“Slag” is a somewhat unusual word, and unless you really know your glassware, the term ‘Slag Glass’ probably won’t ring a sort of bell at first.
Known by various other names, slag glass is instantly recognizable by its opaque glass and smooth blend of colorful streaks, creating a distinct marble effect.
Vintage slag glass can be a collectors item, but contemporary versions are also extremely popular and thousands of items on still made today.
In this guide, we’ll cover everything you need to know about slag glass—from its history to how it’s made, what it’s worth, and how to identify a vintage piece.
What is Slag Glass?
According to the Glass Encyclopedia, Slag glass is made by mixing pulverized silicate slag into glass-pressing. The term ‘slag’ refers to the molten run-off that is produced during iron smelting as the metal cools off.
This glassy residue would vary in tint and color depending on the minerals and chemicals used in steel production, resulting in a glossy, opaque, marbled effect.
It’s no surprise that this style is most typically referred to as marble glass.
Here’s the full list of other names used to describe slag glass over the years:
- Brown Malachite glass
- Marble glass
- Mosaic glass
- Variegated glass
- Vitro porcelain
The smooth, uneven swirls enveloping the glassware give slag glass an unmistakably look, with no two ever being quite the same.
What’s the difference between Slag Glass and Stained Glass?
This is something people will often wonder about. Both beautiful and elegant in their own regard, stained and slag glass are actually completely different.
The addition of iron slag into the process gives slag glass its unique features, while stained glass is made from a simpler process of colouring the glass with vitreous paints.
At first glance the two can appear similar, but a close examination of the glass opacity will often be enough to differentiate one from the other.
History of Slag Glass
The history of slag glass goes back to the late 1890s in Gateshead in the northeast of England.
Sowerby of Ellison Glassworks—the world’s largest manufacturer of pressed glass at the time—was the first to create slag glass, which quickly became popular.
In the early 20th century, Thomas Dugan and Harry Northwood were the first to make imitation slag glass, called ‘mosaic glass’.
This version was considered an imitation because the process didn’t involve using any real iron slag as had been done by Ellison Glassworks. Instead, the glassmaking duo from Pennsylvania combined two colors of molten glass—one white and one opaque—to create the same marbled effect.
Without the need for the residue product of iron ore, the manufacture of mosaic glass exploded in Europe and the US and its popularity has not waned since.
Slag Glass Colors & Composition
The principal feature of slag class is that it’s made from a residue by-product of iron smelting. This ‘slag’ is predominantly binary compounds of elements such as silicon, sulphur, and aluminium reacting with the sides of foundry molds. The slag floats to the top, cools, and is collected to mix and make slag glass.
Brown with silky cream streaks was the first color palate of slag glass, and vintage pieces with this tint are some of the rarest and most valuable today.
George Davidson is believed to have made this brown bugle in the late 19th century, one of the first pressed glass pieces known today.
It wasn’t longer before Davidson, Sowerby and other glass makers began experimenting with different colors, combining streaks of creamy opal with purple, yellow, blue, and green.
Early examples of slag glass already came in a variety of colors, but modern styles of mosaic glass are available in a seemingly endless array of hues and shades.
Slag Glass Identification
There are several things to keep in mind when looking to identify whether something is slag glass or not. Determining slag glass worth will vary in difficulty depending on what you’re looking for.
Always remember to inspect glassware in a room with good lighting.
The swirl effect
Slag or mosaic glass?
This is the most difficult distinction to make if you’re not a professional. Both styles create the same swirl effect, and originals of both are considered vintage and valuable. To determine if an item is true slag glass or if it’s a combination of two molten glasses, consult a vintage glass appraiser.
Vintage or contemporary?
There are three ways to differentiate vintage slag or mosaic glass from modern iterations:
1) Examine the marbling pattern: While both new and older pieces will have a marble look, it’s certainly not enough to spot an antique. Slag glass antiques will not simply have white streaks blended with another color, but possess a richer, deeper tone. The marbling on originals will always be uneven and more creamy than white.
2) Inspect the color: Vintage slag glass can really only be found in brown, purple, green, and blue. Slag glass of other colors is unlikely to be an antique.
3) Check for markings and names: As with most antiques, manufacturers and designers will often leave some form of signature or marking, making it much easier to date and identify vintage slag glass.
An antique piece from the UK would typically have one of these markings:
While American vintages would bear the marks of:
- Akro Agate
- Atterbury& Company
- Challinor Taylor and Co.
- Imperial Glass
- Northwood Glass Company
Be sure to check out this comprehensive guide on antique glass markings to know what to notice when appraising an antique piece of glass.
Slag Glass Valuation
The price of slag glass ranges considerably from five dollars for small slag glass pieces, to antiques that would garner hundreds—if not thousands of dollars.
The value of vintage slag glass, as with any antique, will depend on the quality of its condition, time period, and most importantly, the manufacturer.
Antique lampshades are perhaps one of the most common fixtures for vintage and valuable slag glass. Some famous makers include Tiffany Studios, Bradley Hubbard, and Handel—whose originals in good condition are very sought after.
You may find one for as cheap as $230, but very rare pieces will definitely attract collectors and drive up prices into the thousands.
The Bottom Line
Those who love, buy, or collect antiques will tell you that while price and value are important, they are really only half the battle.
There continues to be a vibrant market for antiques of all varieties due to their breathtaking beauty and exquisite designs. A beautiful piece—no matter the age or value—has the ability to brighten a room and add character to interior decor.
Slag glass pottery and ornaments are perhaps one of the best examples of this, as they are two of the most common types of slag glass found in modern homes.
Whether you’re looking for a decorative piece to spruce up the place, or searching for that next big purchase as a collector, the world of slag glass has something for everyone.
Have any questions? Be sure to leave us a comment.