Holly Keeney on mon 22 jul 02
Can someone please tell me if majolica is microwave and/or dishwasher
safe? I have made a ton of mugs and hadn't thought about it, but now I am
off to try and sell them, so I figure I need to know in case someone asks.
Thank you.
Holly
Pottery by Dai on mon 22 jul 02
This isn't an answer to the question, but an extension to it. I have a
couple of majolica bowls that have become very dull with use. The glaze
actually feels very matt, and looks it. I don't use them in the microwave,
but I do put them in the dishwasher. Two questions: (1) is the dishwasher
causing the glaze to degrade, and (2) if I refire them at ^04 (or my ^06
bisque) will it re-gloss the glaze? Incidentally, I don't do majolica, so
am not very knowledgeable about this process.
Dai in Armstrong, BC
nightfire@telus.net
www.potterybydai.com
Take your work seriously---take yourself lightly. Unknown
Gavin Stairs on tue 23 jul 02
There is a problem with microwaving any piece which has absorbed water
into its pores. The water will be heated, its vapor pressure will
rise, and the vapor (steam) will have difficulty getting out of the
ceramic. The result is a stress on the material which may result in
spallation of the glaze, cracking, or even a full explosion.
This problem exists for most traditional majolica, which is based on
earthenware with a covering glaze and/or fusible slip, etc. In
almost all cases, the glaze coat will not be sufficiently impervious to
prevent liquid water from entering the ceramic body beneath the
glaze. Earthenwares are porous, with a system of pores which are
interconnecting. This is like a sponge, and it is sometimes called
an open cell foam. It does not exist for most stonewares and
porcelains, which are virtually solid, with no pores, and certainly no
network of connected pores throughout the material. That means that
earthenware can absorb water deep into its structure, while stoneware and
porcelain cannot.
There are many counter-examples of majolica and other porous ceramics
which have been microwaved often with no apparent damage. This will
be the case if the water infiltration has not penetrated very far past
the exit opening. Unfortunately, there is no way to guarantee that
majolica will behave this way. Therefore we have to say that
majolica is not microwave safe as a general rule. It is only in
very rare cases that we will observe a true explosion. It will be
more common to see a less dramatic failure, like a cracking similar to
dunting. There is a greatly increased chance of eventual failure of
handles and so forth, which leads to an increased danger to the user from
scalding liquids, etc.
As regards the gradual dulling of dishwashed ware, this is almost
certainly owing to the action of hot, caustic dishwasher detergent
dissolving parts of the glaze, particularly alumina. There has been
much discussion on the list of such glaze weaknesses as a result of Ron
and John's experiments with durable cone 6 glazes. Majolica glazes
are of course usually lower cone glazes, so their work is not directly
applicable. To the extent that the glaze surface becomes deficient
in alumina, it is reasonable to suppose that it will be easier to melt.
However, the glaze will also have lost flux, so it is a matter for
experiment. I would caution that such ware which has been used for
some time, if it is porous, will have absorbed water, and will be
susceptible to the kind of failure described above. In the greater
heat of the kiln, the failure is much more likely. Just as it is, I
might point out, if a mug were to be heated without water or food
inside. Others with more experience have suggested that it is nigh
impossible to refire such ware without damage.
So, the executive summary is: Earthenware (traditional) majolica is
not microwave safe. Stoneware and porcelain ware is generally
safe. Glazed earthenware which has been dulled by dishwashing
cannot usually be refired successfully: if the attempt is to be made, the
ware must be dried very slowly for a very long time.
Gavin
Gavin Stairs
Gavin Stairs Fine Editions
525 Canterbury Road
London, Ontario
Canada N6G 2N5
telephone: (519) 434-8555.
email: stairs@stairs.on.ca
Gavin Stairs Fine Editions is a small, computer press specializing in
book design and fine, hand-made books.
Cindi Anderson on tue 23 jul 02
In "Mastering Cone 6 glazes" they explain that dishwashing soap is alkaline,
and some glazes do not hold up well to either acid or alkaline environments.
