Arts & Crafts Homes

Arts & Crafts Homes
Fall 2008
Mica, Slag, or Slumped ~ What Kind of Glass is That?

The terms for art glass are so numerous and perplexing that I threw in a ringer right in the title: Mica, as most of us learned in elementary science class, isn’t glass at all. It’s a mineral that flakes nicely into translucent sheets, making it ideal as a use for lamp shades, like the famous “coolie” shades make by Dirk Van Erp.

While it’s safe to say you can’t go wrong by referring to most kinds of decorative glass in period lighting, windows, doors, or hand-blown vases as “art glass,” there are terms that are more accurate and specific.

Let’s start with a toughie: the distinction between stained and leaded glass. On European cathedral windows of centuries past, stained glass is colored glass that has been painted with vitreous oxides and fired, with or without the application of silver nitrate. This stained glass is also leaded – that is, individual pieces of the composition (or mosaic) were held in place with lead cames, the rib-like dark matter between the bits of light-filled glass.

Luckily for us, things are simpler now. “If you use the term stained glass today, it is considered by most people to be any window with colored glass,” says stained glass consultant Julie Sloan. “For clear glass windows that are leaded, those are usually just called leaded.”

All colored glass gains its tint from minerals, says Sloan, who is the author of the chapter on stained glass in the forthcoming book, A “New and Native” Beauty: The Art and Craft of Greene & Greene.

Not all stained glass is transparent, however. Translucent stained glasses are better known as opalescent. “Opalescent glass is not transparent,” Sloan says. “It’s milky. It can have more than one color in a sheet. Tiffany windows are made of opalescent glass.

So are the windows made by Greene & Greene, who layered their glass the same way Tiffany did, with copper foil and lead overlay to enhance the dimensional quality of their compositions. The glass Greene & Greene used “is actually a very common glass,” says Sloan. “It’s iridized on one surface so it has the appearance of a rainbow, but Frank Lloyd Wright and Tiffany used the same glass.”

Tiffany called its version of this shimmering opalescent glass “Favrile” – a word it trademarked in 1984. The surface of iridescent glass appears to change color when it’s seen from different angles. While other companies made iridized glass, none have used the term Favrile, Sloan says.

Now that we have mica, stained, leaded, opalescent, iridescent, and Favrile out of the way, what exactly are slag and slumped? According to Jane Powell, author of Bungalow Details: Interior, slag is the name used for the sheets of opalescent glass slipped into Arts and Crafts light fixtures. (The term “slag” comes from the undocumented belief that slag from iron smelting works was added to the glass to give it color.)

As for slumped, it’s glass that’s been fired in a kiln to make it curve. Glass artist Clark Renfort actually climbs in the kiln while the glass is still hot to manipulate the shape of his shades, which are first “slumped” over a bowl. Effects range from delicately folded glass in tulip-like shades, to futuristic shapes that resemble a translucent marshmallow after a swipe through the campfire.