A delicate and beautiful by-product of pyrometallurgical smelting, the repurposing of slag glass embodied the waste-not-want-not ethos of the late 1800s, but, interestingly, this substance that started out as scrap is now quite the collector’s item.
Desirable in both antiquary and geologist circles, it is often bought and sold for astonishing prices.
However, as something of a bygone material, not many are all that familiar with the appearance, properties, or history of slag glass, often making it quite tricky to identify… but it’s by no means impossible!
Today, I’m going to talk you through a comprehensive slag glass identification guide. By the end of this post, you’ll have the answers you’re looking for. So, is that some swaggy slag glass you’ve got, or an imposter? Let’s find out.
Identifying Slag Glass: The Secrets Of The Slag
Thankfully, there are quite a few simple ways to tell slag glass apart from similar materials such as stained glass and obsidian, so don’t worry; you don’t have to be a trained geologist or veteran glassmith to get some conclusive results here!
The Marbling Of Slag Glass: Opacity & Pattern
One of the most distinct qualities of slag glass items is the marbling. Simply put, no other form of glass has the same creamy swirling that is found in almost all repurposed slag glass, but it might not look quite like the marbling you’re used to seeing.
Slag glass marbling isn’t opaque, rather, it’s subtly mixed into the primary color of the glass itself, giving it more of a deep, smokey appearance as opposed to a surface-level marking.
It should also be relatively uneven, as the primary color will be more prominent in certain swirls. If, by contrast, the swirling in your “slag” seems to be consistent in terms of opacity, it’s unlikely that it’s a genuine artifact.
The pattern of the marbling should also seem quite random, as it’s a natural, chaotic result of the repurposing process rather than a synthesized effect. In a nutshell, you’re looking for lots of variation in your slag glass marbling.
Analyzing the marbling of a suspected slag glass item is particularly handy for differentiating it from stained glass, which in certain instances, can look remarkably similar.
On the other hand, the marbling of raw slag glass isn’t quite as prominent, especially if you’ve acquired quite a dark specimen, so let’s continue our investigation!
The Color Of Slag Glass
As mentioned earlier, slag glass is a waste by-product of smelting, a process in which metal ores are super-heated in order to extract the pure base metal by separating it from impurities that accompany it when mined from the earth.
Once the metal melts, these impurities go one of two ways. They either:
- Boil into a gas and escape from the base metal into the air
- Form a crust on the surface of the molten base metal
All those impurities that take option b. become slag, and much like you can peel the skin off custard when it’s been left to sit for a while, metalworkers can skim this winter coat of impurities.
Now, I’m sure you’re probably wondering why I’m giving you this impromptu science lesson, after all, you came to identify slag glass, not to hear its creation story.
Well, the reason I’m telling you all this is that, the type of impurities extracted from the metal defines the color of the resulting slag — amazing, right?
Consequently, we can use the coloring of a piece of potential slag glass not only to confirm it is indeed slag, but, if we chose to, we could use it to trace it back to the metal it was originally skimmed from — sort of an Ancestry DNA test for slag!
With this in mind, as only certain metals were used in the early days of Victorian metallurgy, the color of your slag glass could well be an indicator of its age.
The only colors of Victorian slag glass are:
- Blue
- Green
- Purple
- Dark Brown
So, if your glass is one of these colors, then it could pick you up a pretty nice sum on the antiquities market.
Of these colors, purple is the most common, and dark brown is the rarest, and no matter the hue of your slag glass, if it really is slag, the color will be incredibly vibrant.
Conchoidal Fracture
Okay, so we’ve covered a couple of ways to identify decorative items crafted from slag glass, but what about if you think you’ve got some raw chunks on your hands?
Well, my first suggestion is to look for what are known as conchoidal fractures. These appear as swirls across the glassy surface, indicative of chaotic breakages. They look a lot like the ripples in a lake after you toss a rock in.
The presence of these fracture marks rules out most rocks and less-brittle crystal formations, making it more likely that you’ve found some genuine slag glass.
But we still have to narrow it down to a further degree, as fine-grained formations such as obsidian and flint will share this mode of breakage.
Location, Location, Location
Where did you find your piece of potential slag glass? Do a little digging and see if you can find some geological info for the area. Are there any volcanoes in or around the area?
If so, then there’s a chance that your slag glass is actually obsidian, but there’s still hope!
Color & Abundance
As mentioned earlier, brown is the rarest color of slag, but it’s one of the most common colors of obsidian, so if you found multiple pieces of brown mystery glass, it’s more likely obsidian than it is slag.
However, if there was just one brown piece, and perhaps you came across some other specimens with vivid, brighter colors, it’s looking good.
Opacity
Another amazing way to differentiate slag glass and obsidian is simply holding your hunk o’ glass up to the light. The thin edges of obsidian will allow a small amount of light through, whereas slag glass is completely opaque, no matter how thin an edge is.
Bubbles
Generally speaking, a prerequisite of obsidian formation is an absence of gasses. If there are a lot of gasses present, a bubbly bit of scoria will form instead. In light of this, obsidian doesn’t contain bubbles.
Slag glass, on the other hand, will have lots of perfectly spherical bubbles varying in size.
One-Sided Deposit
It’s perfectly common for a rock or glass-like specimen to come out of the earth with some form of mineral deposit clinging onto its surfaces.
Sometimes, you’ll only be able to see the primary material poking through this dense coverage in one or two places.
However, slag glass tends to only have substantial coverage on one edge, which is where the molten substance initially made contact with the ground when the manufacturing company dumped the slag.
Final Thoughts
Slag glass is a relatively rare find whether in its decorative or raw form, but even if you’ve come to the conclusion that what you have is not the real deal, at least you know more moving forward, increasing the chances you’ll find some eventually.
Besides, one of the great things about slag glass is that the likely alternatives, obsidian and stained glass, are also very attractive materials, so you should be celebrating regardless!