Maria Elaine Lanza on tue 30 mar 99
I realize that the subject of manganese has been discussed at length in
previous posts, however the question came up again for me again after
seeing mugs lined with a really gorgeous black liner glaze that the artist
admitted to be heavily saturated with oxides, including manganese... the
explanation given was that it burns out during the firing... Is that so? I
was under the impression that this ingredient despite the amount used in a
glaze or with any combination is something that should be avoided entirely
for foodsafe ware.
As beautiful as they were in design and overall visual effects... I didn't
get one because of this... If its true that the manganese does burn during a
^6 firing... then I missed out on having another wonderful mug added to my
collection or better still one more to use for my morning cup of coffee.
Please advise, am appreciative of your responses, Marie Elaine
John Hesselberth on wed 31 mar 99
Hi Maria Elaine,
You will probably get lots of opinions on this, so you will ultimately
have to make up your own mind. The primary concern regarding manganese
is with respect to inhaling dust or fumes. There actually is quite a bit
in the food that we eat and, in fact, we need a tiny bit. The
Recommended Daily Intake for manganese is 2 mg. and most people
apparently get 2-8 mg./day from the food they eat. Wheat and rice bran
are particularly high in manganese. That said, I don't like to get my
mineral supplements from pots and I don't like to feed metals to my
customers in any higher quantity than necessary. Manganese doesn't burn
out in firing to any appreciable degree. Even if it were reduction
firing and the manganese dioxide were reduced to manganese metal, it
doesn't boil until 2095 degrees Celcius. That is just a tad higher than
most potters fire--like about 8-900 degrees Celcius higher.
I certainly wouldn't sell a glaze like you are describing unless I had it
tested for extractable manganese and I wouldn't drink from one either.
But I don't consider manganese off-limits for food surfaces--just have it
tested to make sure the extractables are in an acceptable range to you.
There are ways to make an attractive black liner glaze that has extremely
low extractables using only iron and cobalt. The glazes previously
posted here, which were developed by Ron Roy (RR Black #2 and #3), are a
good place to start. Check the archives to find them. In the meantime,
try to get your friend to have her glaze tested before she sells any more
of it. It is easy and cheap and there is little excuse for not doing it
these days.
John
Maria Elaine Lanza wrote:
>----------------------------Original message----------------------------
>I realize that the subject of manganese has been discussed at length in
>previous posts, however the question came up again for me again after
>seeing mugs lined with a really gorgeous black liner glaze that the artist
>admitted to be heavily saturated with oxides, including manganese... the
>explanation given was that it burns out during the firing... Is that so? I
>was under the impression that this ingredient despite the amount used in a
>glaze or with any combination is something that should be avoided entirely
>for foodsafe ware.
>
>As beautiful as they were in design and overall visual effects... I didn't
>get one because of this... If its true that the manganese does burn during a
>^6 firing... then I missed out on having another wonderful mug added to my
>collection or better still one more to use for my morning cup of coffee.
>
>Please advise, am appreciative of your responses, Marie Elaine
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"It is time for potters to claim their proper field. Pottery in its pure
form relies neither on sculptural additions nor on pictorial decorations.
but on the counterpoint of form, design, colour, texture and the quality
of the material, all directed to a function." Michael Cardew in "Pioneer
Pottery"
Tony Hansen on thu 1 apr 99
> after seeing mugs lined with a really gorgeous black liner glaze the
> explanation given was that it burns out during the firing... Is that so?
Yikes! It's there to make the black, if it's black it didn't burn out.
I had a similar experience at NCECA where a well known author told me:
"Every ceramic engineer will tell you that lead is completely harmless
as long as it is fritted"!
--
T o n y H a n s e n thansen@digitalfire.com
Don't fight the dragon alone http://digitalfire.com
Calculation/Database Software for Ceramic Industry
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