The first pressed glass pieces that collectors refer to as "slag glass" were made in the 1890s, using the glass-like by-product of iron ore known as—you guessed it—slag. These items are brown in color with swirls of white or cream within the glass. Other types of metal ores produce different colors of slag when they are refined.
Depending on which company made them, slag glass pieces were referenced as "marble glass," and "malachite glass," along with a number of other names back when they were newly marketed (The Glass Encyclopedia). Slag glass was often used in lampshade production as well as for crafting many different types of decorative objects.
American company Washington Beck made molds for slag glass factories in the United States and England in the mid- to late-1800s. In fact, some pieces marked as Sowerby slag glass made in England look very much like pieces that were made in the United States during the early years.
Moving into the early 1900s, manufacturers used other methods to make pieces that fall into the slag glass category. "Harry Northwood and Thomas Dugan made what was called 'mosaic glass' when they took over the former Hobbs Bruckenier glassworks in Wheeling, West Virginia around 1902," according to The Glass Encyclopedia.
Challinor, Taylor, and Co. also made their own version of this type of glass around the same time. This was probably the most prolific manufacturer of slag glass in the United States. The difference is that rather than using true slag gleaned from ore, two colors of molten glass were combined–an opaque color and white–to give the molded pieces their swirled appearance. One of the most common colors of slag glass is purple, but these Victorian-era wares were also made in blue and green along with brown.
Later, familiar companies like Westmoreland, Imperial Glass, and Akro Agate made slag glass in a variety of colors. Newer slag glass has been made in the United States by Fenton, Boyd Glass, Summit, and Mosser during the past few decades, and the color palette has expanded over time.
Recognizing slag glass is easy due to the distinctive coloration. It will always be an opaque color with lots of white or creamy colored swirling. Figuring out who made a piece, and how old it is, can be a bit more tricky.
Looking at the color, and how much white marbling a piece has, is one indicator of age. Older pieces will be heavily marbled and quite chunky. Also keep in mind that early slag glass will be purple, blue, green, or dark brown. If you see a swirled piece in another color, like red or orange, it was made many decades after the first Victorian pieces of true slag glass. That doesn't mean it isn't valued by collectors, however.
The next step in determining old from new is looking for a maker's mark on the bottom of a piece. Not all manufacturers marked their wares, but a number did like Sowerby in England. Akro Agate is an American company that marked most of their wares ranging from children's dishes to small decorative flower pots. Newer glass by Fenton, Boyd, and Mosser may also be marked.
Some of the most costly slag glass pieces take the form of lampshades by well-known makers like Handel, Bradley Hubbard, and Tiffany. These easily sell for thousands when in excellent condition, with Tiffany being the top of the line, of course. Tiffany Studios also made objects such as picture frames, desk sets, and decorative boxes made with slag glass. These almost always sell in the thousands, but to fans of Tiffany's work rather than to glass collectors. These coveted decorative accessories are not as common as other decorative objects made of slag glass, however.
Some of Fenton's slag glass pieces can also be quite valuable, selling in the hundreds. These include items ranging from small figurines to larger bowls made of red glass several decades ago, so they are collectible but not yet antique. Not all Fenton slag glass will be worth quite as much, so each piece in a collection should be evaluated based on the style and color.
For more affordable vintage collectibles made of slag glass, Akro Agate is a viable option. Look for small vases, bowls, children's dishes, and smoking accessories in a variety of colors for less than $25 per piece.