Tinting By The Sun. Anyone know how?

[Omnivore]

When I was a kid, a lady I knew turned her clear glass baking dishes a beautiful violet in color by placing them on her roof in the searing heat of the summer. When she took them down, however long later, they were a stunning electric violet in hue.

I put a clear glass Pyrex baking dish on the roof a year ago in the beginning of the long, hot summer and it has not changed color like hers did. Anyone know about this and how long should a piece be left out? Ever heard of it before? Should it be regular glass?

[Squink]

You definitely need to have the right kind of glass to get it to turn purple. From one of these links on sun-colored glass:

Glass with manganese content, which was most common in bottle production between 1880 to1914, takes on a rich purple color when exposed to ultraviolet rays.  Glass that was produced between 1914 to 1930 is most likely to change to an amber or straw color when exposed to ultraviolet rays.

It’s pretty stuff, but purpling used to decrease the value of old glass items. Fashions may have changed since the 60’s