John Hesselberth on wed 29 may 02
on 5/29/02 12:03 PM, Michele Williams at fape4kids@NETZERO.NET wrote:
> This is a question for John Hesselberth.
>
> John,
>
> On your website you have a peach gloss glaze that is described as "a
> phase-separated "floating" glaze".
>
> For the newbies like me, would you tell us what that means? Sounds
glorious!
>
> Second, the note refers to "encapsulated cadmium". Is this a way of
making
> cadmium safe for food use or is it still a no-no?
>
> Is this glaze food safe?
>
> Michele Williams
Hi Michelle,
First let me say that many of the glazes posted on my site are not mine and
I have never used them myself. They are glazes that other people tested for
leaching and they gave me permission to post their results. Peach Gloss is
one of those glazes. I have never used it.
Glazes will often separate into two or more phases as they cool. Sometimes
this is a crystalline and a glassy phases (as in properly formulated matte
glazes); sometimes two glassy phases. Taylor and Bull give a little sketch
of 2 glassy phases on page 171 to help you visualize this. They are talking
about the effect of this phenomena on chemical resistance on that particular
page, but this same effect is what gives glazes some of their more
interesting optical properties. I'm assuming from the description of Peach
Gloss that it separates into two glassy phases where one phase appears to
float within the other. Each of those phases has different optical
characteristics because they have different compositions. An analog might
be oil and vinegar salad dressing right after you have shaken it up. Little
globs of oil are suspended in the vinegar matrix. That is also probably some
of what is going on with Waterfall Brown, although I have never done the
testing necessary to prove it. Visually it looks like that is happening.
Encapsulated cadmium is a proprietary technology designed to make cadmium
less soluble to leaching while still getting the benefits of its color. Does
it work? I don't know; the manufacturer says it does. Whether it does or
not, it is still YOUR responsibility to assure that your glazes do not leach
more than the law allows if you put cadmium or lead containing glazes on
pots that could be used for food. In practical terms this means you would be
very wise to have a statistically designed quality control program with a
rigorous leach testing schedule if you were going to use these materials on
surfaces that might be exposed to food. YOU are the one that is liable; the
manufacturer has built in all kinds of disclaimers to avoid that liability
himself. Would I use them on functional pottery? Not on your life! There
are too many other safe, attractive colors to choose from and the cost of a
testing program that would assure they were safe would drive a small-volume
potter right out of business. My recommendation: If you want to have shades
of red on functional pottery then learn to reduction fire to get copper reds
or use chrome/tin reds like the Raspberry glaze in our book or use iron
reds. Leave cadmium to the commercial outfits who have the volume that
supports the necessary testing costs.
Regards,
John
Web sites: http://www.masteringglazes.com and http://www.frogpondpottery.com
Email: john@frogpondpottery.com
"The life so short, the craft so long to learn." Chaucer's translation of
Hippocrates, 5th cent. B.C.
Michele Williams on wed 29 may 02
This is a question for John Hesselberth. =20
John,
On your website you have a peach gloss glaze that is described as "a =
phase-separated "floating" glaze".
For the newbies like me, would you tell us what that means? Sounds =
glorious!
Second, the note refers to "encapsulated cadmium". Is this a way of =
making cadmium safe for food use or is it still a no-no?
Is this glaze food safe?
Michele Williams
Michele Williams
Edouard Bastarache on thu 30 may 02
Hello John & all,
> Encapsulated cadmium is a proprietary technology designed to make cadmium
> less soluble to leaching while still getting the benefits of its color.
Does
> it work? I don't know; the manufacturer says it does.
It does according to Smart.Conseil in France who works for a large
factory. Of course you have to test, so says the law.
Check the archives, I translated some of his texts posted last year.
Edouard Bastarache
Irreductible Quebecois
Indomitable Quebeker
Sorel-Tracy
Quebec
edouardb@sorel-tracy.qc.ca
http://sorel-tracy.qc.ca/~edouardb/
http://perso.wanadoo.fr/smart2000/index.htm
Michele Williams on thu 30 may 02
Thanks, John! I've wondered how to get the floating effect, wondered if
such a recipe is available, etc. Now that you've explained, I've marked it
as "Sculptural use only". I'm I think I'm okay for liability that way
unless someone eats a sculpture.
For your patience with a newbie, my gratitude.
Michele Williams
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