Now that we have all the tools gathered, it is time to start dealing with the required materials.
Therefore in this part of the iron smelting series, I shall devote a few words to the building materials used to construct the bloomery furnace. While I am at it, I shall also be instructing you on how to prepare the ore and charcoal for the smelt.
While I am writing this post from the point of view of a bloomery smelter, most of the information will come of use to anybody who seeks to engage in traditional metalworking, or even other ancient technologies which involve heat.
This post is a part of an instructional series on bloomery iron smelting. Should you have missed them, or should you want a refresher, then you might want to take a look at the previous installments:
Part I: Theory
Part II: Tools of the Trade
With this introduction out of the way, grab a nice cup of tea and dig in.
I. Furnace Materials: Clay, Sand & Organics
“We must eat a peck of dirt before we die.”
Firstly, let us talk about clay, for without a furnace to smelt it in, the ore shall remain mere rusty rock.
People often spill a considerable amount of ink on ore, but the clay you use may be just as crucial. To often I have seen smelters worry about charcoal types, burn rates and slag-tapping when they can barely make a furnace shaft stand upright. Do not be that person. Master the muck!
If all you have is a clay that will start melting away bellow proper iron smelting temperatures, then your smelting operation will be nothing but ‘nasty, brutish and short’. The best ore in the world shall not save ye from disaster then.
Such was my experience when I was meant to run an iron smelting workshop at The Viking Way in Norway. After a well meaning assistant dumped way too much water into the mixture, and I spent a day laboriously kneading the stuff into something usable and building the furnace, I was up for nasty surprise when we fired up the thing. It turns out that the blue-grey marine clay, which you find in Norwegian fjords, melts as soon as you give it a stern look. So my furnace produced dirty glass instead of iron blooms. Learn from my foolish mistakes and inquire about the clay before you smelt in a new place.
What the above personal anecdote should illustrate that not all clay is created equal.
You will need a clay with a high enough alumina and silica content, so that it may contain the infernal blaze. Therefore you will need something with a high enough alumina and silica content, such as an ‘illite’ clay. If it happens to have formed in an area with a lot of granite, then you might be in for a treat. Certain types of kaolin work well as well. As already stated, try to stay away from marine clays.
Should you have some appropriate clay available in your area, then rejoice, for all you need is a spade, a few buckets and some spare time. The dug clay does not need to be specially washed, levigated or sieved. You should just pick out the larger stones, and you are ready for mixing.
Should that not be the case, then you will need to go buy some clay. In which case, dried and powdered is the way to go, since then mixing becomes a matter of combining the ingredients and simply adding water.
Yet before we go into mixing I have to devote a word two to sand.
Although we generally say that furnaces, forges and ovens are built out of clay, this is a misnomer, which often deceives beginners – leading to some initial disappointments. So in order to save you some trouble, let me tell you that these are in fact built out of sand glued together with clay.
Let me elaborate…
Clay, due to its fine nature, binds a lot of water and is very plastic (easily shaped). Most importantly, it contains a lot of chemically bound water. This is what makes it ‘clay’. At the same time, when it dries, it also needs to release all of that water, which leads to shrinking. And rapid shrinking leads to cracking. If the cracks are bad enough, the whole furnace might just fall apart.
Sand does not soak up nearly as much water, therefore it also does not shrink/expand in the same way, which in turn makes it more stable. It also very stable as it is heated up, since the melting point of silica lies around 1600ºC.
Therefore sand is added to clay in order to both reduce cracking in drying and increase its ability withstand high heat.
Most kinds of sand will work. My favourite happens to be coarse granite sand. But when in doubt, one can always buy a large bag of silica (quartz) sand dirt cheap at builder’s suppliers and DIY stores. The only thing that you should stay away is limestone sand. We are not building a lime kiln here. It is best if the sand is a bit coarse, rather than very fine. It should definitely not be powder-fine, since this makes the clay mixture very prone to tearing.
The technical term for things added to clay (such as sand) is temper. Besides sand, the other type of temper with which you should concern yourself is ‘organic temper’.
Do you need to go Organic?
The building of the furnace can be made even easier by the addition of organic fibre to the clay mixture, such as straw, hay, horse manure, or its less discomforting substitute – peat moss. These reinforce the mixture, fulfilling a role somewhat akin to that of rebar in concrete.
During the building phase, this organic temper adds stiffness to the mixture, thus improving the ability to build quickly, without waiting for the shaft to dry a bit, or worrying that the whole thing might collapse on itself. (Provided that you build correctly – more on that in the next post. There is no real cure for clumsiness and stupidity.)
During drying and firing, the same fibres and tubes of organic inclusion provide a path for the water vapour to escape walls. They also provide points of stress relief during shrinking, thus limiting, or at least containing, any large crack.
In short: Putting a bit fibre in the mix is great. Yet it is not actually necessary.
The composition of building materials used varies regionally. There are plenty of (pre)historic cases where only clay and sand were used to construct the furnace. Early medieval Ireland would be an example which I recently studied. Getting the proportions of clay, sand and water in the mixture right, while shaping the shaft of the furnace with a deft hand should be more than enough to build a usable furnace.
The Gritty Job of Mixing Muck
Now that you have your clay, sand and organics, it is time to combine them into something that transcend the mere sum of its parts.
