stickmaker | JOHT: Iron Age

The Joy of High Tech

Rodford Edmiston

Being the occasionally interesting ramblings of a major-league technophile.

Please note that while I am an engineer (BSCE) and do my research, I am not a professional in this field. Do not take anything here as gospel; check the facts I give. If you find a mistake, please let me know about it.

The Iron Age

There are many types of iron. First off, there's the actual element, pure Fe, which comes in several isotopes (same electron shell structure, same charge on the nucleus, but different numbers of neutrons in the latter) which all have the same chemical properties. Wrought iron is nearly pure (through frequently with traces of silicon, which aides the corrosion resistance) and has been in use from before the time of the classic Iron Age up well into the age of steel. (The last wrought iron plant in the US closed in 1969.)

Wrought iron gets its name from the traditional method of production. Iron ore (often bog iron in the old days; nodules of mostly iron formed in bogs, partly by bacterial action) was heated and "wrought" (from wreaked, or bent) to work the impurities out. To perform this process, one or more lumps of iron ore are combined and heated, then pounded to get rid of the unwanted material and forge the remaining iron into a single, solid chunk. During this work the piece is beaten flat, reheated, folded, and beaten flat again, until the smith determines it is ready for forging into the desired shape. Wrought iron was - and still is - a fine material for many uses. It is ductile - that is, easily worked without splitting or cracking - and nearly immune to rust (hence the traditional popularity for such things wrought iron fences and yard furniture). Making large pieces from wrought iron using this traditional method of production is difficult, since the "blooms" produced by wreaking are usually small, maybe enough for a modest belt knife. Anything larger required welding or forging the products of two or more blooms together. For things like furniture or fences individual pieces could be riveted together

If successfully forged, though, a weapon or piece of armor successfully made in this way has some interesting properties. For example, sword blades were made by forging separate strips of wrought iron, heating them and twisting them together, then forging the result into a single mass, often folding one or more times along the way to get the piece to the correct dimensions. The process is not unlike kneading dough or wedging clay, which leads to speculation about how it was developed for iron. The result was a pattern blade. Named because breathing on the metal brought out a writhing pattern created by the twisting, folding and flattening. Some such blades were even acid etched, to bring out the pattern without the fog of breath.

Items of this sort are often referred to as "Damascus" or "Damascene" but most likely the process was developed independently multiple times in widespread locations through a long period.

Such blades had properties quite similar to those of the famed later folded steel fighting blades of Japan, such as the katana sword. Because no two blooms were identical, the twisting and forging fused metals with slightly different characteristics, making a composite which blended those different properties. Selective treatment of the edge made sure it was hard enough to stay sharp. The inherent resilience of the wrought iron in the body of the blade, combined with the springiness imparted by the twisting and folding, kept the blade from breaking or bending easily. Once the knowledge of how to produce steel became widespread the same practice was used to make early steel-edged items of large size, since batches of steel were small during this period. A separately made piece of steel would be forge welded to a wrought iron body to make the working edge. Before that - and still, for many uses - came case hardening, described below.

The process of making a sword or breastplate in this way is very labor intensive, and usually uncertain. With little change in technique for centuries, large items of wrought iron or steel which were well-made were treated as very valuable items. Swords and even knives which had proven themselves in use were beyond normal price, and were often handed down in the same family as working items for generations. Often, if a good-quality long blade was somehow broken, the already forged metal was simply cut and ground and retempered the minimum amount needed to make one or more shorter blades. Thus a fine long sword broken in battle might become a belt knife and short sword. Good metal was simply too precious to melt down and make into something which might not be nearly as good.

Long before people learned to produce iron from lumps found in bogs or iron ore, they were finding lumps of "native" iron. However, iron from such sources is rare. Even rarer is meteoric iron. Some of this is composed of a natural alloy with nickel which is in many ways finer than anything produced deliberately by humans until well into the Nineteenth Century. (Jim Bowie's eponymic fighting knife supposedly had meteoric iron in it.) More than one mythic blade may have been made all or in part from this metal. Including an iron knife buried with Pharaoh Tutankhamun. Who died when bronze was what was used for weapons, and iron was where you found it. In this case, in a meteor crater in a desert.

