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Iron

Written by Rick Boatright

The most dangerous mammal in North America kills over one hundred thirty people each year, and seriously injures another twenty nine thousand. The most recycled material in North America was dumped in landfills until the late 1970s, but now, nearly 100 percent of that material contains recycled content.

The animal? The white tailed deer. The material? Highway asphalt. Things that are very important are often common and overlooked.

Prior to the 1970s the question "What's the most recycled material?" had a very different, but just as surprising answer: Iron. Nearly 100% of all automotive iron, nearly 100% of iron from construction debris, and over 80% of iron from consumer appliances is recycled. Iron doesn't have a memory. The girders and beams from the World Trade center were sold to iron foundries, and will appear as buildings, and refrigerators, and washing machines around the world. Over half of the iron used in the world comes from recycling.

In coming issues of the Grantville Gazette articles will discuss various problems facing the Granvillers, including the "Stainless Steel problem," the replacement of the power plant, constructing boats and bridges and barges, making the steam engines to power those, reproducing the machine shops and building new machine tools, the chemicals industry, coke, medicines, surgery, anesthesia, clocks, navigation and mapping. All of these face a common element in what the 1632 series authors and background researchers have come to call the "Tools to make tools" problem: iron.

In the early 1630s, just before the appearance of the Ring of Fire, the annual production of iron in the part of Europe that interests us was about fifteen thousand tons. One hundred miles of main line railroad needs over twenty thousand tons of iron. The telegraph line from Grantville to Magdeburg needs almost fifteen thousand tons of iron. Small main line railroad steam engines need three to five tons of iron each, and "real" railroad engines run seventy-five tons. Barges, even small barges like the classic UK narrowboat, run six to ten tons of iron per barge. A fifty by twelve foot barge runs around thirty tons. Future articles in the Gazette will detail the rapid increase of iron and steel production in the USE. The projections resulting from the projects named in the books published by early 2004 indicate that within two years of the Ring of Fire, European iron production will have to have increased by a factor of two to three, with a planned increase by a factor of ten by year five.

This leads to the question, what is so important about iron? There are other materials: wood, copper, aluminum, plastics, and alloys like brass and bronze are all common. Why make such a big deal about iron? This article will attempt to place civilization's use of iron in context historically, and physically.

Iron is the fourth most abundant element in the earth's crust. The most abundant is oxygen, which isn't much good for building things. Next is silicon, which we use for computer chips, but not for bridges or boats. Third is aluminum. We do build stuff from aluminum, but winning aluminum metal from the earth's crust turns out to be a very difficult prospect that requires the use of massive amounts of electricity. Most aluminum in the crust is bound up chemically in ways that make it very difficult to separate, even with twenty-first century technology. Iron, on the other hand, comprises about five percent of the earth's crust, and can be separated from its ore with little more than fire and charcoal. Other metals used by civilization are very rare. Copper exists in the crust at sixty-eight parts per million. Lead is even more rare at ten parts per million. One driving force then that makes iron an important part of civilization is that it is common, and easy to produce.

Iron has some very neat properties. It is very strong. Pound for pound, iron is the strongest material available before the twentieth century. It is very workable. Iron can be cast and beaten and rolled and formed into almost any shape. Because it is strong, thin sheets of iron can substitute for thick heavy layers of other substances. Iron can be flexible, and makes great swords and springs. Iron can be stiff and makes great cutting blades and hammers and tools. Iron melts at a very high temperature. Iron's melting point is more than twice the temperature of a normal open fire. Iron doesn't even soften in normal open fires, so it can be used to contain fire and form stoves and pipes and such. Even when heated red hot iron can retain much of its strength. No other single metal does all these things. Copper is ductile, it can be formed into all sorts of shapes, but it is soft. Bronze can be hard, but it is weak, and melts at a low temperature. Lead, gold, and silver are soft, and the latter two are so rare that we make money out of them. Iron is unique and has been the basis of civilization in Europe, Asia and Africa for over three thousand years.

How do you produce iron then? First, select a rock with lots of iron in it. The iron will be bound up with oxygen. The best iron ores are little more than iron and oxygen. They are rust rocks. Most iron ore isn't of this quality, and contains varying amounts of silicon, sulfur, manganese and phosphorus. Oxygen combines with carbon more strongly than it binds with iron. If you powder iron ore and charcoal or coke, and heat the mixed powders, the iron gives up a bit of its oxygen. The oxygen binds with the carbon to make carbon dioxide. In the simplest smelting process, crushed iron ore, crushed charcoal, and a little limestone or sea shells are heated together until they are red hot. As this spongy mass, called a bloom, cools, pure pieces of iron are intermingled with leftover charcoal and the other chemicals left behind. The parts that aren't iron are called slag. The bloom would be hammered and turned and hammered and turned, and the slag would be squeezed out, and the bits of iron would come together to form wrought iron. Wrought means hammered or worked. In the seventeenth century, there were hundreds of hammer mills scattered throughout Europe wherever a seam of iron ore coexisted with a stream capable of turning a wheel and powering a hammer. All the iron available in Europe in the seventeenth century started life as wrought iron. Wrought iron has a carbon content of around 0.02 to 0.08 percent by weight. This is important because the factor that is the most important in describing the strength and brittleness of iron is the carbon percentage. A very small difference in carbon results in a huge difference in the properties of the iron. Consider the next type of iron to be smelted.

