Marty Cugelman & Gail Pasternack on fri 26 feb 99
I would like to know what ingredients in a glaze makes the surface extra
hard. Most of the tests I have done do not pass my scratch test. I would
like to make dinnerware and I feel it has to not only be food safe but
should also not scratch with a spoon, knife and fork. Any simple solutions?
Any recommendations on books to read that cover this topic? I am working at
cone 6 oxidation.
Gail Pasternack
Ballantrae, Ontario
--
Judith Enright on sat 27 feb 99
>----------------------------Original message----------------------------
>I would like to know what ingredients in a glaze makes the surface extra
>hard. Most of the tests I have done do not pass my scratch test. I would
>like to make dinnerware and I feel it has to not only be food safe but
>should also not scratch with a spoon, knife and fork. Any simple solutions?
>
>Any recommendations on books to read that cover this topic? I am working at
>cone 6 oxidation.
>
>Gail Pasternack
>Ballantrae, Ontario
>--
This brings to mind a practice that my mother-in-law adopted with respect
to her use of a set of bone china: She would serve only foods that did not
require cutting-up with a knife; indeed, no knives were set at the table
when this china was used, and we were all admonished to tread ever so
lightly when employing our forks. Of course, the dishwasher was out of the
question. But the entire set looked as new as the day she bought it 50
years ago!
Priscilla was a very nifty person, full of grace and with elegant manners.
She died in January from cancer, and spent her last two months at home
being tended to by a very capable and compassionate hospice team and her
family. She was particular about how things were to be done. Around
Thanksgiving, I had prepared a small bowl of fruit salad, brought it into
her and was gently reminded to place a saucer under it and to use the
correct fruit fork. Went back to get the saucer and fork; the saucer was
okay but I had brought a salad fork. Went back, brought another fork, it
was a fish fork. Went back, scratching my head and asked my father-in-law
(Tom) what a fruit fork looked like. He didn't know either, but he figured
that we had only two choices left (dinner fork and something smaller) so I
tried the something smaller and finally got it right. She and I laughed
about it -- great to see her find humor as she awaited her time.
Tom is visiting us this weekend and bringing Priscilla's ashes. For the
family, I have made a series of miniature lidded "memory" pots and bottles
and will be glazing them using some of her ashes. It will be a special
time, and a sort of closure since she and I had worked together on the
concept the week before she passed on.
A nifty lady, indeed.
Be well....
Judith Enright
Judith Enright at Black Leopard Clayware
BLEOPARD@ricochet.net
(408) 947-1723
John Hesselberth on sun 28 feb 99
Gail, Are your glazes being scratched by utensils or marked by utensils?
If it is true scratching you may not be firing the glaze to
vitrification or, perhaps, have a glaze very high in boron which is known
to be softer than silica. If it is marking it is probably because you
have a mat surface. Mat surfaces are more abrasive that glossy surfaces
and, therefore, are more likely to abrade the utensil leaving behind a
mark--a thin layer of metal from the utensil. I am reading between the
lines, but it sounded as if you may have marking. John
Gail Pasternack wrote:
>----------------------------Original message----------------------------
>I would like to know what ingredients in a glaze makes the surface extra
>hard. Most of the tests I have done do not pass my scratch test. I would
>like to make dinnerware and I feel it has to not only be food safe but
>should also not scratch with a spoon, knife and fork. Any simple solutions?
>
>Any recommendations on books to read that cover this topic? I am working at
>cone 6 oxidation.
>
>Gail Pasternack
>Ballantrae, Ontario
>--
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"
Bill Palmer on tue 2 mar 99
At 11:09 AM 2/26/99 EST, you wrote:
>----------------------------Original message----------------------------
>I would like to know what ingredients in a glaze makes the surface extra
>hard. Most of the tests I have done do not pass my scratch test. I would
>like to make dinnerware and I feel it has to not only be food safe but
>should also not scratch with a spoon, knife and fork. Any simple solutions?
>
>Any recommendations on books to read that cover this topic? I am working at
>cone 6 oxidation.
