Purple Glass in the Piedmont Avenue Neighborhood

Purple Glass

          In the sidewalk on 40th street, next to Ferrari’s deli on Piedmont Avenue, you will find a series of deep purple glass circles. These are the remnants of a light source for the basement and lower reaches of Ferrari’s store.

          In the 19thand early 20th centuries, before electric lighting was common, these breaks in the solid paving overhead allowed daylight to enter cellars and
basements.  The glass was clear at the time it was installed.

          Before 1914, manganese was added to glass during the manufacturing process.  Without it, the glass was muddy looking and opaque or bluish.  The manganese acted as a clarifier, allowing for a crystal clear glass.  The problem with manganese, however, is that when it is exposed to the sun’s ultraviolet rays, it turns purple. Various shades will appear in the glass, from pale lavender to intensely deep purple, depending on how much manganese was in the molten glass and how much exposure the glass had to the sun.

         But in 1914, when much of Europe erupted in war and the U.S. got involved, manganese was no onger available.  The major source, Germany, had cut commercial ties with the U.S. and former shipping routes were no longer safe.  Glass manufacturers turned to other sources for clarifiers and manganese became a part of glass manufacturing history.

          Mountain View Cemetery has a good example of purple glass in the star at the top of the Cogswell burial site and I know someone who lives in a house in Oakland where some of the glass has turned.  However, the 40th street site I’ve described is the only place I know of in Oakland where this purple sidewalk glass appears.  If you know of others, please let me know.

©  Ruby Long

 2013