Stone Age, Bronze Age, Iron Age

Copyright © Karl Dahlke, 2022

Temperatures in this chapter are given in celsius. Since these are approximate, you can simply double them for the Fahrenheit equivalent. 500 °C is about 1000 °F.

In my mid twenties I shared a communal house with three other people. We all worked for Bell Labs, but that's the only thing I knew about two of them as we signed the mortgage together. We were practically strangers. Well we all knew Dorothy, the linch pin of our little commune, and she said we'd get along, so we jumped in with both feet. As it turns out she was right, And Alan, whom I met just three weeks before closing, became one of my best friends and remains so to this day.

Alan and I loved fire, and we use to melt glass in our outdoor firepot, trying to create interesting shapes. “It is deliciously pliable.” he remarked one day as he deformed the heat-softened jar with a stick. Our creations weren't as pretty as those produced by a professional glass blower, but some of them were interesting nonetheless, perhaps a form of abstract art. After my daughter Beth was born I produced my most successful glass sculpture to date, four baby food jars melted down into the bottom of a soup can. The trick, as always, is to put the lid on and let the entire firepot, wood and glass and all, cool slowly over 24 hours so the glass does not crack. I gently pulled the glass out of the bottom of the blackened can, and washed the aluminum oxide off of the base and sides using vinegar and water. to my delight there were no cracks and no sharp edges. It was a very pretty paperweight. The jars were fused together, but you could still see the individual jars clearly, particularly the rims, with the threads where the lids screwed on. Beth kept the jar sculpture for 23 years, until, after several moves, one of the jars broke off and left sharp edges behind, whence we had to throw it away. Maybe she'll have a baby of her own someday, and we'll make another Gerber masterpiece.

As you can see from this experiment, a back yard fire can heat a grapefruit sized object to 500 °C, hot enough to soften a glass jar or bottle so that it sags under its own weight and adheres to itself or to other glass objects nearby, but not hot enough to turn glass into a free flowing liquid. If I had some lead I could melt it into a liquid and pour it into a mold, but I never tried it. Lead isn't pretty the way glass is, especially different colored glasses melted together, and lead entails some health risks if not handled properly. Thus glass was our medium of choice.

I can barely imagine time spans longer than a century, so it is difficult for me to wrap my head around the stone age, lasting 3.4 million years. Generation after generation after generation did exactly as their parents, crafting the same stone tools, and surviving (or perhaps not surviving in the case of our extinct homo cousins) in the same way. There was no understanding of the future or the past, or our place in the universe. I would never have written a book like this; I would be busy learning how to hone sharp flint edges for the benefit of my family and my tribe. So it goes, century after century, millenium after millenium, with only an occasional improvement in tools or in hunting techniques.

I envision nothing but misery if I were condemned to live such a life, but in reality they didn't know the difference, and they probably experienced the same gamut of emotions as we do today.

For 3 million years humans plodded along, but then something interesting happened circa 200,000 years ago - we harnessed the power of fire. To be fair, proto humans may have produced fire much earlier, that is uncertain, but regular and widespread control of fire for warmth, light, cooking, and protection from predators and insects probably did not begin prior to 200,000 BC.

Some 8,000 years ago, curious humans, perhaps like me, used fire for something more. They used the intense heat to soften and melt materials, extract metals, and produce alloys. Ore, as mined from the ground, typically contains metal oxide, sulfide, or carbonate. A process called smelting chemically separates the metal from its ore. The easiest metal to smelt is lead, having a low melting point. The heat of a wood fire is sufficient. When lead ore is mixed with burning charcoal, the superheated carbon monoxide strips oxide and sulfide away. The heat also liberates CO2 from carbonate, and again, CO pulls the remaining oxygen away. the result is lead, the pure metal extracted from its ore. It flows freely as a liquid and can be cast into molds. This metal is too soft to be of any practical use, except for piping and other water vessels. The Latin word plumbum, for lead, gives us words like plumbing, and plumb bob, a lead weight on a string that indicates true vertical. It is also the source of Pb, the atomic symbol for lead. Of course lead pipes produce deleterious health effects, but that's another story.

Tin was extracted in the same fashion, having a melting point even lower than lead, but tin is also too soft to form structural elements or weapons. A significant step forward in early metallurgy was the smelting of copper. When the resulting copper is fused with tin the result is bronze, an alloy with many desirable properties. This metal was so useful that it ushered in the bronze age. Bronze tools were superior to stone tools in almost every way, and bronze could be molded into almost any shape. Civilization would never be the same.

There was a catch however, copper smelts at temperatures 200 degrees hotter than a typical wood fire. More heat is needed. Fortunately artisans had already developed kilns to bake pottery, tiles, and bricks. A traditional kiln was an earthen structure that contained the fire and trapped most of the heat inside. Strategically placed baffles and vents directed the air flow, while a chimney drew the hot gases up and kept the oxygen flowing. The center of the kiln could reach 1200 degrees, hot enough to smelt and even melt copper. It's not clear who first placed copper ore in a kiln, or why, but the resulting metal was incredibly useful for tools, weapons, and construction. Some cultures used copper alone, since tin deposits are rare, but other cultures mixed tin and copper to form bronze, a superior alloy. Countries without tin soon traded for this valuable commodity so they to could produce bronze.

