Matthew Sterner wrote:
"'sun-colored amethyst' ... is glass that has been rendered clear through
the addition of manganese ... When exposed to the ultraviolet rays of the
sun, the glass turns varying shades of amethyst (depending on the duration
of exposure)."

Is it really due to long exposure to the sun, or can it also be a rapid
chemical change that can be triggered by exposure to the sun?  A few years
ago I found a c.1900 pressed-glass (faux-cut) salt shaker which was
perfectly clear when excavated, but within 15 minutes of drying and a very
little sun exposure it turned a wonderful dark amethyst.  It was difficult
to convince the lab folks that it was so recently perfectly clear.

How late was this process used?  The exterior door knobs of glass on my
1929 house have turned a wonderful amethyst while the indoor knobs are
still clear as can be.

        Dan W.