Iron in my mind

What is the history of iron? How did we start turning rocks into tools, iron from ore? I’ve been interested in the history of technology for quite some time and my interest always leads to trying out stuff myself. So I’ve been on a road to make a simple iron tool from scratch for a while now and here is a rather extensive take on how to smelt your own iron.

Title photo by Morgan Riley

Background on my interest into smelting iron

My take on the history of technology derives from my strange interest on thinking how complex machines would I know how to build if I were to be transported back in time. I’ve started thinking about this from the early 2000 and finding information was much more difficult back then. I know (and I’ve usually tried) how to build various wooden machines, with the insight into what kind of wood should be used where. I can create antibiotics from scratch, I know how food can be stored etc. I even know a bit about electronics, glass blowing etc. But all of my knowledge is dwarfed that I never knew, how to make the (iron) tools used to make the things.

Yes I knew in theory on how modern blast furnaces work and could even google them. Their sheer size made them impossible for me to try in practice and since they’ve been the preferred method of creating iron since the 15th century, finding anything written before that was difficult. The big problem when looking into something new, is the lack of the specific terminology. Googling with the wrong terms is quite hard, so I had to start with googleing for the correct terms.

Two things happened: First, I visited the Tekniska Museet in Stockholm, Sweden and found this awesome exhibition of early iron making in Sweden (Sweden has a long history of Iron smelting, and still has one of the most modern iron ore mines in Kiruna). Second, time passed.

More and more content keeps on appearing online and the similarly crazy experimental archeologists begun to transfer their documentation to the web. I found stories after stories of amazing experiments from medieval shows or backyard blacksmiths trying out their methods in creating early iron. In the end, I managed gather a cache of great resources on smelting iron in bloomeries.

The science

Smelting iron differs significantly from melting iron. The old bloomeries didn’t get hot enough to melt iron, but they still managed to produce quality iron ingots for further processing. The bloomery is a furnace, that reduces the Iron molecules in the ore from Iron oxide (Fe₂O₃) into metallic iron (Fe), through the creation of highly reducing carbon monoxide (CO -> CO₂). The metallic iron then falls to the bottom of the furnace and removed while hot. The process also creates slag, composed of liquid silica, charcoal ash and even the furnace wall itself. This slag is removed during the process and even from inclusions from the final iron ingot (known as a bloom).

This method produces wrought iron, that is “easily” malleable, but hard, making it a good material for tools. When the furnace’s temperature rises high enough to melt the iron, the acquire material is called pig-iron, as it has a high carbon content from the used (char-)coal in a more modern blast furnace. Pig iron is brittle and not very useful before puddling. But I’ll focus more to the creation of wrought iron through a bloomery.

The temperature, size and shape of the bloomery, the amount of air introduced to the system, the location of the air introduced, the source material and ratios between ore and charcoal play and even the sequence in which the coal and ore are added are an important part in the quality of the end product. Even though this process was known already thousands of years ago, it is still a very complex process to this day.

A very interesting piece of a reference I finally got my hands on was the study on the smelting of Finnish bog- and lake ore, written in 1794. Carl Rinman (1762-1826) was a Swedish “mountain engineer”, i.e. a geologist. After a throrough scientific education, he was sent to Finland to study the bog- and lakeores and most of all to teach the locals on how to use them in a bloomery to produce iron. He eventually returned to Sweden, developed various new improvements on mining and was awarded with title of a bärgmästare (“mountain master”).

The practice of iron from ore

So now that the science is out of the way, it is time to see what I’m actually up to.

Building a bloomery consists of constructing a clay cylinder, with walls thick enough that it will hold together. A tuyere needs to be placed through the bloomery wall to allow air to be forced into the process. I’ve found numerous schematics and various methods of constructing the bloomery and below is my quick sketch of the finished product.

Iron from ore — A very rough schematic of a bloomery

A wooden mold is usually built, around which the clay is then systematically built. After the shape of the bloomery is reached, the mold is lit up and burned, thus drying and hardening the inside of the furnace. While the wooden mold is almost completely burnt, charcoal is added from the top of the furnace. Once the newly added charcoal is burning, air is introduced from the tuyere to increase the combustion rate.

After the initial burn, iron ore and charcoal are added in turns while a continuous stream of air is kept in the furnace. Traditionally this was achieved using bellows, water displacement, or even channeled wind.

After the fire has had time to work it’s way, an opening is created to reduce the accumulated slag from the slag pool. Once the addition of iron ore is stopped, the bloomery is allowed to burn the fuel (while continuing the air) and the bloomery is broken. This allows for a better access to the formed bloom.

The bloom is picked up and hammered immediately to consolidate the spongy mass and force out any slag left within inclusions inside the mass. Usually, the blacksmith had a forge nearby, so he could heat the bloom and continue to solidify it through hammering.

My progress so far

To be able to create and operate my own bloomery, I’ve decided that this is purely a summer hobby for me. I’ve cleared a spot for the bloomery at our summer cabin and started to source (and hoard) some equipment for it. I already wrote a post about creating the charcoal for the bloomery. The sheer volume of the required charcoal is huge, so any ways I can reduce the overall costs is a benefit.

I’ve also built the base for the bloomery by using Lee Sauder’s excellent written guide for his bloomery. Even though in the old times, people wouldn’t have had the use of symmetric bricks, I happened to dismantle a fireplace that left me with unwanted bricks. I dug up the clay myself and used the natural clay as mortar between the bricks. I used the clay as I found it, even though I read that clay would benefit from some “fermentation” to increase its plasticity.

One layer of bricks — Firs layer of bricks and the beginning of the second

Sloppy mortaring — Close up of the sloppy mortaring using natural clay

Second layer — Finsihed second layer before mortar

Next steps

My next steps will be to fire up some more charcoal, acquire significant amounts of clay and get my hands on iron ore. The iron ore seems to be one of the most difficult tasks at the moment.

The charcoal is relatively cheap, but as I happen to have an abundant source of wood available, I was planning to use the oil barrel method of creating charcoal. That should provide me with enough charcoal for the smelt.

I think I’ll need to buy the clay. I believe that the amount of clay I’ve dug up for the base acts as a proof of concept that I can fins it and use it, but for the bloomery to work properly, I’d rather buy good enough clay to make sure the furnace won’t fail on that aspect.

For the iron ore… I don’t know. If you know where to get some iron ore here in Finland, I’d more than happy to get it. Stay tuned for more.