I sounds like your glaze is not very stable in an alkaline environment. You
might wonder what these glazes are leaching, and are they actually food
safe?
Cindi in Fremont, CA
----- Original Message -----
From: "Pottery by Dai"
I have a
> couple of majolica bowls that have become very dull with use. The glaze
> actually feels very matt, and looks it. I don't use them in the
microwave,
> but I do put them in the dishwasher. Two questions: (1) is the
dishwasher
> causing the glaze to degrade, and (2) if I refire them at ^04 (or my ^06
> bisque) will it re-gloss the glaze? Incidentally, I don't do majolica, so
> am not very knowledgeable about this process.
Ron Roy on wed 24 jul 02
Hi Holly,
Traditional Majolica is low fired and the clay is usually not vitrified
enough to exclude water for atmosphere or use. This means the water will
eventually get into the clay - under the glaze. When the ware is heated in
a microwave it will get hot - hot enough to badly burn hands for instance.
It may also be possible that the super heated steam under a glaze could
result in an explosion - so I would say no for low fire Majolica.
It is possible to make the same looking ware at cone 6 and have the clay
vitrified. Then it would be OK.
Any clay - no mater what temperature it is fired to - if not vitrified
enough to exclude water - will have the same fault - the water in the clay
will get heated and the ware will get hot - including handles.
You can test for this fault - fill a container with water to protect you
oven - say a mug and place it in the oven with an empty example of the clay
you want to test - it can't just be out of a glaze firing because there
will have not been a chance for any water to be absorbed into the body -
best to do it with something that has been around for a while - in use. Or
soak a newly fired piece in water for a few days.
Turn oven on high for 10 seconds - touch the empty mug to see if it's
getting hot - repeat this until the water in the other mug boils - or until
the test piece gets hot. If it does not get hot the clay is vitrified
enough or it hasn't be exposed to water long enough for the water to get
in.
You can also test your clay - without glaze - for absorbency. If the %
absorbency is over 3% you can assume you have a possible problem and need
to test carefully to make sure either way.
If you need to know how to test clay for absorbency let me know.
RR
>Can someone please tell me if majolica is microwave and/or dishwasher
>safe? I have made a ton of mugs and hadn't thought about it, but now I am
>off to try and sell them, so I figure I need to know in case someone asks.
>Thank you.
>Holly
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
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Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Chris Campbell on wed 24 jul 02
Holly -
The majolica mugs I own get so hot in the microwave that they are
dangerous. I would strongly urge you to test yours in your own microwave.
They also crazed in the dishwasher...once again ... test this yourself
before you give any customer assurances of their safety.
p.s. these mugs were made by a very well known majolica potter. They
are not my own amateur attempts.
Chris Campbell - in North Carolina
Chris Campbell Pottery, LLC
9417 Koupela Drive
Raleigh NC 27615-2233
e-mail : ccpottery@aol.com
website : www.wholesalecrafts.com
Ron Roy on fri 26 jul 02
Hi Dai,
You can say it's the dishwasher but it is really the soap (alkalie.) If the
glaze is dulling inside and out it is the soap. If the glazes only dulls
where it touches food then it is acid. If the glaze dulls more on the
inside than the outside then it's the acid and the soap.
Can you refire to get the gloss back. Good question and I could only guess.
Some of the material in the glaze has been removed - so the glaze will not
be the same - one thing is for sure - whatever is happening will happen
again and sooner.
Please tell us what happens if you do refire them.
RR
>This isn't an answer to the question, but an extension to it. I have a
>couple of majolica bowls that have become very dull with use. The glaze
>actually feels very matt, and looks it. I don't use them in the microwave,
>but I do put them in the dishwasher. Two questions: (1) is the dishwasher
>causing the glaze to degrade, and (2) if I refire them at ^04 (or my ^06
>bisque) will it re-gloss the glaze? Incidentally, I don't do majolica, so
>am not very knowledgeable about this process.
>Dai in Armstrong, BC
>nightfire@telus.net
>www.potterybydai.com
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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