Jonathan Nesbitt on tue 9 aug 05
First Hello, I love this web site, and I have probably read every single
archive! I have been making pottery since 2000, and I just bought my
first kiln. Well its my second kiln, the Paragon sits in the corner
waiting for an electric upgrade on the house. Anyways, I have fired my
kiln about 10 times in the last month and each one gets better and
better! My question is that if the glaze is foodsafe, does that still
mean that my earthenware clay fired to 04, then 06 is foodsafe. I don't
make a lot of functional stuff, but I have started leaning more that
way, candy dishes or platters, but mostly I like decorative pots,
vibrant shapes and colors..ok I am off to play, TIA,
Jennifer Nesbitt
Snail Scott on wed 10 aug 05
At 02:21 PM 8/9/2005 -0400, you wrote:
>...My question is that if the glaze is foodsafe, does that still
>mean that my earthenware clay fired to 04, then 06 is foodsafe.
Do you mean bisque-fired to ^04, then glaze-fired
to ^06, as most of the commercial low-fire glazes
recommend? (I'm assuming that you're using low-
fire manufactured glazes.) If you fired according
to the label directions, and it's labelled food-safe,
it should be fine.
-Snail
William & Susan Schran User on thu 11 aug 05
On 8/9/05 2:21 PM, "Jonathan Nesbitt" wrote:
> My question is that if the glaze is foodsafe, does that still
> mean that my earthenware clay fired to 04, then 06 is foodsafe.
A majority of earthenware remains porous, thus will not be food safe unless
covered with an uncrazed glaze.
--
William "Bill" Schran
Fredericksburg, Virginia
Snail Scott on thu 11 aug 05
At 08:42 AM 8/11/2005 -0400, Bill S wrote:
>...earthenware remains porous, thus will not be food safe unless
>covered with an uncrazed glaze.
I will grant that earthenware has its drawbacks
as tableware, but calling it 'not food safe'
seems pretty extreme to me. Even if microbes are
multiplying in the pores of the clay beneath
the (inevitably crazed) glaze, I would think
that relatively few of them will be able to
contaminate the glaze surface.
Earthenware is porous, and generally fragile,
and prone to crazing, but I don't fear to eat
off it.
-Snail
pinacoid3000@LYCOS.COM on thu 11 aug 05
Hi Snail,
No comment about the suitability of earthenware for food but is it fair to
say that such a body will inevitably craze? There are many glazed
earthenware articles that show no evidence of crazing yet are hundreds of
years old. How long is delayed crazing expected to take :-)
Regards,
Andy
William & Susan Schran User on thu 11 aug 05
On 8/11/05 9:23 AM, "Snail Scott" wrote:
> Earthenware is porous, and generally fragile,
> and prone to crazing, but I don't fear to eat
> off it.
I agree and I have no specific issues with using earthenware as functional
pottery. Much of the world uses earthenware to eat and drink from. I don't
fear to eat off it either.
I used to make earthenware functional pots. Visited a client at her home.
She had one of my earthenware teapots sitting up on a stand with a candle in
the stand burning under the teapot. Said she'd been doing that for more than
a year! No cracks, no leaks. I certainly never intended the teapot to be
used in that fashion and I'm certain if the clay were stoneware or
porcelain, that it probably wouldn't survive.
But if one puts their work out in the public domain and realizes that many
folks will be subjecting the pottery to possible use in ovens, dishwashers,
microwave and who knows what, then I think it's incumbent on the part of the
maker to produce ware that can withstand day to day use in our modern world.
With all that said, I continue to believe and advise the need for a glaze
that fits the claybody well enough to not craze for functional ware.
--
William "Bill" Schran
Fredericksburg, Virginia
Snail Scott on thu 11 aug 05
At 11:50 AM 8/11/2005 -0400, Andy wrote:
>Hi Snail,
>No comment about the suitability of earthenware for food but is it fair to
>say that such a body will inevitably craze? There are many glazed
>earthenware articles that show no evidence of crazing yet are hundreds of
>years old...
