Vintage glass pieces come in so many different colors, types, and styles with different names for each variety. Have you ever wondered what someone meant by slag glass?
Slag glass is a type of opaque pressed glass that includes colorful streaks that create a marbled effect. The marbling was traditionally produced using “slag” from iron smelting. Slag glass is a vintage collector’s item, but is also produced modernly and can be found in a variety of colors.
What is Slag Glass?
Slag glass is pressed and untempered glassware, much like depression glass. The molten glass is placed into forms and then pressed to get the right shape and emboss the surface with a pattern. During the molten stage, before pressing, glassmakers have the opportunity to add different compounds and ingredients to produce a unique effect in the glass.
In this case slag, a by-product from iron smelting, is mixed into the molten glass before pressing. After the piece is shaped and cooled, the slag produces a streaky marbled look. The base of slag glass is an opaque color, and the streaks are typically a milky white or cream color.
Slag glass is also sometimes used to refer to marbled glass pieces that didn’t technically use slag as an additive. Glassmakers have achieved a similar-looking marbled effect to slag by simply mixing two different colors of glass together. This wouldn’t be considered slag glass in the strict traditional definition, but many collectors will still use the term to refer to any marbled, opaque, pressed glass pieces. A more specific title for this type of glass would be mosaic glass.
As with many other types of vintage glassware, the slag glass technique can be found in all sorts of different pieces, decorative or functional. Although, the majority of slag glassware was created as decorative pieces. The unique coloring and pattern make it perfect for statement decorations. One of the most common applications of vintage slag glass is in lighting. Lamps, lampshades, and chandeliers were very popular uses for slag glass.
In some of the most stunning lighting pieces, small pieces of the slag glass were laid out in a metal framework. Designers would create intricate stained glass style patterns and scenes within lampshades. Some stained glass pieces may appear very similar to all of the small pieces in beautiful patterns.
However, stained glass will not have any of the milky-colored streaks that slag glass does. The marbled effect is gorgeous on its own, but shining light through a complicated and colorful design is even more stunning. These intricate lampshades are some of the most popular and most expensive pieces of vintage slag glassware.
What is Slag?
Traditional slag glass gets its creamy colored streaks from something called slag. Slag is used to refer to the by-product that shows up when various ores and metals are smelted. In this case, the slag used for marbling glass is produced when iron is heated during the production process. As molten iron cools, a silicate slag forms on top of it. This byproduct can then be pulverized and used as a glass additive.
Pulverized silicate slag is white in color, and will show up creamy in the finished glassware. Since it is only partially mixed to create marbling, slag glass will have different tones of whiteness throughout the piece. Where the slag is really concentrated it will appear milky white. However, towards the edge of the streak, the slag may instead be a lighter and creamier version of whatever the base color was since they were more mixed together.
A well-crafted piece of slag glass with the right ratio of additive and correct amount of mixing will strike the perfect balance between looking too chunky and looking too homogenous. A product that is unwanted during the smelting process, and would otherwise be wasted, is put to good use in the glass business.
It can seem strange to think of putting metal byproducts into a beautiful piece of glass, but it is super common to use a wide selection of additives and compounds to give glassware unique looks. Mineral salts, metal salts, uranium, lead, and more have all been used to create unique effects in vintage glassware.
History of Slag Glass
Slag glass was first created in England in the 1880s as an opaque brown with white streaks from iron slag. Sowerby, a popular pressed glass manufacturer in the UK during this time frame, was the first manufacturer to start producing slag glass. They started with brown glass, but also produced purple, yellow, green, and blue. Other manufacturers quickly caught on and the style rose in popularity soon following its creation.
Two glassmakers in Pennsylvania, Harry Northwood and Thomas Dugan, were the first to imitate slag glass by mixing two colors of glass together and forgoing the actual slag in 1902. They still wanted the white streaks, and so they mixed purple glass with a white, opal-y, colored glass.
Vintage slag glass is most commonly found in brown or purple colors with white streaks, but you can also find plenty of green and blue as well. Most pieces are decorative in nature, sprucing up side tables and bookshelves. Another common use for slag glass was for various lighting pieces. Lampshades and chandeliers were commonly produced out of slag glass. The light shining through the marbled color creates a really unique effect.
Starting in the late 1800s and through the mid-1900s, both England and U.S. manufacturers were creating slag glass both with and with the use of iron slag. The pieces produced are of a very similar style and quality, and it can be difficult to identify exactly where a vintage piece was originally produced.
Challinor Taylor and Co. was an early U.S. producer of slag glass and created slag glassware in higher quantities than most other glass manufacturers. Over time, other companies like Imperial Glass, Akro Agate, and Westmoreland began producing and selling their own versions of slag glass.
Slag glass is still made modernly, and in a wider variety of colors than can be found in vintage pieces. Fenton, Mosser, Boyd Glass, and Summit are some of the common, modern, U.S. manufacturers of slag glass. If you love the look of slag but don’t want to pay extra for vintage, or are looking for a higher variety of options, check out some of the more recently produced glass pieces.