Cast iron became common some centuries after wrought iron. It is made by using a large, hot furnace to smelt iron ore (usually excavated from quarries) in a large batch. As the ore becomes hotter, the carbon from the ore and/or fuel becomes more soluble in the iron, and the melting point of the combination is lower than that of wrought iron. At around three percent carbon the iron will melt enough to run into molds. In the old days these were often nothing more than a channel dug into the dry sand floor of the foundry. These led to blocky cavities likewise excavated. The result resembled pigs lined up at a slop trough; hence the term "pig iron."

Cast iron is stronger and harder than wrought iron. However, it is also brittle, and it rusts easily. Like bronze, it can be cast and filed, but not forged or easily welded. So why wasn't it a big step backwards from wrought iron?

Because it can be cast and filed. Making items for which brittleness wasn't a problem was far quicker and easier with this method. Also, one reason large castings of wrought iron are rare is that - as mentioned above - pure iron has a higher melting point than that of cast iron. Melting enough cast iron to pour into a mold is, simply, easier.

As demand for iron increased and improvements were sought, people learned - independently, several times, in several locations - that blowing air through molten cast iron would remove the carbon, producing wrought iron in large amounts, without all that "wreaking."

Only, if you don't remove enough of the carbon, you get something else...

Wrought iron has little or no carbon. Cast iron has a lot of carbon. In between... lies steel.

Steel was known long before someone figured out how to make it from cast iron. Remember the comment above about treating the edges of forged wrought iron swords? One of the treatments was to put the finished object in contact with carbon (usually charcoal) and heat. This allows a small amount of carbon to dissolve into the forged wrought iron, face or case hardening it. That is, converting the outer layer of wrought iron into steel. Again, creating a metal composite. This one with the sections which have different characteristics more intimately bonded than through welding.

Oh, the stories which could be told of the long struggle to understand and master the process of deliberately making steel with the desired properties. The superstitions, the rules of thumb, the strict rituals and fickle results. Not just for the ingredients and how they should be combined, but for the tempering. (Which type of prisoner was best for running your orange-hot blade through to get the best edge? Which animals' urine should go in the tempering trough?) For over a thousand years, the most successful makers of iron-based swords and armor were not those who had the best case hardening method or the most impressive blade shape and balance, but those who could consistently reproduce their results. Doing this without modern chemical theory, metallurgical knowledge or even thermometers requires a keen eye (as well as ears and nose) and a fanatical dedication to one's craft. (On Forged in Fire the contestants will often choose a premade type of steel known to have a wide range of tolerance for the temperature used in the tempering.)

All of this confusion ended - slowly, gradually, stubbornly and with considerable reluctance - as science finally explained just what was happening, and provided tools to make it happen more easily and consistently. (Though for many uses the old methods are retained, especially the artisanal. Practitioners still have a lot better idea of what's going on, now.)

With the repeatedly improving scientific understanding of the chemistry, the production of bulk steel changed several times through the centuries. However, wrought iron remained the most frequently useful member of the family for centuries after large batches of steel could be made, partly for use as a starting point for making fine steel (solid or case hardened) items. It continued strong for decades after even better methods for producing steel came along, due to such things as its corrosion resistance and resilience. Eventually, however, wrought iron died except for a few specialty mills, which produce it for the art trade. Cast iron continues strong, though (pun intended). New methods of production have made it far more economical than wrought iron, either for making cast iron items, or as a precursor to steel.

Today, in fact, various types of cast iron are doing jobs once the province of wrought iron or even steel. There are cast irons which are corrosion resistant. Cast irons which are ductile. Fine control of the chemistry and heat treatment have greatly expanded its uses.

These improvements in cast iron require moving beyond the simple combination of iron and carbon, with other stuff, such as silicon, added to moderate the effects of large amounts of the latter when mixed with the former. In truth, there has always been other stuff in any iron, except for a few very pure samples normally restricted to laboratory use. Historically, variations in trace elements native to the ore or coming from the particular process - and even individual crucible, mold, hammer or anvil - used to make it had observed but at the time unexplainable effects. Those were present as accidents, though, and any benefit they provided was a mystery to those working the metal. Today it can be safely said we have now tamed iron. Especially cast iron, the metal once decried as "ruined iron," and can make it do the job of true iron, the wrought iron of old. Sometimes even better than the original could.