Slag or slag glass is fairly common in the rock and mineral world, believe it or not. Most rock shops will have a bucket or two of this material and it will always catch your eye because the colors and patterns are vivid in nature.
I get lots of subscribers to my newsletter emailing me about a huge “enter the name of a precious gemstone here” they found while buying antiques or bidding on parcels of rocks online. Most of the time these junior rockhounds and crystal collectors have a piece of slag but they swear it’s a rare gemstone.
With that being said, I thought I would discuss slag glass and educate everyone on what it is, how to ID it quickly, and why it’s kind of cool to collect but make sure you don’t overpay.
Let’s get the basics out of the way, which shouldn’t take long. Slag glass is the term used to describe a colored, pressed opaque glass made using ‘slag’ leftover from the iron smelting process.
Slag Glass Rock Varieties
Slag Glass rock varieties come in different hues (hue is another word for “color”) and patterns. The original slag glass created in England was well-known for having a brown base mixed with streaks of creamy white, which led to Brown Malachite and Brown Marble. Another early example was Purple Malachite Glass produced by Sowerby, sold in the US under the name ‘Blackberries and Cream.’ Sowerby created several other color formulas, including Pomona (green), Giallo (yellow), and Sorbini (blue).
Mosaic glass made in Pittsburg blended purple and white for an opal hue.
Different companies offered their products in similar colors and hues but listed them under different names, which you’ll notice below.
While brown, white, cream and purple slag glass was common, it was rarer to find shades of blue, green, and brown. In recent years, Slag Glass can be found in orange, red, and pink.
Blue Slag Glass
Sorbini was blue slag glass produced by Sowerby, but the Lealand Lake Superior Iron Company made Leland Blue Slag Glass, aka Leland Blue and Leland Blue Stone. This company was located at the mouth of the Leland River in Michigan, and heated Michigan ore with limestone to refine the ore into materials for manufacturing steel, but beech and maple hardwood-based charcoal was involved too. The combination of ingredients produced beautiful shades of blue. About 2 percent of the slag produced here created the blue prized by collectors.
Red Slag Glass
Swirled pieces of Slag Glass in shades of red or orange were made many decades after the first Victorian pieces of genuine Slag Glass. You can find raw material that is translucent and opaque which make for great cabochon material.
Green Slag Glass
British companies Sowerby, Davidson, and Greener produced Slag Glass during the Victorian era (1880s/90s). Sowerby marketed their products under ‘Malachite,’ which was used for all the colors they made, even though it is generally only applied to green glass.
Other companies produced green slag glass as well in European countries but Sowerby is the most well known.
Purple Slag Glass
Imperial, the manufacturer, was one of the most prolific makers of Slag Glass in various hues. They made glass in green, brown, red, and purple colors but Sowerby is most well known for producing the Purple Malachite you see above.
There is one other company based out of Pittsburg named Mosaic Glass that manufactures purple and opal or white glass.
Black Slag Glass
Black Slag Glass material was typically gray, brown, or black, sometimes producing an iridescent effect. This variety often had rough or sharp surfaces, with some samples producing a glassy surface with several vesicles or bubbles.
Imperial Slag Glass
Imperial Glass produced many glass patterns during the company’s 80 years which can be seen in all sorts of glass and glassware items. However, it wasn’t until the 1960s that Imperial’s line of new Slag Glass items became an industry standard because of their quality and beauty.
It might not be a rock or mineral but you should know that Imperial glass pieces are incredibly valuable.
How To Identify Slag Glass
It’s fairly easy to identify slag glass in raw form but it does get tricky when people fashion them into cabochons, carvings, or faceted stones. You’ll need to take a closer look which means you might have to breakout the 10x power loop.
Opacity & Pattern
One of the most profound qualities of Slag Glass is the marbling. No other form of glass produces the same creamy swirl in repurposed Slag Glass.
You want to remember that the marbling you see in Slag Glass does not make the material opaque. Instead, it’s subtly mixed in the glass’s primary color, producing a smokey, deep appearance. The primary color should be more prominent in specific swirls, and it should be relatively uneven.
The pattern should seem random because it’s a natural, chaotic result of the repurposing process and not a synthesized effect. Variation is the key!
Slag Glass Color
The type of impurities extracted from the metals during the smelting process defines the colors of the resulting slag. The color of Slag Glass confirms if it is genuine, but it can also provide insight into a sort of ancestry. For instance, only certain metals were used in the early days of the Victorian era, which helped indicate the piece’s age. The only colors used during the Victorian era were blue, green, dark brown, and purple. Regardless of the color or period, the Slag Glass is from, the color will be very vibrant if it is true slag.
The presence of conchoidal fracture marks will rule out most rocks and less brittle crystal formations. Conchoidal fractures will only be noticeable in raw slag glass chunks, so this will only be an identifying factor if you’re handling the raw material.
Slag Glass will have a lot of perfectly spherical bubbles that vary in size.
If you’re working with or trying to identify black Slag Glass then you need to keep in mind you can be easily fooled into believing it is Obsidian. You should spend some time comparing black slag to Obsidian if you want to get good at spotting the differences. The main difference is the absence of bubbles in authentic Obsidian.
Slag Glass Value
The value of Slag Glass antiques can range from $20 to $20,000. Unsigned decorative pieces like compotes and vases can sell from $5 to $50, even from the nineteenth century. Lamps range between $200 and $2,000. Prices increase significantly if the pieces are signed by a prominent manufacturer like Tiffany or Bradley & Hubbard.
Raw Slag Glass won’t fetch nearly as much but prices can range between $5 and $20 depending on the size and condition.