Snail Scott on fri 15 mar 02
At 01:19 PM 3/14/02 -0600, Connie wrote:
>In stores...I see alot of tea sets...etc. that
>have crackle glazes on them...I thought that is was
>unsafe to have crackle glaze on a food surface?
I've seen that around a lot lately, too. I don't
know how it's made, but I know a lot of commercial
dinnerware is actually high-fired before glazing,
(like china, for instance), then the glaze firing
is done afterward to a lower temperature. If this
is the case with these newly fashionable crackle-
wares, then the clay underneath may be fully
vitrified. I also suspect that since the glaze
layer on dinnerware fired this way is less fully
bonded to the clay than it is in 'normally'-
fired ware, the crazing may have less of a
detrimental effect of the clay underneath, too.
Just my speculation, though.
This is a BIG difference between porcelain and
china, though many lay-people use the terms
interchangeably. Porcelain dinnerware is bisque-
fired low, the glazed and high-fired, just as
most craft potters would do it. China was
invented as a 'porcelain substitute' originally,
back when European makers were trying to
replicate the look if the Asian imports. Bone
china is actually overfired by most normal
standards - it's got lots of fluxes in it to
give it that translucent look, but unlike
porcelain, it slumps badly when it approaches
the temperatures which give that effect. So,
bone china is often fired in special molds, to
allow the ware to slump to the right shape and
not warp. Naturally, it can't be glazed while
this happens, so the glaze is applied afterward
and fired to a lower temperature that won't
induce slumping. If you compare a broken piece
of china with a broken piece of porcelain, you
can see the difference, especially at the
interface of the clay and glaze. Porcelain is
closely bonded to its glaze, and the interface
is almost a transition rather than a boundary,
but china has a very distinct line of
demarcation.
I don't know that the trendy new crackle
wares are made by a similar method, but if they
are, the vitrification of the clay under the
glaze would make food-safety issues fairly
negligible, since only the glaze cracks
themselves could hold bacteria, and since no
moisture would absorb into the clay, I wouldn't
worry much. (Actually, I don't worry much about
it anyway...our world is chock-full of bacteria,
and what might live in the interstices of a
crazed glaze seems pretty trivial to me, even
if it was on earthenware.)
-Snail
Craig Martell on sat 16 mar 02
Snail sez:
> I also suspect that since the glaze
>layer on dinnerware fired this way is less fully
>bonded to the clay than it is in 'normally'-
>fired ware, the crazing may have less of a
>detrimental effect of the clay underneath, too.
>Just my speculation, though.
Hello Snail:
My opinion is that you've speculated correctly. When ware is fired hi and
glazed at a lower temp there is no chance of developing much of a
clay/glaze interface. On "hard paste" porcelain the body and glaze are
both matured at hi temp and there is a very distinct clay/glaze
interface. When glazes craze on hard paste porcelain or any hi fire ware
for that matter, the crazing extends into the interface and scores the
claybody which weakens the pot. On soft paste ware which is fired hi and
glazed low this scoring of the interface and body does not happen.
regards, Craig Martell in Oregon
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