Solarization of glass

Mary Fahey <maryf [at] hfmgv__org> writes

>I am interested in obtaining information regarding the solarization
>of glass. Specifically I would like to know if it should be a
>consideration in terms of limiting light levels in glass displays.
>I'd appreciate any information regarding this subject.

Dr. Robert H. Brill, the Research Scientist at the Corning Museum of
Glass, wrote this "hand-out" for the Docents in 1988.  I reproduce
it here with his permission:

                  Desert Glass or Solarized Glass

    Certain types of colorless, transparent glasses, when exposed to
    sunlight for extended periods of time, develop a pink or violet
    color.  Bottles, insulators, and fragments having this color are
    often called "desert glass", but the scientist prefers the term
    "solarized glass".  Other well-known examples are the famous
    purplish windows on Beacon St. in Boston and the little circular
    glass disks in older sidewalks.  Occasional examples are also
    found in the ancient world.

    The major constituent of most glasses is silica, which is
    usually introduced as a raw material in the form of sand.
    Although silica itself is colorless, most sands contain iron as
    an impurity which imparts a greenish color to the glass. (In
    ancient times glassmakers used very impure sands, with higher
    iron contents than those of sands used today, so most ancient
    glasses have a more pronounced greenish color.)  By adding
    certain other ingredients to the molten glass, it is possible to
    offset this greenish color and produce water-white glasses. Such
    ingredients are known as decolorizers and one of the most common
    is manganese dioxide (MnO2).  In chemical terms, the manganese
    acts as an oxidizing agent and converts the iron from its
    reduced state (which has a strong greenish-blue color) to an
    oxidized state (which has a yellowish but much less intense
    color).  In the course of the chemical reaction, the manganese
    goes into a chemically reduced state which is virtually
    colorless.

    Manganese is believed to have been first used as a decolorizer
    as early as about the 2nd century B.C.  It was probably
    introduced as the mineral pyrolusite.  From Roman times onward,
    glasses often contained about 0.5 to 1.0% MnO.  Later on
    manganese dioxide was sometimes called "glassmaker's soap".

    If pieces of colorless glass containing reduced manganese are
    exposed to ultraviolet light for long periods of time, the
    manganese may become photo-oxidized.  This converts it back into
    an oxidized form which, even in rather low concentrations
    imparts a pink or violet color to glass.  The ultraviolet rays
    of the sun can promote this process over a matter of a few years
    or decades, thus accounting for the color of desert glass.
    Variations in hue and intensity are caused by variations of
    chemical composition and conditions of exposure.  The effect has
    been reproduced under laboratory conditions.

    Other chemical elements which are subject to photo-oxidation may
    also undergo color changes when exposed to ultraviolet light.
    Since about the turn of the century, some of these, such as
    selenium and cerium, were occasionally used as decolorizers and
    therefore can produce solarization colors, just as manganese
    does.  The colors developed by these two elements are said to
    range from yellow to amber.

        Robert H. Brill, July 19, 1988.

Some additional notes to consider: Not all that many glasses have
manganese as a decolorizer, because most clear glasses, especially
today, start with much purer raw materials.  However, for those that
do have manganese and become solarized, the process is irreversible,
and can occur on one side of a glass and not the other.  Or in the
case of two outdoor light globes from my 1920's home, the purple
color occurred below the screw-in threads on the glass (because the
screw threads were protected by the fixture).  I have also seen a
glass doorknob that was regularly exposed to the South turn purple,
while the doorknob on the inside did not. > In general, glass
objects in cases are protected from ultraviolet light because the
glass (or plastic) covers cut out most of the ultraviolet light.
Additional protection can be provided by ultraviolet sleeves or
screens over Gallery lights or by ultraviolet film on outdoor
windows.  Some of these films will cut out all the ultraviolet
light, without radically limiting the overall light levels.

Stephen Koob
Conservator
The Corning Museum of Glass
One Museum Way
Corning, NY 14830
607-974-8228
Fax: 607-974-8470


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                  Conservation DistList Instance 16:12
                 Distributed: Tuesday, August 20, 2002
                       Message Id: cdl-16-12-003
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