Topic: Slag glass manufacturing and its relation to iron processing?

[krsilber]

On a trip to Colorado recently I collected a bunch of stuff I thought was obsidian (I'm a rock hound, pick 'em up wherever I go).  Turns out it's probably a byproduct of iron processing.  Flux was introduced to the smelt to bind with the silicon dioxide, which is an impurity in metal and has to be gotten rid of.  The resulting slag (molten silicon-rich stuff) floats on the molten metal, and is drained off.  Depending on how it's cooled, it can turn into glass.

There are various modern reports that this slag was once used in the manufacture of marbled glass, which is how it got the name "slag glass," but I remained skeptical until I found an encyclopedia entry from 1886 that alludes to it.  I thought it kind of interesting, and thought I'd share it and see if anyone else has come across more info about it.

Mod: Long URL converted to short link
Google book entry

[Frank]

A lot of early glassworks were based in iron works, one of the reasons that so much of the glass terminology is derived from iron working technology.

[krsilber]

How early are you talking?

Then it would make even more sense that they used iron slag in some of the glasses.  I've read a couple places that "slag glass" is a newer (late 20th C) term, so it's interesting to see it in a 1886 book.

If regular and slag material were only partially mixed to create things like agate glass it seems awfully lucky that their coefficient of expansion was compatible.

[Frank]

1610 in Scotland but while it phased out in UK the US continued to use for a long time. I think you will find more info on the original meaning of slag glass in US terms in the Glass-Study and also some UK patent abstracts which 'might' describe the processing well.

[krsilber]

Hmmm, afraid I haven't kept my membership active (by the way, Frank, are you still interested in a cut glass motif catalog?  [I named my first sculpture series after a cut glass motif!  Vesica.])

Oh, so 1610 - we're talking way early!...although it's all relative.  I've never heard of a glass factory in the US that was in an iron processing plant, must have been early here by our standards.  Did they share furnaces?  You'd think there would be all kinds of contamination

Here's a quickie photo of some of my slag.  Even the glassy stuff is extremely hard and heavy.  Some is a lovely green.  Apparently the ancients used glassy slag in jewelry making.

[Frank]

No-one's membership has expired as it is still in early stages and the problems that we have faced since moving to France have stolen all of my time for web projects. I am slowly getting myself back into it and am working on a huge document for the glass-study. Member fees are intended to finance growth of the library so while it is not growing no fee renewals...

Yes on cut glass project...

Slag... it seems the use of slag must have died out at some point and was being reintroduced in the 1870s in Europe and US. Disadvantage is you cannot get very clear glass, advantage is a huge cost saving in materials. Good for some containers, mineral wool (US) or silicate cotton (Europe) for insulation and other uses. Furnace not shared but when removing the slag it is hot and by having glass furnace close to hand it can be loaded into that hot, saving on fuel for re-heating.

c1875 US report

...we read of an iron manufacturer offering to furnish to any glass-maker who wishes to establish glass works in his neighborhood three million of tons of slag for nothing.

1867 UK patent

Steel slag, or the steel cinder obtained in making steel &c. ... is used in the manufacture of glass by fusing it with silica, lime, and lead or zinc oxides or other glass-making materials.

Clearly no-one thought to check prior art in that case!

[krsilber]

Very interesting.  Thanks, Frank!  (Will email within the next couple weeks re: cut glass)