I was exaggerating a bit for effect, true, but...
Since the poster was, I assumed, a novice using
a commercial low-fire clay body with commercial
low-fire glazes, the chance for crazing is fairly
high. Few novices try multiple clay bodies or
various bisque temperatures to get better results,
and often take crazing for granted. Also, even
a well-fitting glaze seems to craze after repeated
use and washing, as the stresses of use accumulate.
I don't know why moisture seems to cause swelling
of a fired clay object, but that often is the
apparent (if not actual) effect. I've seen very
few pieces of uncrazed earthenware, and those few
I suspect of being either not-daily-use ware (like
those Italian majolica presentation plates), or simply
having hard-to-see crazing due to densely opaque
glaze, or of being borderline in the definition of
earthenware.
I suspect (though can't prove) that absorbency
and glaze fit are potentially equal factors in
earthenware crazing. Just as some clay/glaze combos
have better fit and thus stave off crazing, some
earthenwares are much less absorbent than others,
and I would expect that less-absorbent ones will
contribute to less crazing. Most commercial low-
fire bodies of my experience are quite porous at
^04, but often behave well up to ^1 or so, when
they start to be much less porous but could still
be called earthenware.
When we say 'earthenware', especially with regard
to museum pieces, we seldom know what the composition
of the clay body is, or how hot it was fired. Some
earthenwares are 'earthier' than others, and I
believe that typical commercial low-fire bodies are
generally on the 'more porous' end of the
vitrification scale for glazed earthenware.
My point, however, is that even a badly crazed
glaze seems like a very limited vector for health-
threatening bacteria. (Anyone out there have
actual test data to confirm or deny this? Idle
speculation is fun, but facts are best.)
-Snail
pinacoid3000@LYCOS.COM on fri 12 aug 05
Hello Snail,
Whilst theres nothing in your post I would disagree with my experience of
earthenware appears different to yours. My kitchenware is a rag tag
collection of pots but are largely earthenware. Decades of faultless
service only being marred when clumsy hands have dropped them!
I have seen old and new pieces that have crazed, and have always put
crazing on food dishes down to bad workmanship due to incompatible body
and glaze or excessively high porosity
A good bet for durable earthenware would be:
i. Thermal expansion of body to be slightly higher than glaze to ensure
some compression
ii. Not excessive water absorption, may be 5% ish
iii. Inclusion of some calcareous and magnesia rich raw materials which
have been shown to produce a body with lessened moisture expansion
On the subject of water absorption it might be worth noting that many so
called vitreous bodies do show an extent of open porosity
Regards
Andy
Snail Scott on fri 12 aug 05
At 06:10 AM 8/12/2005 -0400, Andy wrote:
>Hello Snail,
>
>Whilst theres nothing in your post I would disagree with my experience of
>earthenware appears different to yours. My kitchenware is a rag tag
>collection of pots but are largely earthenware. Decades of faultless
>service only being marred when clumsy hands have dropped them!
>
>I have seen old and new pieces that have crazed, and have always put
>crazing on food dishes down to bad workmanship due to incompatible body
>and glaze or excessively high porosity...
Actually, it sounds as if your experience is much
like mine! Well-made earthenware works well as
functional pottery, and a properly vitrified low-fire
body is quite strong.I merely suspect that many
commercial bodies are not actually at their best
if fired to the standard ^04, resulting in (as you
mentioned) overly absorbent, fragile ware prone to
bad glaze fit.
By the way, you seem to have looked into the
moisture-expansion issue more thoroughly than I
have. Can you decribe the mechanism that causes
it? I have trouble visualizing how fired ceramics
(even if underfired) can expand like a sponge,
though it certainly does so.
-Snail
pinacoid3000@LYCOS.COM on fri 12 aug 05
Hello again Snail
Thanks for the question as it forced me to remember back a few years.
