for about 17 hours.
For the glazing: Used Duncan Concept underglazes and Duncan clear overglaze,
meant to be used, according to the labels on wares bisqued at ^04 and then
glaze fired to ^06. The ramp was the same as the bisque without the candling
and with the peep holes closed. After the cone slumped, I didn't open the
kiln for almost 24 hours.
The bottoms of the pieces are unglazed -- that is, besides the foot rings,
the center part is also unglazed.
I guess what I'm wondering is whether this type of claybody and glaze can be
used at all for functional ware? Is it impossible to get any kind of
vitrification I just realized that I didn't do an absorption test on the
bisque ware -- I guess I should have. Though I did notice that when I
sprayed the bisque with water before applying the underglaze, thanks to
Snail's suggestion, the pieces seemed to suck it right up -- sluuuurp.
I'll soon be using a different claybody that Ron Roy has helped me formulate
that is still a ^04 for bisquing but which should be able to take a ^1 or ^2
glazing. Will that be any more suitable for functional ware? Also, I don't
mind not having the things be dishwasher safe, but do want to be able to use
them for eating and drinking. My first big project after my throwing skills
are up to the level I need, will be to make a complete set of dinnerware for
our everyday use.
I'd appreciate any insights you can give me with these questions.
Thanks so much,
Martin
Lagunas de Barú, Costa Rica
www.rice-family.org
Snail Scott on mon 30 sep 02
At 08:21 AM 9/29/02 -0600, you wrote:
>...I took a
>close look at the pieces that I hadn't put in the dishwasher -- turns out
>that they are now crazed, too. So it seems it was a matter of time rather
>than the diswasher until crazing appeared.
>
All glazes on earthenware will eventually craze. That's part of
the reason you don't see it much for commercial dinnerware. It
will craze quicker with exposure to lots of water. So, whenever
you have earthenware pottery used for food, hand-wash it! And
that means wash each piece and set it to dry; don't leave them
sitting in a sink full of water for an hour beforehand. Washing
dishes in a dishwasher does pretty much exactly that - keeps
the pottery in contact with water almost continuously for an
extended period of time. And dishwasher detergents are extremely
caustic, much more so than hand-dishwashing detergents. After
all, they have to do most of their scrubbing chemically! So,
they will gradually eat away at glaze surfaces. Unstable glazes
can show noticeable degradation after just a few washings, but
even quite stable glazes will eventually dull or change. Even
commercial ware. So, better to handwash ALL handmade pottery,
even the stoneware. (It might not craze, but the dishwasher
still isn't good for it.) With earthenware, even if you handwash
it and keep the soaking to a minimum, it will craze with humidity
and time. That's just the way it is. They'll still make fine
functional pieces.
If the crazing gets really filthy and nasty-looking, you can try
refiring. Most of the gunk will burn out in the re-fire. Some
people get rather worked up about the unsanitary nature of the
stuff that takes up residence in the crazing, and which may make
its way into the porous clay of the pot. Personally, I figure I
meet nastier bacteria every day, and I don't worry about it. If
the food-contact surface is clean, that's good enough.
>
>Something else....
>The finish is not as glossy as the other pieces, I guess it's a semi-gloss.
>Reasons for this not crazing yet?
>
It probably did craze, but it's much harder to see when the
glaze isn't glossy.
>
>And two other pieces of information: all the crazing is in the overglaze.
>The painting I did with the underglazes has not crazed yet at all. And the
>clear glaze I use I had diluted, 2/3 glaze, 1/3 water. Could that have
>contributed to the crazing?
>
When yo say the underglaze didn't craze, do you mean that the glaze
COVERING the underglaze didn't craze, or that the bare underglaze
didn't craze? If your underglaze is not vitreous, it won't craze by
itself because it's not glassy enough. If it is vitreous, it probably
did craze, but you can't see it because it's opaque. If it has a
clear over it which didn't craze, it could be either of two effects
(or both) playing a role.
Dilution won't matter. Thicker glazes show heavier crazing patterns,
but that's just 'cause they are thicker. Thin glazes craze too, but
the lines are closer together and smaller, i.e harder to see. If you
do underglaze painting which has noticeable thickness, the glaze
over it can thin out in firing. And if the underglaze is less
absorbent than the bare clay, it will end up with a thinner layer
of glaze from dipping.
Also, the layer of glaze which is closest to your pot actually
reacts with the clay (or underglaze) under it. This effect is most
pronounced the higher the firing temperature, since the clay itself
is closer to a vitreous melt. Basically, to a greater or lesser
degree, (according to the temperature and glaze,) the area where the
clay and glaze touch is actually a mixture of the two, with the
glaze fluxes partially melting the clay surface, and the clay
releasing some of its silica and (especially) alumina into the glaze
melt. So, very thin glazes are actually chemically different than
thick glazes, since a greater percentage of their total substance is
affected by the clay/glaze interface. And, if your underglaze has
more fluxes than the plain clay body, it will likely form more of a
'mix' with the glaze above it, and may stabilize it somewhat against
crazing in that area.
-Snail
Reno, NV
| |
|
