Answered By: Regan Brumagen
Last Updated: Jul 19, 2016     Views: 69

From Dr. Robert Brill, Research Scientist Emeritus:

Glass solarization occurs when manganese dioxide, used to reduce the greenish tint in some types of glass, becoming oxidized due to exposure to ultraviolet radiation.

Assuming that the lamp is made up of strongly-colored pieces of glass, they may or may not have any manganese in them.  That would depend on whether the glassmakers routinely added manganese to large batches of their glass and then used that to make different colors of glasses — or whether they made separate melts from batch for each of the different colors.  If they were doing the latter — melting different colors straight from batches — then they might not have bothered to decolorize the glass.  Such glasses would not contain manganese (or other decolorizers), and thus would not become solarized. 

On the other hand, if prepared by the former method, they might contain manganese and could be subject to solarization.  In that case, changes in color might be difficult to detect, because they could be masked by the original stronger colors. 

If you feel curious enough, you might look for differences within each color of glass where they are found on the "sunny side" and "shady side" of the lamp.  If the lamp had been in the same location and orientation for decades, you might see some solarization effects in the lighter colored glasses.  But that is unlikely.

(Moreover, light from an incandescent bulbs on the interior of the lamp are probably too low in UV to cause much solarization.)

All of the above may be moot, because the controlling factor in solarization is usually long-term exposure to UV.  Even though it appears nearly colorless, the double-paned window glass could possibly filter out much (if not most) of the incident UV.  This would be the case if the window glass has even a slightly yellowish cast when viewed in thick section.  (The usual bluish or greenish aqua tints in glass are ordinarily due to iron which serves effectively as a UV filter.)

Solarization is quite common in glasses exposed to direct sunlight in outdoor situations.  However, there are not many examples of medieval stained glass windows that definitely show solarization.  There are examples of pink glasses used to represent flesh tones.  However, it is unknown if they are solarized colorless glasses or if they were originally pink glasses used deliberately to represent flesh tones. 

Most medieval stained glasses do contain manganese. In any event, any solarization would only be noticeable in the colorless glasses, and would be masked in the stronger-colored glasses. 

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