Ron Roy on sun 20 sep 98
Hi Laura - I like your question - wish more potters would be interested in
the why of it all.
We have to understand - all glazes leach - deteriorate with use. The
question is how long does it take and what is released and under what
circumstances.
Safe and durable are different. Safe can mean a glaze that is durable but
has toxic oxides in it that are released so slowly they are not a concern.
Safe can also mean a glaze which is not durable but, because there are no
toxins present, no toxins will be released.
Some glazes cannot be made safe because they rely on toxic materials and/or
a basic imbalance for a certain effect - matte glazes are usually short of
silica for instance.
We try to ascertain durability by looking at the amounts of alumina and
silica in a glaze. Using the Seger molecular formula we count the molecules
of oxides in a fired glaze. For instance one set of limits for cone 6
glazes specify Al2O3 0.2 to 3.5 and silica 2.5 to 3.5 to have a durable
glass. If the amount of silica is only 1.5 we can safely assume the glaze
will be susceptible to attack by acids - vinegar for instance - which is
how glazes are tested for durability - in vinegar for 24 hours.
If a glaze is short of alumina then the glaze becomes susceptible to
leaching by alkalies - like dish washing detergent and alkaline foods. If
you have a dish washer you can notice this with some of your older drinking
glasses - they have a dulled surface. In fact one of the tests for
durability is to immerse a glaze in detergent and then look for a dulled
surface.
To answer your question - can anyone upset the durability of a glaze by
changing it. Of course. If the glaze had 2.6 moles of silica and you added
enough flux or took out enough silica you could lower the silica content of
a glaze and turn a stable glaze into one that will deteriorate with use.
You can also do this in other ways - adding so much colouring oxide that
the oxide can no longer stay in solution as the glaze cools will result in
some of the pure oxide being right on the surface where it can be released
easily into food.
RR
>When you have a glaze that is comprised of nontoxic materials and is
>food safe,when you change the amount of ingredients (including oxides)
>can you knock it out of the "safe range" for functional ware? I do not
>understand limits. I think the answer is yes but I can't say why.
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
Laura FREEDMAN on mon 21 sep 98
Ron, thank you so much for your explanation. I have always been concerned
about glaze materials since I started glazing and not really understanding how
it all "goes together". My conscience is a mile wide and I wouldn't want to
hurt anyway. I now have a clearer understanding. Perhaps someday I can get
into one of your glaze classes. I would love to be able to create my own
glazes from scratch and feel confident of what I'm doing.
Laura in south central PA where it is muggy and hot today.
>
Ron Roy on tue 22 sep 98
Hi Laura - I like your question - wish more potters would be interested in
the why of it all.
We have to understand - all glazes leach - deteriorate with use. The
question is how long does it take and what is released and under what
circumstances.
Safe and durable are different. Safe can mean a glaze that is durable but
has toxic oxides in it that are released so slowly they are not a concern.
Safe can also mean a glaze which is not durable but, because there are no
toxins present, no toxins will be released.
Some glazes cannot be made safe because they rely on toxic materials and/or
a basic imbalance for a certain effect - matte glazes are usually short of
silica for instance.
We try to ascertain durability by looking at the amounts of alumina and
silica in a glaze. Using the Seger molecular formula we count the molecules
of oxides in a fired glaze. For instance one set of limits for cone 6
glazes specify Al2O3 0.2 to 3.5 and silica 2.5 to 3.5 to have a durable
glass. If the amount of silica is only 1.5 we can safely assume the glaze
will be susceptible to attack by acids - vinegar for instance - which is
how glazes are tested for durability - in vinegar for 24 hours.
If a glaze is short of alumina then the glaze becomes susceptible to
leaching by alkalies - like dish washing detergent and alkaline foods. If
you have a dish washer you can notice this with some of your older drinking
glasses - they have a dulled surface. In fact one of the tests for
durability is to immerse a glaze in detergent and then look for a dulled
surface.
To answer your question - can anyone upset the durability of a glaze by
changing it. Of course. If the glaze had 2.6 moles of silica and you added
enough flux or took out enough silica you could lower the silica content of
a glaze and turn a stable glaze into one that will deteriorate with use.
You can also do this in other ways - adding so much colouring oxide that
the oxide can no longer stay in solution as the glaze cools will result in
some of the pure oxide being right on the surface where it can be released
easily into food.
RR
>When you have a glaze that is comprised of nontoxic materials and is
>food safe,when you change the amount of ingredients (including oxides)
>can you knock it out of the "safe range" for functional ware? I do not
>understand limits. I think the answer is yes but I can't say why.
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
Ron Roy on wed 23 sep 98
Hi Laura,
I do a week long glaze course in NorthBay every summer - good deal for you
guys cause of the low Canadian dollar. Last year the course and
accomadation was aroung $300 Can. Jus watch the list - I will post the
details in the spring.
I have also saved your message to remind me to let you know.
>----------------------------Original message----------------------------
>Ron, thank you so much for your explanation. I have always been concerned
>about glaze materials since I started glazing and not really understanding how
>it all "goes together". My conscience is a mile wide and I wouldn't want to
>hurt anyway. I now have a clearer understanding. Perhaps someday I can get
>into one of your glaze classes. I would love to be able to create my own
>glazes from scratch and feel confident of what I'm doing.
>
>Laura in south central PA where it is muggy and hot today.
>
>
>>
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
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