Gail Dapogny on thu 8 nov 01
Could someone send to me the latest message (Wednesday, I think) written by
Ron Roy on barium? I was about to print it for posting at our guild, and
somehow it got lost (deleted?). Ah, computers!
I would be most appreciative..
---Gail (gdapogny@umich.edu)
Gail Dapogny
1154 Olden Road
Ann Arbor, MI 48103-3005
(734) 665-9816
gdapogny@umich.edu
Ron Roy on fri 9 nov 01
Hi gail,
thanks for asking - from the horses mouth regards - RR
This is a complicated subject and it will help those of you who are trying
to decide if you want to use Barium in glazes if you consider the following
points.
White glazes do not need Barium -if you need another mid range flux
Strontium is a better choice because - you need less of it and because of
that you get lower expansion rates - which help avoid crazing. Strontium
for Barium subbing is not a good choice if your expansion is already low -
if that is the case sub in calcium but do it by calculation.
I'm sure it is possible to keep barium in a glaze if the amounts are on the
lower side and the glaze is stable. Lower side in my estimation is below
10%. Keep in mind - shiny (well melted) with enough alumina and silica can
mean stable but you can't tell by looking - have the glaze tested.
When crystals form - some rob the surrounding glaze of silica making that
part of the coating less stable - adding copper affect glaze leaching badly
- so increases the chance of all ions getting into many foods.
Then you have to decide how much barium - leaching into food is too much.
You may be able to make a good guess for the average adult but what about a
small child or a one month old fetus? Keep in mind - Barium Carb is an
effective rat poison - how does the soluble barium in the carbonate compare
with the barium ions leached out of a glaze into food?
Barium kills by paralyzing muscles - particularly heart muscles - the
effect is like a heart attack - so it may not be suspected as barium
poisoning. An important factor when you think that some who use your ware
may already have heart problems. I don't know what the long tern effects of
barium are - I do know it replaces calcium in bones.
Barium favours turquoise from copper and vivid blues from Barium - used a
lot in high barium glazes - chances are they are going to leach a lot of
barium - and change colour with some foods. Remember - heat makes the
leaching happen faster. These types of glazes - unless proven stable -
should not be used on food bearing surfaces.
Strontium with iron gives richer yellows and browns in my limited experience.
I won't use Barium simply because I don't know the answers to these
questions - I do know it's toxic and I can't find any data that says there
is no problem.
What I do know - there are lots of Barium glazes in use - on functional
ware - that are unstable. If there is ever a case of death or injury traced
to one of those glazes we are all going to suffer loss of sales. As it is -
I have seen cases of high barium glazes which have lost their colour after
sitting over night with acidic liquids in them. I am sure that customer did
not run back to the potter and buy another to replace it - or any more of
any kind from that potter. It may even be so that they never bought another
hand made pot again.
I don't know when it will happen here - it has happened in some countries
already - that barium will be on the restricted list. It is surely not in
the same category as lead and cadmium and maybe even some of the oxides we
use - but it is being misused in many functional glazes and it is toxic.
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
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