I apologize for my late entry into this strain, but I see I need to
 finish the rewrite of my article on manganese for Historical
Archaeology.

 Manganese was indeed used by the Romans and by many other people
 since, but it was used as a colorant not as a *de*colorant.  Most
sand contains iron impurities that turn glass various colors
including aqua, green, light blue, and light amber (depending on
varying factors).  Minute quantities of manganese mask aqua and
similar colors creating a colorless glass.  Heavier use of manganese
creates varying shades of purple, amethyst, and similar shades.

 The amount of manganese present in the glass and other added
 ingredients affect how long manganese-bearing, colorless glass must
be exposed to ultraviolet radiation before it undergoes a chemical
change creating SCA (the change can be reversed by heating the glass
to ca. 450 degrees, but that is very close to the plastic point).
Experiments in 1908 demonstrated that some glass changes color in as
short a time as one week (some collectors say even less), but some
requires two weeks or more just to show a small shade change.  The
amount of manganese in the glass strongly affects how dark the purple
shade will be.  More exposure will not create a darker shade unless
the glass contains a (relatively) heavy amount of manganese.

 The early use of manganese in the U. S. as a decolorant in bottle
 glass began about 1880 but was not widespread until later.
Confusion in terminology makes it difficult to determine exact dates,
but manganese was a major player in colorless glass by at least 1889,
possibly sooner. Information about the 1914 date began in 1963 when
Grace Kendrick published The Antique Bottle Collector.  She was a
pioneer in the field, and much of her information has proven
erroneous.  I checked import records, and Germany was *not* one of
the major players in the manganese market.  All sources for that
information (at least that I have found) eventually trace back to
Kendrick and were repeated when archaeologists did not yet have our
own literature to fall back on.  Btw, Smoke's information about
manganese use in the U. S. is all correct but is not directly
connected to the cessation of manganese use.  Both India (prior to
the war) and Brazil (after the war) were our major sources for
manganese.

 The big reason for the industry change to selenium (the major
 decolorant still used today) was caused by the change from
hand-blown to machine-made bottles and closely follows the dating
schemes set forth by George Miller in conjunction with various other
researchers (including his latest [and excellent] paper from the SHA
convention). In other words, ca. 1920 is a good date for the general
reduction of SCA production.  However, manganese was still used as a
decolorant *in a limited way* as late as 1933 (yes, I have
documentary evidence that will appear in the article).

 Although I am not prepared to go into details (that will be a future
article), forget about straw-colored glass (assumed to be selenium)
as a dating tool.  It shows up at least as late as the 1940s and
could have appeared at least as early as 1890!

Bill Lockhart
-----------------------


Bill Lockhart
New Mexico State University
Alamogordo, NM
(505) 439-3732