primalmommy on mon 18 aug 03
John, thanks for your explanation.. that helps clarify things. Does
shivering happen right away? Upon cooling? can you tell when something
is going to shiver? The surface of the falls creek has that nice buttery
rutile-clayish feel to it.. I can't picture shards and slivers popping
off.. but what do I know?
I have to admit I was wishing today someone would do a safe-glazing
workshop for guilds and schools. I have been to more than one workshop
where the glaze info offered sounds way too scary for me to want to try.
And I often look at liner glazes of beginner potters or those who aren't
familiar with /didn't make/don't understand glazes, and wonder... What's
that glaze? "I dunno, it was in the bucket"..
Not sure that makes me hysterical. I buy organic food too, which some
people think is silly. Different strokes.
Personally, I miss Monona Rossol. It doesn't surprise me that she
prefers to write for a magazine. I don't know how long I could advocate
for safety on a list where every statement I made - no matter how well
researched and documented -- meant abuse heaped on my head, name
calling, and accusations of paranoia and fearmongering. Everybody wants
to shoot the messenger because it's easier than relearning your old
standard glazes or wondering if you've sold toxic pottery. Probably
better for her to write a column and get an occasional "letter to the
editor".
Disclaimer: I know not much about glazes. I mix my own and I have
"mastering ^6 glazes". I never met Monona.
And I haven't written a book and I'm not selling a damn thing. How
annoying to have the contributions people have made to this list -- or
potters anywhere -- written off by others as "sales pitches" and self
serving.
I would rather "live in fear" than live with such a cynical view of
human nature.
Yours, Kelly in Ohio... crabby and sore after more garage-remodeling,
tired of the big framing hammer, the mouse poop and the sawdust...
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primalmommy on mon 18 aug 03
OK this is beginner stuff but I have to be sure:
If a glaze is tested and declared "stable", does that mean food safe?
I have been using a Falls Creek Shino (^6) from John Hesselbarth, found
it online... it is "stable". But then I read scary stuff about lithium
and wonder. I remember Ron Roy once saying the concern with fired
lithium is NOT leaching, but shivering... (?) Or do I?
I am afraid to use it inside of things, anyway.. I am not likely to buy
another big load of Albany Slip so this is probably my last batch
anyway. I have tried a lot of other cream-colored-breaking-reddish
glazes without much luck, and my potter friend Dave cayton (the guy in
LaCrosse withthe cool moustache) has been equally disappointed trying
albany substitutes. Sigh.
Anyway... does anyone know if "stable" means "OK for a baby's orange
juice cup"? (That's my standard for pottery... I sleep better with that
one than I would with the "I use it and I haven't died yet" approach.
Yours, Kelly in Ohio... where I spent the weekend cat-walking on 2x12
rafters in the garage-soon-to-be-woodshop with a makita and a box of
screws... while jeff (who is a great builder but dislikes heights)
shoved 4X8 ft plywood sheeting up there for me to screw down for
flooring. Jeff came home from ACC in Tennessee jonesing for a wood
turning shop... and as supportive as he has been of my pottery
addiction, I feel like I have to make it happen! Anybody got a second
hand lathe, or some turning tools you'd like to get rid of? We'd drive a
long way for a good deal...
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John Hesselberth on mon 18 aug 03
Hi Kelley,
It's not beginners stuff. You, of course, are asking a question for
which there is no exact answer. We chose to use the terms "stable" and
"durable" in our book because there is no agreed-on definition of what
"food safe" means. The FDA says it can be used for food if it doesn't
leach more than certain amounts of lead and cadmium. By that
definition, any glaze you make will be food safe because I know you
wouldn't have either material anywhere on your property. At the other
end of the scale are people (for example Monona Rossol) who will say we
should use drinking water standards as the definition of food safe. She
would propose that if no more of whatever material leaches into 4%
acetic acid in 24 hours than would be allowed in drinking water it
would be OK. That is a very harsh standard--but you may choose to
follow it. The only real life situation where you might get that kind
of exposure is if you stored orange juice in a pitcher at room
temperature for a couple days or more. Or maybe in you made pickles in
a ceramic container--and then drank as much pickle juice as you do
water. I think it is more strict than is reasonable, but no one has any
data to prove me right or Monona wrong. I would point out that nearly
all the glazes in Mastering Cone 6 Glazes meet that standard, though,
except for a one or two of the copper-containing ones. That is, where a
standard exists. There is no water standard for cobalt or several
other things we potters use.
