How did ancient civilizations make furnaces hot enough to melt metals like copper or iron with just charcoal, wood, coal, clay, dirt and stone?
In: Engineering
Wood fires are all it takes to melt most early metals. All you have to do is make the fire hot enough with strong airflow.
Prior to the (or in absence of the) bellows, long tubes were blown into by apprentices or other servants to keep a constant, fast airflow on the coals.
Copper has a low melting point. You can melt copper coins on a modern stove burner.
A simple clay brick furnace with a bellows attached to a tuyere can get hot enough to melt, or at least soften, iron to be shaped or poured into a mould.
Primitive Technology on Youtube has a few experiments with iron bacteria.
Iron furnaces made of clay and dirt – even if they do work, won’t last for long.
But stone is pretty much universal till today. Correct air flow, charcoal and bellows helped to increase temperatures even more. Worst that could happen to stone furnace is freezing temperatures at night – rapid cooling might crack or even explode the stones. That’s also why they were usually covered with a roof – since rain tends to cool things down way too rapidly.
Everything in nature has (freezing and) melting point, even stones (think lava), and finding out suitable material or type of stone was more or less matter of experimentation. But people, having used rocks (heated in campfire) to warm themselves at night for almost half a million years, were pretty knowledgeable about those by the time of metal smelting.
Especially with copper and tin, you don’t need to get it super hot, relatively speaking, in order to melt it. Pile up some dirt and stone to make a cylinder, put some wood inside the furnace you just made, dump your copper and tin inside to cook, and then put a pot of water on top for a bit of tea (optional).
A regular cooking oven used to bake bread gets to 400f. Copper needs about 2000f to melt- hotter than your bread oven but, relatively speaking, not super super hot.
Iron is significantly harder than copper, needing about 2800f to melt – almost 50% hotter – but once people had been making bronze for a while, iron was basically the same principles at work.
Regular wood fires, without any special effort, can get as hot as about 2750f, give or take a bit. So copper is well within the range of “Just throw more wood on it,” while iron is -just barely- at the top end of the range of what a wood fire can melt.
Among other things people have said, some people also burned bone for a hotter fire. Plus there were surface coal deposits on some places. But really, if you have enough fuel and you can control the airflow you can do all kind sof things with a fire.
copper, gold had relatively low melting points which could be melted with the fanned flames of wood and coal .
Iron doesn’t need to be heated to liquid temperatures to work with. Iron can be forged when heated to 1800-2000F. Through forging, you can hammer it into various shapes and even join two pieces together.
How hot a fire can be is determined by the vessel you’re starting it in. The type of fuel you’re using. And how that fire is getting fresh air to use for the burn.
Early civilizations found ways to improve from just making simple fires on open ground to making specifically shaped furnaces with special fuel and forced air delivery to make a fire burn hotter and for longer.
Furnaces with thick walls of clay, mud, and stones insulate the fire so heat isn’t lost to open air or make it difficult to be near such a hot fire. Better shapes allowed air to flow more effectively into the fire and make it burn hotter.
Better ways to deliver air got more oxygen into the fire for a hotter burn. Tuyeres (clay tubes to focus airflow), bellows, etc.
But one of the biggest changes was fuel. Charcoal, and how to reliably produce it, was a huge game changer for early societies working on the foundational metallurgy we’ve built on over the course of centuries and millennia. Charcoal burns hotter, longer, and cleaner than wood.
These things all contributed to tools like bloom furnaces. Which were able to reach temperatures to not just soften metals to make them able to be pounded into useful shapes… But also could make them so hot they melted entirely and could be poured into molds to make jewelry, hand tools, weapons, and armor.
Lets look at temperatures first.
Charcoil burning temperature 980 C. With propper air pumping it goes to 1200.
Copper melting point around 1100.
Bronze (copper and tin) melting point 913 C
Iron 1538C.
So charcoil is enough to completly melt copper to nake more usefull bronze. That allows to to puur liquid bronze into forms for mass production ( eg arrowheads, swords, sizors etc)
Iron is a bit different story. While it is possible to malt iron in furnance it creates a lot of problems for iron quality. Which greatly limits iron use (it easy to bend, has no retraction, does not keep edge etc). Thats why iron require forging with hammer to reduce impurities. That for long time was expensive ( mire expensive than making bronze tools) activity which produced low quality tools. Basicly sole iron usage untile 8 century CE iron mainly used in chainmails as non of it negative traits were important in chainmails.
Pretty easily. Especially copper.
I’ve personally built my own cheap and fast “forge” to mess around with as a teen. It was as low tech as it gets.
I dug around a 1 foot deep hole, about a foot wide and 5 feet long. Then I took a pole and made a diagonal hole that opened up into the bottom of the trench.
The hole was so I could pump air into the base of the fire.
Then I literally just filled the hole with wood and kept burning it.
Now I used an air compressor to pump air into my hole as a bellows, but a more crude bellows would work fine.
Worked fine for the random small bits of scrap iron fixtures and stuff I had lying around.
Wasn’t an amazing setup, but I made it entirely with a shovel, wood, and a piece of pipe in a handful of hours. I have 0 doubts in an ancient civilization’s ability to make something far more efficient and better than that.
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