Karl P. Platt on tue 16 sep 97
I can only do a brief thing here as I have a million things to do
today....... If there's interest enough we can take all of this up in
robust detail later.
This is a very interesting subject in every respect. Phototropism in
rutile rich glazes and bodies was first reported by Parmelee and Badger
in 1934. Their original work contains the formulae they studied and I,
unfortunately, do not have my copy available at the moment -- it's some
8,000 miles from here. Oh well......As such it would be worthwhile to
contact the library at the American Ceramic Society in Westerville,
Ohio, from which you can easily obtain a copy of this work
(www.acers.org also gets you there).
Parmelee, C.W. and Badger, A. E., Coloration of Glazes by Light,
Journal of the ACerS, 11:359, 1934
You would also want to locate a copy of :
Williamson, W. O., The Reversible Darkening in Daylight of Some Glazes
Containing Titanium, Journal of the British Ceramic Society, 39:345,
1940.
If you live near a university with a well established Ceramic
Engineering/Materials Science school, they may have these articles, too.
It should be made very clear that Rutile is a form of the TiO2 crystal
-- TiO2 can exist as Rutile, Anatase or Brookite -- much as SiO2 can
exist as Quartz, Crystobalite or (maybe) Tridymite.
That is, in referring to Rutile we are not referencing the (highly
fugitive) TiO2 rich material sold (foisted off on) to studio ceramists
under the same name -- this stuff can contain TiO2 in a variety of its
forms in addition to (generally) large and variable amounts of Fe. That
is, one would expect more consistent results using a pure-ish form of
TiO2. However, Parmelee/Badger did see phototropism with some of the
commercial products they used.
The effects are related to the crystalline material present and have
nothing to do with any TiO2 that is dissolved into the glassy part of
the glaze. Phototropy has been reported in a fairly diverse range of
compositions which do not necessarily include any TiO2. In the main,
however, the effects depend not only on the presence of TiO2, but also
on having a small bit of some polyvalent element present -- Fe, for
example. CeO2 added to the glaze in some very small quantity would also
serve to improve the photosensitive qualities.
The change of color would seem to be caused by photoreduction (of TiO2??
something in the TiO2 lattice??), i.e., UV kick loose an electron from
something in the glaze and it is assumed elsewhere in the glaze. As
reduction cannot occur without causing oxidation elsewhere it helps to
have some material present which is friendly to being oxidized
(accepting an extra electron). Fe certainly satisfies this, as does Ce.
Fe(oxide) - Mn(oxide) reactions driven by UV in sunlight are responsible
for producing the purple color one sees in old glass that's been exposed
to sunlight. The most beautiful example I've seen of this was a skylight
on the Texas State Capitol in Austin, TX. I've been trying for years to
make this color artificially.
My work with developing images in ceramic glazes (and glasses) by
exposure to laser radiation sometimes uses TiO2 rich glazes and I always
mask off one side of a tile an put it on the roof for a couple week to
assure they're photolytically stable. Many fast-fire tile glazes use
TiO2 for an opacifier. A couple have been photosensitive, but
fortunately to a small degree. I've only seen a couple of these glazes
and must say that the irradiated colors are usually not very attractive.
Photosensitive/Photochromic glasses are based on very different behavior
given by the presence of the nobel metals Cu, Ag and Au in the glass. Of
interest to ceramists one might want to consider that any "unstruck" Cu
red glaze is photosensitive. That is, if a negative were placed on the
glaze and exposed to (strong) UV, and then reheated it is possible to
develop a photographic image. The hows, whys. etc of this go into
another (long) post. For now suffice it to say that these effects have
never (in my experience) been exploited expressivly. Am I mistaken or is
it true that studio ceramists are given to looking back in history
rather than to the present in respect to obtaining the technological
vocabulary used?
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