A couple of examples of Pats salvaged door knobs, the first knob with the violet color is a true antique manufactured with glass that contained impurities that reacted with sunlight. Originally the glass was white but as it aged with exposure to the UV rays from the sun eventually changed to the violet color.
In the late 1800's, manufacturers created glass mixes that used manganese instead of lead to create a brighter glass. In time, it was discovered that glass containing manganese would turn lavender after months of exposure to the sun.
Often you'll find other glass items that have overtime turned violet, we often find old canning jars.
The bottom glass door knob is of a later manufacture made with improved glass chemistry, the violet discoloration was deemed inappropriate but has now become a desirable collectible - go figure.
I do not buff the surfaces of violet glass knobs, but have done so with the white glass, after years of handling the surfaces become marred but after a few minutes of buffing they look like the crown jewels.
I use a felt wheel with jewelers rouge for the final buff.
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