Pontil scars on dating glass
The base of the second bottle whose lip was shown above right is displayed here.The the lower left corner you can see evidence of the diagonal mold seam which at one time bisected the base.Beginning collectors often confuse an Owen's ring with a pontil mark and it is easy to see why this happens.
Both require more specific research to refine the bottle's date beyond observation of technology. Bottles with a purplish or amethyst hue are made from manganese dioxide glass and date circa 1880 to 1920.
The middle picture shows an open pontil on the base of a cylindrical medicine bottle.
The third picture shows the base of a milk bottle from just after the trun of the century.
The food and household product industries on the other hand widely adopted the screw cap quickly.
One unique closure to the late 1930s was a three point screw top.