If you take iron and carbon and heat it above red hot (to about 1200 degrees Celsius) something interesting happens. The iron begins to absorb the carbon, and starts to melt. The iron-carbon mixture has a melting temperature far below the melting temperature of pure iron (which is around 1500 degrees C). If you make a tall chimney like structure, and layer charcoal, flux and iron ore in it, and pump air with a bellows through it so that it gets above the critical temperature, molten iron would run out of the blast furnace. Sadly, the iron produced has three to five percent carbon in it. Cast iron is very different from wrought iron. It is hard and brittle. If you hit it with a hammer, it will crack or shatter. Microscopically, cast iron is a mat of fibers of iron crystals, iron carbide crystals, and graphite. It is very rigid and very tough. It doesn't soften much before it melts, and it can not be worked by hammer and anvil into a shape like a knife, a sword, or a gun as wrought iron can. Cast iron was known in Europe in the middle ages, but was not used much beyond pots, pans, cannon, cannon balls and bells. Casting iron was called founding and so businesses which cast iron are called foundries. Cast iron is perfect for making things that need to be very rigid.

Cast iron is not very expensive. Generally, items made out of cast iron are cast in sand. A wooden copy of the item is made, and sand is formed around the master. The master is removed, and molten iron is poured in. After cooling, the sand is shaken off and re-used. Grantville will use far more cast iron than the Europeans were using before they arrived. They know neat things to make from it, like Franklin stoves, frying pans and the Eiffel tower. But for all its strength, cast iron is brittle. Guns made from cast iron fail because they are not elastic. They can't expand with the explosion of the powder and then spring back to shape. If they are not made very thick to withstand the pressure, cast iron guns explode after a few uses, so they have to be very heavy for their power.

Iron makers from the middle ages learned to transform cast iron into wrought iron by burning the carbon out. They would use a fining furnace, where they would break the cast iron into small lumps and heat the lumps with a stream of very hot air. The iron would melt, and carbon would burn out and the decarburized iron droplets would sink to form a bloom below the hot zone. Then, they would forge the bloom just like they would in a hammer mill. Wrought iron made this way was more expensive than iron made directly from the ore, but the two step process could be done with some iron ores that the one-step process was not effective for. This was expensive.

In the late 1700s, an Englishman, Henry Cort, developed another technique for transforming cast iron into wrought iron. Molten cast iron was poured into a stone basin in a reverberatory furnace and exhaust gasses from a hot fire were run over the top of the basin. A worker with a long rake stirred the surface of the puddle of iron, and carbon monoxide in the gasses would combine with carbon in the iron. The resulting pure iron melted at a higher temperature than the cast iron it was suspended in, so it would form semi-solid bits of wrought iron. At first, these puddlers would gather these into a single mass which would be wrought like any wrought iron. Later puddlers would keep mixing the mass of iron, as it became more and more viscous. Skilled workers would recognize when the hot iron had "jelled" enough to have had enough of the carbon burned out of it.

Blast furnaces produced bulk cast iron efficiently, but the puddling furnace was a major bottleneck. The process was slow. It required huge amounts of fuel. Only very strong men could stand the heat, and work the thick, heavy liquid metal and tell when it was ripe to be withdrawn. Many attempts were made in the 1800s to mechanize the process, but they all failed.

So far we've talked about two types of iron. Cast iron, with carbon content over 2%, and wrought iron, with very little carbon at all, less than 0.1%. What about iron in the middle of the range? We know that wrought iron is flexible, and can be forged into all sorts of shapes. We know that cast iron is rigid and brittle. It should come as no great surprise that iron between 0.1% carbon and 2% carbon is intermediate in its properties. It is stiffer than wrought iron, but less stiff and brittle than cast iron. It has a higher melting point than cast iron, but less than wrought iron. Clearly, this is what we want to use to make stuff. Iron intermediate in carbon between wrought and cast is called steel.

Even today, the basic chemistry of iron is such that it is difficult to move directly from iron ore to steel. In 1632, we have to come at it from one end or the other. We can take wrought iron and add carbon to it, or we can take cast iron and reduce its carbon. Several techniques were developed in antiquity that resulted in steels of different carbon content and different microstructure. One common element is that all these were small batch processes that were labor intensive. Steel was very expensive.

The oldest known steels were produced by cementation. Sheets of wrought iron were packed with charcoal or other carbon sources in a closed ceramic container and heated red hot (1000 to 1100 degrees C) for five to seven days. The carbon would be absorbed into the iron in the solid state. The process was very slow since the iron is solid, and the carbon atoms have to ...