Consider three conditions of a body and glaze
i.) TE of body is lower than glaze. On cooling from the kiln the glaze
will contract more and if the resultant stress is sufficiently high
crazing will occur
ii.) TE of body is greater than glaze. On cooling from the kiln the body
will contract more than the glaze. If the contraction is too great
peeling / shivering will occur. However if the difference is just right,
at 500oC around 0.06% difference, the glaze is put in compression. An
ideal situation as not only does it help resist crazing but increases the
overall mechanical strength.
iii.) TE of body and glaze are equal. The system is finely balanced, and
whilst the glaze has not helped increase the strength, nothing may happen
unless the conditions change
I understand that when a porous pot experiences water ingress two
consequences are:
a.) Water is adsorbed on the surfaces causing a reduction in surface energy
b.) Water can diffuse into the lattice of the glass and crystal phases
Both these give rise to a volume increase, which admittably is small, I
seem to recall a value of 0.03%. However in the system described in iii)
such an increase would be sufficient to put the glaze in tension, and
hence craze. However in situation ii.) the increase is insufficient to
leave a glaze in tension, and hence no crazing
That=92s the theory. What Im more confident to discuss is practical methods.=
The use of an autoclave is a recognised life cycle testing of pots and
their craze resistance. In effect the pots are put in a humid environment
at a specified pressure and hence moisture is forced in to the pores
The inclusion of a small amount of calcareous and magnesia rich raw
materials can favourable change the moisture expansion characteristics of
bodies: feldspathic glasses increase more than Ca or Mg crystalline
phases. Also calcium and magnesium compounds can have a mineralizing
effect that promotes the formation of cristobalite which will also help
increase craze resistance
Hope that helps, and thank you again for the mental stimulation
Andy
Ron Roy on sun 14 aug 05
Hi Jennifer,
There are problems with making earthenware functional - especially in
todays kitchens.
When potters start asking me questions like this I try to explain the
technical issues to them - and I advise them to switch to cone 6 - where
the same kind of colour are possible - simply because the technical
problems are easier to solve.
At the heart of the matter is the fact that earthenware clay is all but
impossible to make vitrious at earthenware temperatures. This means that it
is difficult to prevent water from being absorbed into the clay. It can
happen at stoneware temperatures as well but at least it is possible to
correct the fault.
Clay that has absorbed water will heat up in a microwave - so any pottery
that may end up in a microwave oven should have a warning impressed into
the clay - warning that it may get very hot when heated in a microwave
oven.
Water that is absorbed into earthenware clay can rehydrate the clay and the
resulting increase in size will likely craze any glaze over time. There are
special low fire clays - high in Talc - that will resist the rehydration
and also raise the expansion of the clay - to help prevent crazing and
delaysed crazing.
This absorption can also result in food liquids being absorbed into the
clay - which can result in smelly pots over time. Not such a bad thing in
the past - but not acceptable in many of todays kitchens.
Bacteria can grow in this situation - so such pots are not ideal for
storing food.
I'm not saying it can't be done - but it needs a lot of understanding to be
done properly - and I don't think most studio potters have the control or
knowledge to do it for todays kitchens.
It is also more difficult to make stable glazes at lower temperatures -
because you need silica and alumina to make durable liner glazes - but you
also need more flux to melt glazes at lower temperatures - so the silica
and alumina are lowered.
My suggestion is to start developing a cane 6 line for your functional ware.
RR
>First Hello, I love this web site, and I have probably read every single
>archive! I have been making pottery since 2000, and I just bought my
>first kiln. Well its my second kiln, the Paragon sits in the corner
>waiting for an electric upgrade on the house. Anyways, I have fired my
>kiln about 10 times in the last month and each one gets better and
>better! My question is that if the glaze is foodsafe, does that still
>mean that my earthenware clay fired to 04, then 06 is foodsafe. I don't
>make a lot of functional stuff, but I have started leaning more that
>way, candy dishes or platters, but mostly I like decorative pots,
>vibrant shapes and colors..ok I am off to play, TIA,
>Jennifer Nesbitt
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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