David & Diane Chen on sat 1 may 99
Maybe if I were a bit further along in glazes I wouldn't need to ask this
question. However, isn't there a way to determine if something is going to =
be a
problem before you work out all the problems, decide to love the glaze? =
Then
you send it to this man at Alfred and the recipe to John H, and ta da, it's =
a
horribly poisonous or even mildly unsatisfactory glaze. I know I can read =
about
how hazardous each ingredient is but then there is the issue of how it =
reacts
with the other things in the glaze. And to make matters even more =
confusing,
this morning in the posts, I read that barium under 7=25 might be okay. =
Yes, I
could avoid it completely, and am really just using barium for illustrative
purposes, but IS there a way to find out prior to spending so much time in =
the
development phase?
Diane
Massachusetts
John Hesselberth on sun 2 may 99
David & Diane Chen wrote:
>----------------------------Original message----------------------------
>Maybe if I were a bit further along in glazes I wouldn't need to ask this
>question. However, isn't there a way to determine if something is going
>to be a
>problem before you work out all the problems, decide to love the glaze? Then
>you send it to this man at Alfred and the recipe to John H, and ta da, it's a
>horribly poisonous or even mildly unsatisfactory glaze. I know I can read
>about
>how hazardous each ingredient is but then there is the issue of how it reacts
>with the other things in the glaze. And to make matters even more confusing,
>this morning in the posts, I read that barium under 7% might be okay. Yes, I
>could avoid it completely, and am really just using barium for illustrative
>purposes, but IS there a way to find out prior to spending so much time in
>the
>development phase?
>
>Diane
>Massachusetts
Hi Diane,
No, sorry, but with our current state of knowledge there is no simple way
to assure that the glazes you formulate will be durable without testing
them. We are all struggling to learn how to do this. You can increase
the odds they will be durable by staying within published limit formulas,
but you still have to test. I have seen durable glazes outside of limits
and non-durable glazes inside of limits. Unfortunately most of the
serious research on glazes stopped about 50 (or so) years ago and
leaching of lead and crazing were the only durability issues studied with
any thoroughness. The only other way I know of is to stay completely
away from questionable materials and even then you should test for
durability in use.
To increase your odds to the maximum I would 1) stay within limits, 2)
stay away from lead, cadmium, antimony, barium, chrome, nickel, uranium,
vanadium and copper (have I missed any?? what about zinc??) 3) use only
low levels of cobalt and manganese (say less than 2%), 4) use at least
three or four fluxes from the list of K, Na, Ca, Mg, Li and Sr, 5) learn
to like tans, oranges, browns (from iron) or blacks (from iron plus
cobalt) or blues (from cobalt) or whites (from zirconium, tin or
titanium). And then still test. I probably have missed a couple things
here, but I hope someone on the list will chime in to complete it if I
have.
The above might give you a pretty boring set of glazes. On the other
hand, depending on your needs and your style of pottery this might be
just great. Maybe we should have a Clayart contest to see who can
formulate the most attractive glazes from the above guidelines. In any
case this would be a pretty good set of guidelines to follow for a person
who is just starting in glaze formulation. Then as you learn some of the
chemistry and the safety issues you could gradually expand to using other
materials. On the "stay away from" list above, the only one I use is
copper and I work hard to keep it from leaching at high levels. I
probably would use chrome and nickel if I had a need. The others, I just
wouldn't use or don't see why I would want to.
I'd be interested in other's thoughts on the above guidelines to yield a
"high probability of being safe and durable" glaze.
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"
David & Diane Chen on mon 3 may 99
John, Your post is going to be taped to my wall. Thank you. One question,
though, where do stains fit into this glaze development issue?
Diane
John Hesselberth on mon 3 may 99
David & Diane Chen wrote:
>John, Your post is going to be taped to my wall. Thank you. One question,
>though, where do stains fit into this glaze development issue?
>
>Diane
>
Hi Diane,
Stains are nothing more than mixtures of colorant oxides which have been
compounded and calcined in a way that makes them more reliable and
predictable than using the raw materials. The general composition (the
metal oxides they contain) of each one can be learned from the
manufacturer, but they normally don't disclose the specific percentages.
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"
Ron Roy on sun 9 may 99
Hi Diane,
There really is no way to tell if a glaze will leach significant amounts of
toxic material into food except by testing them.
I do believe we are on our way to being able to make better guesses because
of the work of John Hesselberth and Dr. Hale at Alfred.
There are just too many factors involved for us, as potters, to be sure.
There are ways we can eliminate the problem however. Simply do not use
materials with know toxicity in glazes that will be in contact with food.
This is not as onerous as it seems - there are lots of oxides that do not
present a problem and using them can produce beautiful functional glazes.
We have been using glazes out of books for so long we seem to have gotten
off the track completely. I do think we will find many acceptable glazes
which will be both functional and interesting now that we are looking for
them.
In the meantime - I encourage all of you who are interested in contributing
to send your glazes to Alfred and have them tested - and give Dr. Hale the
recipes and permission to use the results in his data base. Who knows -
this may be your chance at immortality.
RR
>----------------------------Original message----------------------------
>Maybe if I were a bit further along in glazes I wouldn't need to ask this
>question. However, isn't there a way to determine if something is going
>to be a
>problem before you work out all the problems, decide to love the glaze? Then
>you send it to this man at Alfred and the recipe to John H, and ta da, it's a
>horribly poisonous or even mildly unsatisfactory glaze. I know I can read
>about
>how hazardous each ingredient is but then there is the issue of how it reacts
>with the other things in the glaze. And to make matters even more confusing,
>this morning in the posts, I read that barium under 7% might be okay. Yes, I
>could avoid it completely, and am really just using barium for illustrative
>purposes, but IS there a way to find out prior to spending so much time in the
>development phase?
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
| |
|
