Spark test shows carbon, should be very low in wrought iron. Tells nothing about slag
Here is an old wrought iron bar, most likely off a shipwreck. Found it on the shore of Lake Huron, northern Michigan. Use it as my "wrought iron example"
In s-l-o-w process of writing a small book on metals used in old guns. What i wrote on wrought iron is:
“Wrought iron cannot be cast, and cast iron cannot be wrought” was the ancient view of these metals.
Pure iron is an element, with the chemical symbol Fe, from the Latin word for iron, ferrum. It is magnetic, and not very strong. Iron has little commercial value in its chemically pure form, and is usually seen mixed with, or alloyed with, some other elements. The nearest thing to pure iron one might commonly find in the hardware store is “black iron wire”, typically 1008 steel. That is, mostly iron with 0.08% carbon by weight.
The first iron used was meteoric iron, which contains a few per cent of nickel. Tutankhamen had one dagger forged of meteoric iron, and another of gold. At the time the two metals may have been comparable in value.
The oldest form in which iron was refined from ore is called “wrought iron”. This term means just what it says, that it is iron which has been "wrought", or forged, into shape—it is not cast iron. Wrought iron is fairly pure metallic iron mixed with maybe 3 percent by weight of slag fibres. This slag is a glass made of iron oxides, and calcium silicates. It runs through the iron as long stringers. It is lighter than iron, so that 3% by weight means more than that by volume. If you look at an old piece of rusted wrought iron you will see a “grain”, or long lines, on the surface. Those long, stringy marks are from the slag fibres, which did not rust.
This slag has a positive effect on the properties of wrought iron. One is that when wrought iron is brought to a forging heat, glowing white hot, some of the slag melts on the surface and acts as a flux. This permits two pieces of wrought iron to be readily forge welded together. Herodotus says the art of welding was discovered by Glaucus of Chios. Indeed, for nearly three thousand years forge welding was the only kind of welding. Welding iron was done then without any melting of the metal.
Wrought iron is absolutely magic stuff in the forge. I made my first forge weld in wrought iron years ago on a friend's forge - just before I burnt my first iron.
Oh yeah - those slag fibres do protect the iron somewhat from rusting. Had that bar above been mild steel it would long ago have crumbled to rust. The last wrought iron was puddled in Western Pennsylvania about 1960. I DO NOT KNOW, BUT STRONGLY SUSPECT that the market for wrought iron disappeared as the use of galvanized (zinc coated) iron became more economical, for rust protection.
