The storms in the DC region tuesday night knocked down a power line near my house. The live line touched the sidewalk, and arced all over the place until it was shut off. Investigating the results later, the area of sidewalk where the line hit appears to be melted: there is a roughly eliptical crater with a major axis of about a foot, and a minor axis of 8 inches, about 3 or 4 inches deep. The periphery of the crater is studded with cylindrical holes, ranging from 1 to 3 inches in diameter, and from 1 to 6 or 7 inches deep. Mounded around the crater is a ring of what appears to be dark green glass.
Now I know that concrete is mostly cement and sand. The silica in the sand appears to have melted and re-formed into this blobby green stuff. But what about the limestone in the cement? Did it vaporize? Is it the cause of the green color in the resultant sample? Also, I thought that concrete was pretty much fire-proof. I guess a do-it-yourself arc furnace produces enough heat to melt even concrete.
Any insight into what happened and what I have would be appreciated.
I would think the metals would pretty much contribute to the color of the glass, and the gypsum would just be an impurity. There wouldn’t seem to be enough to really worry about.
Damifno what would happen to the calcium from the silicates
The silicates have a melting point of 1400 °C or higher, although there’s a lot of uncertainty in this because it depends on the precise composition of the mix. Gypsum, Ca-sulfate, melts at ~1450 °C. Any limestone in the mix would undergo thermal decomposition at ~1000 °C to form calcium oxide (quicklime) and CO[sub]2[/sub]. The green color of the glass resembles that usually attributed to iron in the melt.
That’s a great picture, you ought to buy the slab and mount it in your front yard as “found art.”
My husband, the firefighter calls this phenom “Spalding” Indeed, the temps produced exceed 1800°F, so Squink is correct. One other thing in the green, is probably the melted copper from the line itself.
My FF also has a general warning about downed lines. Never assume they are dead. Even if they have been shut down, high powered lines can “loop back” making a dead line live.
The power grid is designed to cover as many customers as possible. If a break in a line cuts off some customers, the substation can reverse flow direction and feed the line from the opposite direction.
I hope I explained this understandably, second hand, you know.
Nope, Iron normally gives a green glass. It’s a common contaminant in sand that gives rise to the green color of, for example, coke bottles. To make clear glass the iron must be removed from the sand.
While copper salts can result in a green glass you’ll also get reds, depending on conditions:
Enamels
I don’t see any reds in Emilio’s photo, and the conditions appear to have varied wildly. Iron contamination of the sand in the concrete is the simpler hypothesis, especially since the material has the color of a melted coke bottle rather than the bluish-green of some copper glasses.