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leaching and toxicity

updated mon 23 nov 98


Joseph Herbert on thu 19 nov 98

Carrie wrote: Will the toxicity leach through the porcelain and the glaze and
into the inside? Or are you
safe using a glaze like this as a decorative accent?

Carrie, as far as a glaze is concerned, leaching is a passive activity.
Leaching is done to the glaze by an acid liquid removing metal ions from the
glaze surface. So a glaze just sitting there doesn't "leach." Notice that
there is a liquid component to this leaching activity. Since porcelain is, by
definition, nonporous, no liquid can move through the wall of the vessel to
carry (leach) the metal ions from the outer surface to the inner surface where
the ions could be taken up by food.

Secondly, toxicity is not a thing, it is a condition. The metal ions, which
cause toxicity in humans and other animals, are good and useful in other
places that are not in the human body. The danger is in the transfer of the
metal ions into the human user in some soluble form. There are several
barriers to prevent this result and, if we keep them in place, all is well.
If not, then there is danger.

1. The glaze surface is impervious to attack by any leaching agent. (Well-
balanced Glaze)
2. There is no transfer of a leaching agent into the user. (Glaze not in
contact with food)
3. No leaching agent is ever present. (Vessel not used for food or drink)
4. The glaze contains no toxic agent. (No lead, barium, etc., in the glaze)

A well made, Well fired balanced glaze that is free of toxic metals uses two
of the barriers and is very safe. If you are flirting with death in the name
of beauty, you need to apply additional care.

Joseph Herbert

Ron Roy on sun 22 nov 98

Hi Carrie

When I first saw your question I did as I always do - wait to see what
others answer and then - if I have something to add I do - saves me some
time at least.

I was going to answer - you don't have to worry about the glaze on the
outside - but now that I see the different answers I am not so sure - if
the clay is vitrified enough to stop water getting in to it then there
seems to be no problem.

There are new rules in Canada now - and I think else where as well - if a
glaze contains lead or cadmium the glaze must be a certain distance down
from the rim on the outside.

But back to the clay part - if the ware is not sufficiently vitrified to
exclude water and say the inside glaze is crazed - could not leachate get
into food and would it be problematic?

It comes down to firing clay to a proper vitrification for use with food.
This means we all should be testing our clays to make sure they are good
enough for the purposes we intend them.

When I see clay manufactures advertising clay with a maturing range of say
6 to 10 I know it will not be vitrified enough at cone 6 to be functional
in all the situations our customers will assume.

I maintain the bodies at a clay company and know - continual testing and
of a clay body is necessary to keep it properly vitrified at ONE cone -
never mind several.

It is next to impossible to make the kinds of clay we use at lower cones
vitrified enough to exclude water in use. They should in fact be labeled as
"limited functional."

Comments welcome as usual - RR

Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849

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