By Rebecca Ruger
It’s likely not our most favorite color to find, though certainly there may be rare or extraordinary pieces, but after white sea glass, green may indeed the most common color found. And green is a not a one-off color; there are probably more different shades of green to be found in bottles and glass than any other color, rivaling even the multitude of amber glass variations (which can also at times be considered shades of green).
The many different greens are made by numerous coloring agents, impurities, or glass making processes. Green glass can be produced by iron, chromium, and sometimes copper.
Like so many other glass recipes, the process used in the actual making of the glass will directly affect the color outcome. For example, a yellowish-green is produced when chromium oxide is used under oxidizing conditions, but the most common green— we glassers may call it Kelly, common, or grass green—is the result of chromium oxide used under reducing conditions. Some may refer to the very common Kelly green as emerald, but these are two completely different colors, as emerald glass is devoid of the yellowish tint of Kelly green, being removed in the glass making process by the addition of tin oxide and arsenic.
Different colors and shades of green can be found in just about any type of bottle, from any period, and thus provide no easy diagnostic utility to inform you about the green sea glass you’ve combed from the beach. However, some limited assumptions can be made by the different greens: the very bright 7-up green can be assumed to be almost exclusively a 20th century color; and olive glass—when found thick and chunky—can easily give a glasser confidence to date it more than 100 years old.
The chromium oxide of our Kelly green is purportedly found in no glass recipe books until the 19th century. It was a popular Depression glass color and before that, beers bottle were almost wholly green until the 1930s when it was determined that brown bottles actually filtered out light better than most greens, preventing the beer from turning ‘skunky’— not just a randomly appointed term, chemists actually found that as sunlight breaks down the alpha acids in hops which reacts with sulfur to form a chemical nearly identical to the chemical make-up of a skunk’s spray.
Kelly green, like the equally immeasurable amounts of brown and white glass and bottles made, will be very difficult to categorize or classify definitively unless there are remaining or accompanying bottle marks, engravings, or other helpful dating and identify tools still visible on the sea glass.
This article appeared in the Glassing Magazine March/April 2018 issue.