Jeff Lawrence on mon 26 oct 98
Hello all,
My investigations of my glazes with my buddy Ray have raised a question I'd
enjoy hearing your comments on.
Several discussions of glaze durability and heavy metal leaching have
described the phenomenon of a colored glaze soaking overnight in orange
juice or vinegar and being bleached of color in the morning.
My inference from these accounts was always along the lines of, "Gosh, that
heavy metal colorant got sucked out of the glaze into the oj." I imagined
the resulting juice a witch's brew of toxins.
Ray's maltreatment of my glazes -- bathing them in molten salts at the harsh
extreme and merely chipping them off the pot at the gentler extreme --
indicates that color changes occur pretty easily as the result of other
changes besides constituent parts leaving the glaze. As my malmixed red
devitrified over the course of a few days, the red color disappeared in an
area around the developing crystals. Since these specimens are in inert
immersion oil, the leaching mechanism above seems unlikely.
This in turn makes me wonder if the color changes induced by the overnight
soak in vinegar are the result of leaching or of some non-leaching change in
the glaze matrix. Has anyone ever tested their witch's brew for the
dissolved presence of the suspected leachates?
Never too overworked to spend time on tangential reflection,
Jeff
Gavin Stairs on tue 27 oct 98
At 09:46 AM 10/26/98 EST, you wrote:
....
>This in turn makes me wonder if the color changes induced by the overnight
>soak in vinegar are the result of leaching or of some non-leaching change in
>the glaze matrix. Has anyone ever tested their witch's brew for the
>dissolved presence of the suspected leachates?
One of the recommendations from Ron Roy and others is to look for
discoloration in the solvent as well as on the pot. Color changes in the
liquid are undoubtedly due to some leached material. The same is true of
discoloration of foods left overlong in a pot. This is similar to the
rapid color change in, say, onions left in contact with a (non-stainless)
steel knife blade. A relatively small amount of iron will turn the onions
black. Maybe we can try the onion test as an assay for iron release... Hm.
Assaying the liquid is relatively expensive. Something like x-ray
diffraction spectrometry can give a broad range of element analyses, but
the application of this to leachate analysis is not straightforward.
I'm sure that some color changes are due to changes in lattice, and
certainly the loss of some fugitive colors, like copper opalescence, can
come from changes in oxidation state rather than from physical removal of
the copper. But any such change means that the structure is not durable,
is susceptible to infiltration of chemical agents, and may well leach or
disintegrate over time. Of course if the treatment amounts to a refiring
or annealing or other reformulation of the pot chemistry by intent, and not
normal (or normally abnormal) use, then this is not a defect, but a
treatment. I would say that if the color of a pot changes upon being
placed in a molten salt bath, or in an oven at high temperature, or upon
being torched, then this is not evidence in itself of a structural or
chemical problem. However, if it changes on soaking in or contact with any
foodstuff over a reasonable period of time, or upon boiling or freezing or
any other kitchen heat treatment, then this is evidence of a possible
problem, and needs to be assessed as to danger in likely or reasonable use
by a buyer or subsequent user.
Gavin
Gavin Stairs
Stairs Small Systems (S3)
921 College St., # 1-A
Toronto, Ontario, Canada M6H 1A1
(416)530-0419 stairs@stairs.on.ca
Ron Roy on thu 29 oct 98
Hi Jeff,
Gotta make this short. The leachate is what is analysed to detect what has
come out of the glaze - so it is a witches brew - sometimes. In fact when
fruit is on say a less than stable cobalt glaze you can see the colour in
the fruit or so it is my understanding.
During the formation of crystals in a glaze - as the molecules are
arranging - they will rob the surrounding glaze of what they need. If what
they need is silica they can rob enough to make the surrounding glaze
susceptible to acid attack. The glaze surrounding the crystal can be
reduced in size (from the acid) so that eventually you would be able to see
the crystals projecting above the surrounding amorphous glaze. The crystals
- being silica rich would resist the acid attack. See Taylor and Bull
"Ceramics Glaze Technology."
Now I know that my understanding of this is on a rudimentary level and
would appreciate any corrections or augmentation from those who do
understand this.
Regards - RR
>Several discussions of glaze durability and heavy metal leaching have
>described the phenomenon of a colored glaze soaking overnight in orange
>juice or vinegar and being bleached of color in the morning.
>
>My inference from these accounts was always along the lines of, "Gosh, that
>heavy metal colorant got sucked out of the glaze into the oj." I imagined
>the resulting juice a witch's brew of toxins.
>
>Ray's maltreatment of my glazes -- bathing them in molten salts at the harsh
>extreme and merely chipping them off the pot at the gentler extreme --
>indicates that color changes occur pretty easily as the result of other
>changes besides constituent parts leaving the glaze. As my malmixed red
>devitrified over the course of a few days, the red color disappeared in an
>area around the developing crystals. Since these specimens are in inert
>immersion oil, the leaching mechanism above seems unlikely.
>
>This in turn makes me wonder if the color changes induced by the overnight
>soak in vinegar are the result of leaching or of some non-leaching change in
>the glaze matrix. Has anyone ever tested their witch's brew for the
>dissolved presence of the suspected leachates?
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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