Other Names for Slag Glass
Most styles of vintage glassware have a plethora of names that they go by, and slag glass is no different. As with any glass type, or art in general, different circumstances and people have adopted different names for slag glass. Some of these names can be used synonymously, but some of them have slightly different meanings that a serious collector should be aware of.
Some of the other names for slag glass come from its historical beginnings. With the first versions of slag glass being an opaque brown with cream to caramel-colored streaks, slag glass is also known by the names of brown malachite or brown marble vitro porcelain (“brown marble”).
One of the most common alternate names for slag glass is marble glass, for obvious reasons. The key identifying feature of this type of glass is its similarities to the patterning found in natural marble. This being the glass version rather than stone, it makes sense to call it how you see it. Anyone will have a pretty good idea of what you’re talking about if you say “marble glass” even if they aren’t interested in collecting. On the other hand, “slag glass” can easily leave people confused.
It can also be called Malachite glass, in reference to the stone. Malachite stones have wavy striping in a green-tinged milky white on top of a darker green. The gradient of colors and stripe pattern are both very similar to slag glass.
Mosiac glass is sometimes used interchangeably with slag glass, although there are some technical differences. The strict definition of slag glass means that it has powdered silicate slag from iron smelting mixed into the glass to create streaks. A similar look can be produced using multiple colors of glass mixed together, each individually colored using various compounds. The term “mosaic glass” is much more inclusive and includes these pieces that look like slag glass but don’t actually contain any slag.
Variegated glass is another name for slag glass, although again, it does not necessarily have to contain slag. Oftentimes the pieces labeled as variegated glass have more stripey streaks in contrast to the more organic swirls of a true marble pattern.
Slag Glass Colors
Vintage slag glass is a lot more restrictive in color options than recently produced pieces are. All slag glass will be a marbled mix of some opaque color with opaque white. Older pieces will usually have a higher ratio of white within the piece as compared to modern pieces that are a little less generous on the slag. The original slag glass is brown with creamy streaks, a similar pattern to how your cup of coffee looks when you first pour in milk or creamer.
Vintage pieces can also be commonly found in a dark purple color. White melts into a pale lavender when it intersects with the opaque purple. Sowerby was a popular producer of this “blackberries and cream” slag look. A similar look was created around the same time by U.S. manufacturers Northwood and Dugan. Instead of slag, they mixed purple glass with a white-colored glass that did not contain any iron slag.
Brown and purple are the most common colors for vintage pieces but Sowerby, as well as other manufacturers, also created some pieces in blue and green with the typical white streaking.
As technology and manufacturing changed over time, new colors have been introduced to the slag design. Now, you can find slag glass pieces in practically any color. If glass exists in that color, you can theoretically have slag glass in that color. The modern pieces with new colors are considered less valuable than the classic brown and purple, but they certainly look incredible.
How to Identify Slag Glass
Telling slag glass apart from other styles is actually quite simple. Slag glass has something that cannot be found in imperial glass, uranium glass, milk glass, depression glass, carnival glass, or any other type of vintage collector’s glassware. The white marbling pattern on slag glass is a dead giveaway.
This does use the broader definition of slag glass that includes white marbling produced through any method. Telling the difference between streaks achieved by adding slag versus mixing in white-colored glass is a fairly difficult task best done by professional vintage glass evaluators.
Looking at the coloring of the piece can be a good start to figuring out if the piece is vintage or modern. If the base color is brown, purple, blue, or green there’s a decent chance that it is a vintage make. If the piece is pink, orange, or red it is not vintage. Typically vintage pieces will be worth the most, but some collectors may still value the piece.
Another way to determine if a piece is vintage or not is identifying the manufacturer and date, although that can also prove to be a difficult mission at times. Not all glass pieces are clearly marked. If the glassware is marked by the manufacturer, you can use that to reverse search information about your piece.
Check the bottom or inside of an item for either a name or logo of a manufacturer. The mark will usually be somewhere inconspicuous and be printed fairly small. A mark for Sowerby, Davidson, or Greeners will indicate a vintage U.K. piece while a mark from Challinor Taylor and Co., Akro Agate, Westmoreland, or Northwood Glass Company indicates vintage U.S. glass.
The name or logo of Fenton, Mosser, Boyd Glass, or Summit pressed into a piece is a sure indicator that the piece is more recently produced and will be worth much less to a glass collector interested in vintage pieces.
There’s a fairly large price range for slag glass, with intricate vintage lampshades fetching the highest price. Color, age, and manufacturer are definitely important in determining the value of a vintage piece. However, since each piece will have very unique marbling patterns, the estimated value can vary greatly based on personal preference. Two vintage brown Sowerby figurines can sell for two completely different processes depending on how the marbling came out. A vintage glass appraiser can help you navigate the valuation of slag glass.