Have you perused the terms commonly used for art glass? Most people find them numerous and perplexing, so much so that I threw in a ringer right in the title. For starters, mica isn’t glass at all, as most of us learned in elementary science class. Rather it’s a mineral that flakes nicely into translucent sheets. Thus it is an ideal material for lamp shades, as demonstrated in the famous “coolie” shades made by Dirk van Erp.

Although you can’t go wrong using the term “art glass” to refer to most kinds of decorative glass in period lighting, windows, doors, or hand-blown vases, there are terms that are much more accurate and specific. The first thing to discern is the distinction between stained and leaded glass. The stained glass used in European cathedral windows of centuries past is actually colored glass that’s been painted with vitreous oxides and fired, either with or without the application of silver nitrate. This stain glass is considered to be leaded. Individual pieces of the composition or mosaic were held in place with lead cames, which you can recognize as the dark matter holding together the bits of light-filled glass.

Fortunately, nowadays things are much simpler. Today, the term “stained glass” is considered to be any window with colored glass. Any clear glass windows that are leaded are simply just called “leaded.”

Colored glass gains its tint from minerals. But not all stained glass is transparent. The term “opalescent” is used for translucent stained glasses. But opalescent glass is not transparent—it’s milky. Also, this type of glass can have more than one color in a sheet. Tiffany windows are a great example of opalescent glass.

Another notable example of this is the windows by Greene & Greene, who layered their glass the same way as Tiffany, but they used copper foil and lead overlay to enhance the dimensional quality of their compositions. Like their wood furniture and wood dining chairs Greene & Greene’s glass is actually very common. Iridized on one surface so it has the appearance of a rainbow, it’s been used by Frank Lloyd Wright and Tiffany.

Favrile became the brand-name of Tiffany’s version of this shimmering opalescent glass, a term that was trademarked in 1894. What makes this iridescent glass so special is that it appears to change colors when it’s seen from different angles. Although many other companies made their own versions of this iridized glass, no one else used the term Favrile.

Now that we’ve clarified mica, stained glass, leaded glass, opalescent, iridescent, and Favrile, what exactly are slag and slumped? Slag glass refers to the sheets of opalescent glass used in Arts and Crafts light fixtures. (The word “slag” actually comes from the undocumented belief that the glass got its color from slag from iron smelting works.) As for slumped glass, that’s glass that’s fired in the kiln so it will curve. Some glass artisans will actually climb into the kiln to manipulate the hot glass to their liking. To create glass shades, the hot glass is actually “slumped” over a bowl. With this technique, artisans can achieve effects ranging from delicately folded glass in tulip-like shades to futuristic wild shapes.