So Ron and I went after stable glazes knowing that no glaze should
change color or dissolve in use--whether or not there is anything toxic
in it. And, as noted above, if they are stable by our definition they
come very close to meeting that very stringent water standard. Pots are
supposed to last forever and should, in my opinion, not fade in use. As
an aside I have 2 mugs I purchased that are leached out on the
inside--the color is virtually gone--after about 2 years of regular
use. Needless to say my opinion of that potter has gone from high to
low. She did not practice acceptable craftsmanship in my opinion. Did I
get poisoned? Almost certainly not.
So what about Falls Creek Shino? Would I eat off it from a leaching
standpoint? Of course. Would I feed my granddaughter from it? Yes. What
I would be careful about is to check it very thoroughly for shivering
propensity. It has been reported here a number of times that
lithium-containing glazes can exhibit peculiar behavior with respect to
shivering and/or crazing, particularly when the lithium is sourced from
lithium carbonate. I know Ron recommends no more than 2% lithium
carbonate in a glaze and this one has 6.5%. It is a material for which
we do not have complete understanding of its behavior in glazes.
So does stable mean "food safe"? No, because there is no agreed on
definition of what that term means. Do I think a stable glaze (by our
definition in MC6Gs) is food safe, personally? Yes, but that is my
opinion only. I am toward the conservative end of the spectrum and, as
you have seen in the last couple days on Clayart, there are people who
would be a lot more liberal than I with their definition of food safe.
I hope this hasn't confused too much, but the bottom line is that you
will have to decide for yourself.
Regards,
John
On Monday, August 18, 2003, at 09:50 AM, primalmommy wrote:
> OK this is beginner stuff but I have to be sure:
>
> If a glaze is tested and declared "stable", does that mean food safe?
>
> I have been using a Falls Creek Shino (^6) from John Hesselbarth, found
> it online... it is "stable". But then I read scary stuff about lithium
> and wonder. I remember Ron Roy once saying the concern with fired
> lithium is NOT leaching, but shivering... (?) Or do I?
http://www.frogpondpottery.com
http://www.masteringglazes.com
Ron Roy on tue 19 aug 03
Hi Kelly,
John covered this in his usual complete way - but just to add a couple of
points.
Heat increases acid attack - so a casserole full of tomatoes in the oven is
something to keep in mind when talking about acid attack.
The - no more than 2% Lithium Carb - business came from some experiments
that David Hewitt did when he discovered a glaze that was crazing and
shivering on the same clay.
We tried to design some experiments that would show us what was happening
and I did the dilatometery. We could not deduce anything from those
experiments.
The best we could come up with was - Lithium Carb is more soluble in water
than any of the other carbonates we use. It also has a rather low
expansion. The soluble part of it migrates around in ware as the glaze
coating dries - we guessed that the concentrated Li2O was setting up
different layers of expansion in the ware and that must be the reason for
the strange fit results.
David made a guess that 2% should be the upper limit because he has never
seen the problem in glazes with that little.
The freezing and boiling water test is designed to aggravate low glaze
contraction problems - cooling further than room temp simply puts clay and
glaze under more fit stress - pouring boiling water into the still frozen
ware will make the inside glaze expand a bit before the clay does and split
the ware - showing the mis fit.
All this and you need to keep in mind - this is a problem with some clays
and not others. Only glazing the inside of ware - with the glaze a little
thicker than normal and the clay a little thinner is a good way to test for
glazes under compression - especially using the freeze/boil test.
How all this applies to the Lithium Carb problem is still up in the air as
far as I'm concerned.
RR
>I have been using a Falls Creek Shino (^6) from John Hesselbarth, found
>it online... it is "stable". But then I read scary stuff about lithium
>and wonder. I remember Ron Roy once saying the concern with fired
>lithium is NOT leaching, but shivering... (?) Or do I?
>
>I am afraid to use it inside of things, anyway.. I am not likely to buy
>another big load of Albany Slip so this is probably my last batch
>anyway. I have tried a lot of other cream-colored-breaking-reddish
>glazes without much luck, and my potter friend Dave cayton (the guy in
>LaCrosse withthe cool moustache) has been equally disappointed trying
>albany substitutes. Sigh.
>
>Anyway... does anyone know if "stable" means "OK for a baby's orange
>juice cup"? (That's my standard for pottery... I sleep better with that
>one than I would with the "I use it and I haven't died yet" approach.
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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