Shattering Glass Myths, Part 2 of 3

In the inagural post to this blog, which was Part 1 of this short series, I looked at the ways we tend to think of glass (feels solid, but isn't really considered one) and gave a general overview of some of the differences among glass types--all glass is not "just glass." Continuing on the myth-shattering mission . . . .


Myth #3: The Clear Fulgurite

There's a scene in the opening moments of the movie Sweet Home Alabama where lightning strikes sand on a beach. The storm, part literary device and all Hollywood artistic license, produces a glowing mass in the sand that the movie's two main characters, just kids, gaze at briefly. Later, that item, supposedly a fulgurite, is on display--a sensuous, clear, sculptured object meant to symbolize a certain relationship in the movie. Symbols aside, does lightning produce this sort of item when it strikes sand? Yes, and no. The shape in the movie is the closest truth, but even then, it's far too elegant. Fulgurites, the shapes born when lightning and sand (and which I'll devote an entire future post to), are root-like, branching, and can even be spiked, gnarly (I've been waiting to use that word), or just tubular. But they are not clear. No way, no how. The reason is sand by itself is too impure to make a clear glass from a lightning strike. And don't let anyone tell you that a bolt of lightning is so hot that it burns out all the impurities. Lightning, according to the National Weather Service, doesn't technically have any heat, itself, but it can heat the air it passes through to around 50.000 degrees F--5 times hotter than the sun. The sand doesn't get that hot, but it does reach melting temp, and does so instantly. What you will find--if you are lucky enough to ever see a fulgurite--is most likely a tan- or gray-colored root-like shape, and it will be fragile. Some places on Earth produce black or dark brown fulgurites. But nowhere are they clear. (So beware of the sites online selling "authentic" clear fulgurites.) And this leads directly to the next misconception about glass.


Myth #4: Sand alone makes glass

Ordinary sand, by itself, does not make glass as we know it when heated although sand high in silica is usually 2/3 to near 3/4 of a batch. And though it contains one of common glass's three main ingredients, the other two--soda ash (sodium carbonate) and lime (calcium carbonate) must also be added to create a useful glass. Henry Halem, in his classic text Glass Notes: A Reference for the Glass Artist, notes also that since melting sand requires temps of about 3360 degrees F, it's not feasible as an ingredient alone (10). That's why even ancient cultures figured out ways to bring down the melting temperature of sand by adding soda or other ingredients. Then they figured out that to make the glass useful--what Halem calls "durable" (10)--they had to add lime, just one of several possible "stabilizing" agents (Halem 11). So you may be wondering, "What's the basic recipe for glass, then?" Well, there isn't one. And while it's not "basic," what follows will give you an idea of some of the possible ingredients for a batch recipe from 1919:**

Red Sulphide

Sand--100 lbs.

Soda ash--30.3 lbs.

Potash--30.3 lbs.

Zinc--30.3 lbs.

Borax--1.5 lbs.

Selenium--8.8 oz.

Cadmium Sulphide--8.8 oz

As you can see, just as there's much more to a cake than flour, there's a lot more going into this batch than plain old sand!


In Part 3, we'll look at an often-painful myth about glass and heat, and the price of art glass. See you then!


** Scholes. Modern Glass Practice, qtd from Kirkpatrick and Roberts, Journal of the American Ceramic Society, 1919, p. 896. Qtd. in Halem, 32.


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