RE: Only OLD glass turns purple???
About the only reason I can think of as to why this glass batch may have contained some manganese is from the cullet source. I worked for Ball and I know we had to buy our cullet from many different sources and each batch of raw materials had to be mixed with a certain percentage of cullet (broken glass) in order to melt properly in the continuous flow furnace. If possible, we avoided using window glass as cullet because it was too brittle and they had to alter the other raw materials to compensate for that for making containers. I know we (Okmulgee, OK Ball plant) used Selenium at this time (mid 1970's) as a decolorizer in our batches of raw materials purged into the furnace.
One of the early Kerr plants was located at Sand Springs, OK about 40 miles from where I live. They used sand from the Arkansas River in their glass and sometimes in many of their earlier jars in the teens and twenties, the colors ranged to a smokey gray color. It remained in operation as a Kerr facility until the mid 1990's when Ball bought them out. They closed that Kerr plant and merged it with the Okmulgee, facility. For a couple of years, we made Kerr fruit jars at the Ball plant!
I'd say this particular jar we are talking about is very unusual in this color, whether it was naturally SCA or purple boxed. Unusual? Yes. Commanding a big premium? Up to the individual. For all we know, there are MANY of these clear jars that would turn purple like this one has. Even a small batch of glass in today's furnaces is 350 tons, and that makes a LOT of jars!
Always something else we learn each day about our interesting hobby. :o)