Adding Manganese to glass was a method of helping clarify it.  When folks
purchased it, they had no idea it would eventually turn purple.  I have
never heard of the color change taking less than a couple of years.
Collectors place their "Purple" glass stuff in south facing windows since
the collector value increases with the depth of the purple color.  The
process was used from the 1890s and ended exactly in 1914 with the
beginning of World War I.  All the best and cheapest Manganese came from
Germany.

After that, glass makers started to add Selenium to glass as a clarifying
agent.  This ended at the beginning of the depression when it was cheaper
to color glass than to clarify it (thus depression glass).  The selenium is
a polarizing agent.  Look at the glass fragment face on and it looks clear.
Look at the glass from the side broken edge and it will look as amber as
honey.

Both these color variations are caused by sunlight - specifically the
ultraviolet.  It is always hilarious to look at old site forms where
someone puts "solar glass" down.  One knows they meant the Purple (amethyst
for the pretentious) glass.  They knew just enough to make them dangerous.

Smoke.


Smoke (Michael A.) Pfeiffer, RPA
Ozark-St. Francis National Forests
605 West Main Street
Russellville, Arkansas 72801
(479) 968-2354  Ext. 233
e-mail:  [log in to unmask]

It is easier to get forgiveness than permission.




                    "Daniel H.
                    Weiskotten"          To:     [log in to unmask]
                    <weiskotten@E        cc:
                    ROLS.COM>            Subject:     Sun Colored Amethyst Glass
                    Sent by:
                    HISTORICAL
                    ARCHAEOLOGY
                    <HISTARCH@asu
                    .edu>


                    01/22/02
                    10:06 PM
                    Please
                    respond to
                    HISTORICAL
                    ARCHAEOLOGY






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.