Serious antique glass collectors will be familiar with "slag glass" which became popular in the late 1800s and is still made today. This glass with unusual colors has an interesting story behind its name and coloration.
What Is Slag Glass?
Slag glass is a term used to describe a type of colored pressed opaque glass made using the "slag" left over during the iron smelting process. This antique glass is known by other names including:
- Brown malachite
- Brown marble vitro porcelain
- Mosaic glass
- Marble glass
- Variegated glass
What Is Slag Glass Made Of?
Slag glass is created by using pulverized silicate slag, an ingredient that forms on the top of molten iron as it cools down. Slag glass was originally created in the United Kingdom in the 1890s by adding this slag substance during the glass-making process. Sowerby in Gateshead, England is believed to be the first glass foundry to create slag glass. Slag glass was also created by taking two colors of glass and combining them to create what was called "mosaic glass" in 1902 in Pennsylvania by glassmakers Thomas Dugan and Harry Northwood.
Slag Glass Colors
The original slag glass created in England was known for having a brown base color mixed in with streaks of a creamy white color. This color pattern led to the "brown malachite" and "brown marble" names. Another early slag glass was the purple malachite glass by Sowerby, which was sold in the United States under the name "blackberries and cream." Sowerby also created several other color formulas including Giallo (yellow), Pomona (green), and Sorbini (blue). The mosaic glass created in Pittsburgh was a mixture of purple and either white or an opal shade. You'll find slag glass in blues, browns, and greens as well, though these colors are rarer than the brown/white/cream and purple formulations. Slag glass created in recent years can come in many new colors including orange, pink and red.
Antique Slag Glass Light Fixtures
One item where slag glass was used often was in the creation of antique oil lamps, chandeliers and lampshades, particularly during the Art Deco and Art Nouveau periods during the late 19th and early 20th century. This is one of the most common uses for slag glass, and you'll see many slag glass chandeliers and other fixtures in antique shops.
Patterns and Designs for Slag Glass Lighting
Many manufacturers used colored slag glass to create elaborate patterns in the lamp bases and shades, while others used the glass to create everyday scenes and landscapes. The lamps and shades often included intricate bronze and brass metalwork featuring scrolling floral, foliate, relief and decorative patterns. Egyptian patterns were also common due to the interest in the tomb of King Tut in the early 1920s. Popular slag glass lampshades shapes included mushroom, dome and flower petals. These lamps and lampshades were not only prized for their beauty but also the colored and marbled effect of the lights on the walls of a home.
Identifying Stained Glass vs. Slag Glass in Light Fixtures
Many antique lamps of these periods were made with stained glass, rather than slag glass, and a careful review of the glass opacity and patterns is necessary to determine the difference. This is because the top manufacturers of the day often did leave any identifying brand marks on the fixtures. Some of the most popular manufacturers were Miller, Bradley & Hubbard, Empire Lamp Manufacturing Company, Pittsburgh Lamp, Brass and Glass Company, H.E. Rainaud, and Tiffany Studios.
How To Tell if an Item Is a Slag Glass Antique
Slag glass is often used as a catch-all term for any type of pressed glass that is opaque and colorful, but not all glass that fits this description is true slag glass. You can determine antique slag glass with a few steps:
- Look at the coloring for a marbling effect. It should not just be white streaks mixed in with another color, or one solid color. You should see creamy or white uneven marbling mixed in with the base color. It sometimes is compared to tortoiseshell or malachite.
- Examine the color. Antique slag items are usually brown, blue, green or purple.
- Look for names and marks of manufacturers. Well-known antique slag glass manufacturers include:
- Sowerby, Greeners, and Davidson from the United Kingdom
- Atterbury & Company, Challinor Taylor & Company, H. Northwood Glass Company, Akro Agate and Westmoreland from the United States.
- Slag glass antique lampshades can also be found by Tiffany, Roycroft and Steuben and will have those companies' marks on their base.
- Modern day slag glass manufacturers in the U.S. include Fenton, Mosser, Summit and Boyd Glass.
How Much Is Slag Glass Worth?
Slag glass antique items can run anywhere in value from a low of $50 to a high of $1,500. Typically slag glass antiques will be vases, dishes, bowls and decorative figurines and picture frames.
How Much is an Antique Slag Light Fixture Worth?
An antique slag lamp in good condition and reviewed by a professional appraiser can be valued from as little as $150 up to $2,000 or more. You can find antique slag lighting fixtures for sale on websites such as Etsy for as low as $20 or as much as $16,000. Tiffany, Roycroft or Steuben lampshades made with slag glass can command prices of up to $20,000.
Valuing Slag Glass Antiques
Slag glass stands out from other types of glass antiques for the beautiful marbleized color patterns. Though it's easily mistaken for other glass products produced around the same time period, such as stained glass, slag glass has its own unique look and slag glass items can fetch a wide range of prices. Work with a qualified appraiser to help you identify the manufacturer and type of slag glass antiques you have, including antique glass insulators, to determine the true market value.