In a recent Museum Monday talk (also visible in this YouTube video), I discussed the topic of cobalt sea glass, a popular color to find on the beach. Created through the addition of cobalt oxide to glass to create the deep blue shade, this color has a long history dating back to ancient Egypt. Richard LaMotte’s book has a great section on cobalt glass, and for more information you can also check out this Beachcombing magazine article.
Probably the three most common sources of cobalt glass found on beaches in the U.S. are the Noxema, Vicks and Bromo-Selzer bottles. The Maryland Glass Corporation produced all three of these bottles from 1907-1980, and the “M inside a circle” on the bottom of a bottle is a trademark (c.1921) that can identify these finds. Bottles produced before 1921 will have a number or no mark, while after 1921 the M will be present.
Cobalt glass was used primarily in medical bottles to distinguish it from other types of bottles in the cabinet for safety purposes, and thus was also a common color for poison bottles. More rare ink or soda bottles were manufactured using cobalt glass, and since cobalt oxide is expensive, it is prized as a decorative tableware color. More rare is cornflower blue, a lighter shade of glass in which less cobalt oxide would have used during manufacture—it is found in earlier Milk of Magnesia bottles.