>
>Gail Pasternack
>Ballantrae, Ontario
>--
>
>There is a good list of the relative hardness of materials in a book called,
Pocket Ref, published by Sequoia Publishing in Morrison, Colorado. There
are very few materials harder than 7 (quartz) on the Mohs scale. One of
those is Zircon at 7.5. Floor tile manufacturers load their glazes up with
zirconium opacifiers to harden them beyond the ultimate floor tile
scratcher, sand(often mostly quartz).
Bill Palmer
Technical Support Manager
ClayPeople
112 Ohio Ave., Richmond, CA 94804
Toll Free 888-236-1492 ext. 324
Local 510-236-1492 ext. 324
Fax 510-236-2777
Email ceramic@slip.net
http://www.claypeople.net
Bill Palmer on tue 2 mar 99
At 11:09 AM 2/26/99 EST, you wrote:
>----------------------------Original message----------------------------
>I would like to know what ingredients in a glaze makes the surface extra
>hard. Most of the tests I have done do not pass my scratch test. I would
>like to make dinnerware and I feel it has to not only be food safe but
>should also not scratch with a spoon, knife and fork. Any simple solutions?
>
>Any recommendations on books to read that cover this topic? I am working at
>cone 6 oxidation.
>
>Gail Pasternack
>Ballantrae, Ontario
>--
>
>There is a good list of the relative hardness of materials in a book called,
Pocket Ref, published by Sequoia Publishing in Morrison, Colorado. There
are very few materials harder than 7 (quartz) on the Mohs scale. One of
those is Zircon at 7.5. Floor tile manufacturers load their glazes up with
zirconium opacifiers to harden them beyond the ultimate floor tile
scratcher, sand(often mostly quartz).
Bill Palmer
Technical Support Manager
ClayPeople
112 Ohio Ave., Richmond, CA 94804
Toll Free 888-236-1492 ext. 324
Local 510-236-1492 ext. 324
Fax 510-236-2777
Email ceramic@slip.net
http://www.claypeople.net
Michael Banks on wed 3 mar 99
Gail,
One of my industrial ceramics texts (Singer & Singer) lists the following
glaze constituents as providing beneficial effects on hardness (in order of
favourable effect): boric oxide, alumina, baria, magnesia and zinc. Note
that these are minor additions and their effectiveness is dependant on their
individual solubilities in the glaze. Limit formulae (for gloss glazes) for
each component give a rough guide to their solubilities.
For example, if you add alumina, baria, magnesia and zinc beyond their
individual solubilities, matt glazes result. The hardness of matt glazes
depends on the hardness of the mineral crystals causing the matting of the
surface. In most cases these minerals are actually harder than the glaze,
but matt glazes are not neccessarily desirable. For example on the surfaces
in contact with food, for a number of reasons.
Boron initially enhances the hardness of gloss glazes, but only up to a
certain amount added. There is a point where all the available sites to
boron (or as I've heard them called on clayart "holes",) in the glaze
lattice are occupied. If more boron is added, soft borate glass rich
inclusions are exsolved, softening the overall glaze surface.
One of the best ways to get the hardest glaze possible is to select a glaze
which uses the above fluxes, but with the highest silica content possible.
Obviously, the higher you fire, the more silica you can squeeze into a
glaze. The ultimate would be to fire to 1750 degrees C which would allow you
to use a pure silica glaze with a moh hardness of 7. Lesser mortals firing
to cone 6-10 can do no better than maximising silica (or zircon as noted by
Bill Palmer) content of their glazes, consistent with achieving a gloss
surface. Of the matt glazes, alumina, zirconia and magnesia matts are
probably hardest and can exceed moh 7.
If you're very serious (fanatical) about hard glazes, adding beryllia
(beryllium oxide), or beryl (beryllium aluminium silicate) is the way to
go. Beryllium silicate glass has a hardness of moh 8. But beryllium
compounds are very toxic and are to be avoided.
Michael Banks,
Nelson,
New Zealand
>----------------------------Original message----------------------------
>I would like to know what ingredients in a glaze makes the surface extra
>hard. Most of the tests I have done do not pass my scratch test. I would
>like to make dinnerware and I feel it has to not only be food safe but
>should also not scratch with a spoon, knife and fork. Any simple
solutions?
>
>Any recommendations on books to read that cover this topic? I am working
at
>cone 6 oxidation.
>
>Gail Pasternack
>Ballantrae, Ontario
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