Updated: Jan 31, 2021
Part of the seduction of working with glass is the color of it. Stare at a cobalt blue vase, a deep red bowl, or a shocking lime-green pile of frit, and you'll be wondering how these colors came to be.
When I was a kid, an elderly couple lived just over our back fence. The lady interested me the most of the two of them--or, rather, something she had interested me the most: a screened-in back porch full of glass bottles in every shape, size, and color. I would wait for a sunny morning and race across my dad's garden, step over the sagging back fence, and knock on her back door.
"Can I sit on your porch for a while?"
"Sure, honey," she'd say. "Just be careful of the bottles."
And so right there on that porch is my first real memory of the magic of glass, and without even knowing it, I formed a question in my mind that has persisted unanswered until I began my glass love affair five years ago: What makes glass the colors we see?
Most often, it's the use of metal oxides, and these elements are added to glass in very small amounts when compared to the entire batch of formers, stabilizers, and fluxes. For instance, Cobalt is only added in ratios as low as several parts per million to the glass batch to produce a light blue (Glass Packaging Institute), and of course, the more added, the deeper the blue becomes. Gold makes red glass; copper makes blue or green glass. Uranium (yes, the same element used in The Bomb--future Blog article here) makes for an eerie hot-green glow, especially under a blacklight--you DID keep yours from the '60's, didn't you? You don't need to be a chemist to understand the chemistry of colored glass--I've found it can just be appreciated for the beautiful thing it is.
And these elements can be combined for other colors or effects. There are the sulfur-bearing glasses, such as Bullseye's French Vanilla Opal, and when it's combined with the copper-bearing Light Aqua Marine blue, the result is a green glass. In fact, you can see this reaction and many others in the Bullseye article "Get a Reaction." It's fascinating to see the possibilities.
In industry and packaging, glass colors have specific purposes. ambers and browns block ultraviolet rays, and so do some greens, though not as well. This explains why nearly all beer bottles are brown, amber, or green--except for a few brands such as Corona and Pacifico, which explains that "skunky" smell (and some would say, taste) that Corona can have when exposed to the sun. (Corona brewmaster, take note.)
There's a wealth of info around on colored glass--its history, its uses, its batching--so just type that into Google or your favorite search engine. You'll be amazed. I sure am.