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Hardware 101 > A Blacksmith's Glossary of Terms

About Iron

Iron - chemical symbol FE - the most commonly occurring (by weight) element on earth is, at the same time, the rarest in its elemental form. While the magma core of the earth is made mostly of iron and the red hills of the Rockies and brown and gold fieldstone all contain iron, the iron exists only in its various oxide forms. The only naturally occurring pure iron is found in fresh meteorites (and some very rare chemical situations - like bog iron in the New Jersey cedar swamps). Meteorites quickly (in geological terms) oxidize on exposure to air and turn to rust - iron oxides - just like the Rocky Mountains.

Cast Iron - the product extracted from iron ore. Iron oxide-rich ore is heated to a molten state in the presence of limestone within a large flask. Oxygen makes a chemical transition from the iron oxides to the calcium forming a "slag" which floats to the top of an increasingly iron-rich molten pool. When done, the bottom of the flask is opened and the iron pours out - historically into sand patterns to produce cast iron stoves and such, or into troughs on the foundry floor to form rough billets. The billets could be re-melted and cast into shapes at other foundries or could be hammered into wrought iron in forges. Good news is cast iron flows easily into molds and can be used to quickly form intricate articles from iron. Bad news is that cast iron is brittle and cracks rather than bends. Brittle because it is not pure iron - impurities are not completely leached from the ore and carbon from the heating fuel is absorbed into the iron pool. Quick cooling of the iron in a mold also increases brittleness - smaller, thinner pieces (like hinges) are usually more brittle than larger, heavier pieces (like machines) that "hold their heat" when poured and thus take longer to cool.

Wrought Iron - literally "hammered" iron. Early on, billets of cast iron were transferred to iron forges. The cast iron billets were heated to near-molten and flattened under a heavy water-powered drop hammer - kind of a see-saw with a couple of hundred pound chunk of iron "hammer" on one end of a heavy wooden beam and a rotating wooden cam acting on the other end to lift and drop the hammer in time with the turning of the water wheel. Valley Forge took its name from a small shop such as this which the British burned, unopposed, on their way to Philadelphia in 1776. Once hammered thinner and longer the billet would be doubled backed on its length, re-heated and re-hammered. This repeated exposure to the oxygen in the fire and the compressing blows of the hammer would burn and drive impurities from the billet. The iron being forged would become increasingly malleable with each heat to which it was subjected. The finest forging iron was multi-layered and nearly pure iron. To the best of my knowledge, there is no new wrought iron being produced today. Well, maybe Japanese Samurai sword smiths still make their own - but if it's in Home Depot and says "Wrought Iron" it isn't.

Steel - basically a mixture of iron and carbon, although other metals may be added to change the characteristic of the steel (add chrome, get stainless steel; add nickel, get armor plate). The carbon content of steel is closely controlled in its manufacture - the more carbon the "stronger" the steel. Steel varies from iron in that it can be hardened. Heat steel red-hot and quench it and it gets hard, heat iron red hot and quench it and it gets cold.

Blast Furnace - originally the Bessemer converter, invented pre-Civil war. Where steel comes from and why there's no wrought iron anymore. In this process, the iron ore is reduced to a molten state, the slag is poured off, then air (oxygen) is forced into the melted mass. Any impurities, such as carbon, are burned from the melted iron in the presence of the free oxygen. While still in the furnace and in a molten state the appropriate additives are added in very controlled measure to the near-pure iron in the furnace to produce the type of steel required.

Tempering - as mentioned above, steel can be hardened. When heated to red-hot, the iron and carbon in the steel arrange themselves into a lattice network. When quickly quenched, the lattice is frozen in place and forms a rigid structure; the more carbon, the greater the rigidity. When quenched, the steel is as hard as it will become. The harder the steel, the more brittle it is. A file is absolutely hard. It will cut lots of mild steel because it is so much harder - but put a file in a vise and hit it with a hammer and the file will readily snap in half. Same with a drill bit, cock the drill and the bit will easily snap. Tempering is a process of controlled heating of hardened steel that will relax the lattice within the iron and carbon, thereby softening the steel. The higher the temperature, the softer the steel becomes. A chisel for steel is tempered at a high temperature - softened enough so it won't snap, but still harder than the steel it will cut. A clean piece of steel will turn a very specific color based on the temperature to which it is raised. Heat one end of a piece of steel in a fire and you can easily see bands of color move up the steel coincidental with the heat migrating up the piece. It begins as a very light yellow, turns to a darker straw color, through red and on to purple. Blacksmiths have always used these colors to identify the temperature of the steel and thus control the temper (hardness) of the work piece. Spring steel is often a pretty bluish-purple color - because it's tempered to 750* and that's the associated color - soft enough to bend, but hard enough to retain “memory” and spring back to its original shape.

Forged - iron or steel heated and hammered to shape.

Drop Forged - a piece of red-hot iron or steel placed within an impression and then struck with the matching impression mounted on a heavy top hammer or ram. Typically the work piece is struck through successive impressions to squeeze the material to finished shape. Adjustable wrenches are drop forged. Nice detailing like on a cast iron piece, but not brittle. Makes a good strong tool, heavy gear, or cam shaft.

Hand Forged - iron or steel heated and hammered to shape without the use of "closed dies". Drop forging uses "closed dies". Other than moving the piece from one cavity to the next there is no human input to the shape of the finished piece, the dies do it. Hand forging involves hammering the heated metal on an anvil - the movement of the material and the finished form are determined by the smith as the material is moved under the hammer. Each piece is slightly different. If you find "Hand Forged" and a barcode on plastic packaging be suspicious.

Forge Welded - how they used to weld all the time. Two surfaces of iron or steel are heated in a fire to nearly melting (actually just melting on the surface) then hammered together. A lot like taking two ice cubes and allowing them to sit out until the surfaces are wet, then putting them back in the freezer stacked together. Come back and cut the two ice cubes across the surface where they re-froze and you can't see a seam where they re-joined. Same with forge welding - two melted iron faces puddle together then "re-freeze" - a very strong weld with complete joining of the surfaces involved (when everything goes right).



 

 

 

 

 

 

 

 

 

 

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