Preindustrial smiths were aware of seven elementary metals: lead, tin, gold, silver, copper, iron, and mercury. Six of these are mentioned in the Bible: Ezekiel 22:20 NIV, “As silver, copper, iron, lead and tin are gathered into a furnace”, and of course there are numerous references to gold. KJV consistently refers to copper as brass, but the copper zinc alloy we know as brass today is modern, and was unknown at that time. Schollars agree that the KJV references to brass actually mean copper, and recent versions of the Bible reflect this understanding. There are some references to bronze, including Daniel's statue, and these were almost certainly the bronze alloy, copper + tin, which was in common use. Mercury is not mentioned, but there is another element in the Bible, brimstone, the stone that burns, which we now know as sulfur. I felt a brimstone once during my travels through the west; it felt like an ordinary rock, but when I got close to it I could smell the sulfur. The last candidate, the only other element that was seen in pure form at that time, is carbon. Charcoal and diamond are both mentioned in the Bible. The first always appears in the context of a fire, John 18:18 “the servants and officers were standing around a charcoal fire”, and the second is a specific gemstone. Well perhaps charcoal qualifies as carbon, and if so, then there are eight elements in the Bible.

Isacc Asimov suggested that gold was the first metal to be fashioned into ornaments and jewelry, because there is no need for smelting. Although gold nuggets are rare, they appear as pure gold in nature, and do not require chemical separation from ore. The melting point, 1064 degrees, is a tad high, bug gold is ductile and malleable at lower temperatures. Once heated, it is easily hammered into new shapes. since gold and silver are rare, they were primarily statements of wealth and opulence. Like lead and tin, these precious metals are too soft to serve as the blade of a sword or the strut of a chariot. copper smelting was the key to the bronze age, a quantum leap forward in technology.

The third stage of technological development is the iron age, and once again it's all about heat. Iron and its steel cousins are superior to copper for most applications, especially in weaponry, but iron melts at 1535 °C, beyond the reach of a traditional kiln. Smiths began to explore the properties of iron around 3000 BC, but only through meteorites, where iron appeared in pure form and required no smelting. These rare iron fragments could be heated in a kiln and hammered into small beads, acting as ornaments for the rich. Meteoric iron is much too rare to support industrial applications, but iron ore is abundant across the earth, thus iron smelting was the key.

By 1200 Bc smiths had developed bloomeries, hardened pottery vessels that contained the iron ore and trapped and controlled the heat. These crucibles, combined with the heat of the kiln, extracted wrought iron from ore, and in a follow-on process, wrought iron was merged with charcoal to produce steel. this represents a cascade of tools and technology. Kilns produce ceramic bloomeries, which are then used in the smelting of iron, and later in the careful manufacture of steel. One of the most interesting Nova episodes is Secrets of the Samurai Sword, which aired on October 9, 2007. This is a fascinating look at early metallurgy and craftsmanship. The skill required to forge the iron blade using little more than fire and clay might reasonably be compared to a concert pianist or a world class basketball player. Purchase this episode for your library, or at least read the transcript. Then imagine yourself as a smith 1000 years ago.

“There is really still only one way to learn the art of sword making, through apprenticeship - no easy career path. They rise at dawn, help with household chores, and work with their master six days a week.”

Note that fire is the key to everything. Without fire, would we ever get past the stone age? I don't think so. I speculate on this in the next chapter.


Glass, sand, and quartz are essentially the same substance, silicon dioxide, SiO2 _ but the atoms are arranged diffferently in these three materials. By simple observation, people knew that heat could turn sand into glass - at least the heat of lightning. When it strikes the floor of the desert, it leaves filiments of glass in the sand where the bolt hits. More recently, in 1945, the atom bomb turned the sand of the New Mexico desert into bits of glass, light green, or occasionally red, depending on the impurities in the sand. Traces of this glass can still be found today. However, people could not turn sand into glass with the heat of the kiln. Fire is simply not hot enough.

It takes 2 thousand °C, 4 thousand °F, to turn sand into glass. As described above, the best technology, developed by smiths across the centuries, brings iron close to its melting point of 1,500 degreees C, but this is still well below 2,000. However … put some soda ash and limestone in with the sand, and it becomes glass at around 1,100 °C. It's like a melting agent. That temperature can be reached with a good fire and some bellows. It's the same as melting copper, or gold, or tin. Thus glass is part of the bronze age. While one smith melts copper and tin to produce bronze, another produces glass. Trace metals, such as iron or chromium, can be added into the mix to produce colored glass.

Once sand is transformed into glass, it melts over a simple campfire, as described at the top of this article. If you've ever watched a glass blower at work, it is a thing of beauty.

energy Cost of Smelting Iron

The biggest energy cost of making iron isn't mining it, or melting it, although those are substantial; it is unburning it. Iron ore, as found in the ground, is mostly iron oxide, that is, rust. In other words, iron that is burned. A chunk of iron that sits in the ground will eventually oxidize, over a million years, even if it is several miles down. Water and oxygen reach it, even in trace amounts, and it rusts. Think about a log, and how much energy is liberated when it burns: heat and light. Imagine putting all that energy back to unburn the log and get wood back again. This is what we do for iron, and other elements too but particularly for iron, as it has a strong affinity for oxygen.

The process of "unburning" iron is called reduction, or smelting; the iron oxide is reduced to get iron back again. This is the opposite of oxidation. Oxidizing iron, that is, turning it to rust, releases energy, and that energy must be put back to reduce iron, prodeucing the pure metal that we need for industrial purposes. This is the conservation of energy, as described in an earlier chapter. No amount of technology can circumvent this reality. Yes, some smelting processes are more efficient than others, but they all have a minimum energy cost, according to the chemistry of reduction.

Consider, however, there's an entire planet of iron beneath our feet. The core is iron, almost pure iron. Trillions and trillions of cubic miles of iron. The heat is so intense, that it drives the lighter elements, like oxygen, up to the surface - over geologic time. There's no water down there, no oxygen, just iron and other trace metals. So just go down and get it! Nay, we'd have to descend 1800 miles to reach it, and we're lucky if we can mine half a mile down. So smelting continues, to support our evergrowing appetite for iron and steel.