I’ve decided to explore hardware fabrication for my upcoming hutch build. Actually, I’ve gone a little further and have bought myself a small pile of wrought iron rods, reclaimed from a grain elevator in the midwest. This is real wrought iron:
I say real wrought iron rather pointedly, as the term ‘wrought iron’ has become mangled out of all recognition, and there are those folks who, as they don’t apparently read very much, take the word ‘wrought’ in by ear and think it is ‘rot’, or ‘rod’, iron. And even when most of those who mention the term ‘wrought iron’ are not actually understanding what it is. No surprise since it has become a very uncommon material. Many are taking the term ‘wrought/rot/rod’ in the sense of ‘having been worked by a blacksmith’, however the sad truth is that probably 95% of what is marketed as wrought iron has not been worked by a blacksmith, nor is it even iron. Rather it is mild steel, and has been cold bent. And even if them metal has been heated, hammered and formed by a blacksmith, it is likely that 95% of the time the metal was mild steel, not iron.
So, let’s go through it then. What’s the difference between iron, wrought iron, cast iron and steel? Well, it mainly comes down to carbon.
Iron. It’s an element on the periodic table, symbol Fe, atomic number 26. It’s the most common element forming the earth. Pure iron is actually softer than aluminum, however you can’t obtain pure iron by smelting. The process of smelting is the production of a metal from an ore using heat and a chemical reducing agent. When iron is smelted, carbon gets added to the mix, and this radically hardens the iron, up to 1000 times harder depending upon how much carbon – and other impurities – get added.
Wrought Iron. This is the purest form of commercial iron, containing 0.10% to 0.25% of carbon and less than 0.25% of impurities like sulfur, phosphorus, silicon and manganese. In the past, wrought iron was also referred to as ‘commercially pure iron’ however this term now applies only to iron with a carbon content of 0.08% or less. Wrought iron was typically produced in a bloomery, a type of furnace producing a porous mass of iron and slag called a bloom. Wrought iron has fibrous inclusions in it (see the above picture) which are termed ‘slag’ or ‘stringers’. These slag stringers are merely mixed in, they are not alloyed (a chemical union). Wrought iron that contains an excess of sulfur is termed ‘redshort’, which makes the iron tends to crack when heated – thus redshort wrought iron is useless for forging. If there is an excess of phosphorus, like the stuff I’ve bought, then it is termed ‘coldshort’. Coldshort wrought iron can crack if bent cold, but may be worked fairly easily at high temperature. Phosphorus in the iron gives it great corrosion resistance. The 1600-year old, 7m. tall Iron Pillar of Delhi, is 98% coldshort wrought iron. No black paint needed it would seem. As wrought iron lacks much carbon, it cannot be hardened through heat treatment. It has excellent weldability. Sheet wrought iron cannot bend as well as sheet steel without being heated up beforehand. Wrought iron can be melted and cast, however the process of doing this removes the slag stringers, so the resultant product is no longer wrought iron.
Steel. Take iron and combine it with other alloying elements, most typically carbon, and you obtain steel. ‘Carbon steel’ is what you get when carbon is alloyed with iron to a total of 0.2%~2.1% carbon by weight. As the carbon content rises in the steel, what is obtained is improved hardness and strength (through later heat treating), at the expense of ductility. Also, the more carbon one adds, the poorer the weldability and the lower the melting point. There are many types of carbon steel – the one seen on the market as ‘wrought iron’ most commonly is actually mild steel. Mild steel, aka ‘plain carbon steel’, contains 0.16–0.29% carbon, which makes it malleable and ductile, but incapable of being hardened by heat treatment. Mild steel rusts readily.
Cast Iron. This material is characterized by a carbon content of 2%~4% by weight. It also contains silicon at 1%~3% by weight. Cast iron is produced by melting iron or iron alloys in a furnace until it liquifies, then pouring into a mold. Typically, cast iron is made from pig iron, which is iron ore that has been smelted with coke (for carbon) and limestone (as a flux), and has a carbon content in the range of 3.5%~4.5%. Most types of cast iron are brittle, and have excellent machinability, resistance to deformation and wear. The engine block of your car is likely to be cast iron.
In the above descriptions I have simplified the picture somewhat, and avoided going down the various sub paths and branches which affect the qualities of the myriad forms of iron and steel that are produced – I refer to the various other possible alloying elements which can greatly affect working properties of a material.
So, on first glance it would seem that just a percentage up or down of carbon, mostly, makes all the difference to whether something is iron or steel. Just a few percent may seem like no big deal, however in a similar sort of way a difference of just a few percent in the arrangement of the four elements in DNA makes the difference between a slime mold and a human being.
Wrought iron was the precursor to modern steel, and was used extensively up to its production peak in 1860 or so. After that, steel began to supplant wrought iron, most especially because it was far cheaper to make. Wrought iron took, in most processes used, a lot of skilled labor to produce. Wrought iron hasn’t been produced by large mills for the past 100 years of so. The last commercial production of wrought iron happened in Sweden in the early 1970’s. If you want to work with real wrought iron, it will be a material obtained from salvage or made by those folks who want to build their own kiln and smelt their own ore, as some are actually doing.
Compared to mild steel, wrought iron has improved ductility and far superior corrosion resistance. The slag stringers in wrought iron give it a grainy, stringy quality than cannot be matched by the homogeneity of mild steel. Wrought iron can be forged further without cracking or splitting than most mild steels. Wrought iron fences, gates, etc., will last for hundreds of years and don’t require painting. Try that with mild steel.
I’m going to try and make drawer handles out of the wrought iron I have purchased. I don’t have a forge, or much in the way of metal-working equipment, but why should I let a lack of experience or knowledge slow me down? I’ve found that the blacksmiths and metal artisans I have communicated with in recent days have been most willing to share their knowledge, so when I run into problems, there are people I can call on. Forward into the abyss!