Things shall get very hands-on now. Needless to say, do not bring your Sunday best for this.
This mixing can be done on the ground, where some kind of tarp can be useful in containing the scatter and turning the mass of muck. Alternatively, this can also be done in a large tub or even a wheelbarrow, which makes transporting the muck easier. Both methods have worked fine for me in the past.
First you shall break up the clay, which is more easily done when it is a bit on the dry side. If you are working in the height of summer, and the clay has gone completely dry, then take the time to smash it up finely. It will make the next steps much easier.
Next you shall add plenty of sand. How much? Probably more than you think. Remember the part about grains of sand glued together with clay.
Equal parts of sand and clay by volume is a good starting point. When working with dry clay and sand, I got great results when using even twice as much sand as clay. You are not trying to make an easily workable potter’s clay here. When you drag your finger through the mixture, it should feel very gritty and not smooth at all. A bit of experience goes a long way when it comes to judging this.
Mix the clay and sand, then start adding water. A mattock, gardening hoe, or even a wooden pole can be useful in doing this. Yet eventually you will have to stick your hands in there as well.
Try to get the clay and sand distributed before you add the water. When adding water to the mixture, do so in increments and rather add less than more. It is very easy to add too much water. Once you have made a too wet mixture, it may take a lot of dry ingredients and work, before you get something usable again.
Aim for a consistency where the mixture can be shaped into a ball, but starts breaking if pressed too much. This way, it can be used to build tall without it deforming under its own weight (too much). If the mixture can be easily shaped,then something is probably wrong. Your mixture is either too wet or does not contain enough sand.
Once an agreeable mixture is formed, you may knead in the organic fibres. This will usually take up some of the moisture in the clay, so you might need to add a bit more water. At the same time, if you made you mixture a little too wet, then some organic fibre will usually make it just right.
When using things such as straw/hay, you may chop it finely, or leave it in long strands. When using long pieces, it adds a lot of strength to the muck, allowing you t o build very quickly. On the other hand, it makes it harder to combine the ingredients. Try both ways and see which you prefer.
The process of preparing a heat resistant clay mixture is nicely shown in a video on building an earthen bread oven, made by Jas. Townsend and Son. The only difference that I would recommend is that you make your mixture a bit dryer, since you will be building a tall shaft and not a supported dome. Did I not tell you that these skills have many applications?
One Last Tip:
You may try mixing all of the clay at once, but I prefer to do it in smaller batches. I find that it makes it easier to adjust the rations and quicker to mix. This in turn makes the whole ordeal feel a bit less like wrestling a mountain of mud.
There is no hiding from the fact that the whole process of mixing all the materials is an ardous task. Probably it is the most annoying part of the smelting process. So if you can convince a few people to help you out, then it will go by a lot quicker.
Also, the mixture which made does not need to be used up immediately. Sprinkled with a bit of water, wrapped in some plastic and stored in the shade, it will keep for days. Therefore you can do all of this well ahead of building the furnace, or you can do it as you go along. It is in fact good to make a bit more than you need, in case you need to make some later repairs to furnace.
Congratulations! You now know how to make metallurgical muck. In the next part of this series, you will be learning how to shape it into a bloomery furnace.
Now that we have the building materials covered, it is time to tackle the issue of preparing the materials consumed during the smelt – namely ore & charcoal.
II. Iron Ore – Much Ado About Rusty Rock
Ore needs to be processed before it can be charged into your blazing furnace. Should you not do this properly, then even the best ore will give you sub-optimal results.
Luckily, getting the ore ready is not hard to do, it just takes a bit of time. Also, it involves making a bonfire at some point.
Iron ore comes in several forms ranging from fine sand to dense solid rock, with many shades in between. I have already described the most common types in the first post of this series. Now let me tell you how to prepare them for smelting.
Due to the varied nature of ores, the minutiae of of ore preparation will be dictated by the type of ore used.
Nevertheless, the main steps remains the same. These consist of:
All of these work towards improving the quality of the ore.
The first two increase the relative content of the ore – by removing some of the unwanted materials.
The other two work towards making the ore easier to smelt – by transforming the ore into a reducible form, and increasing the surface area of the ore.
Iron smelting might not be the cleanest of operations, yet washing does come into play at some point.
Iron ore recovered from the ground generally comes covered in dirt. Since we are interested in reducing the iron oxides, there is no point in iron content of the ore by tossing a load of crud into the furnace.
Some ores might be a bit rich for regular bloomery smelting, but these should be fluxed in a more controlled manner.
All types of ore of ore which come in solid lumps, will be improved by first rinsing off the dirt. The exception would be some fine grained bog ores and magnetite sand, which due to their size are hard to process in this manner.
Washing was historically often done in streams, but a strong jet of water from a from gardening hose (or even one of those pressurized cleaning things), will get the job do the trick just as well.
You do not need to get the ore squeaky clean. You just need to wash off most of the mud.
Of course, there is also the lazy man’s way of doing this.
If you have the spare time and the right sort of weather, you can also get away with just spreading the dirty ore out in the open, and letting the rain do the work.
I certainly was lucky enough to get my ore washed by rain, when I went gathering bog ore.
Separating the wheat from the chaff.
After washing and drying, it might turn out that a few common stones have crept their way into the ore pile. This would be the right time to pick them out.
Should you be able to recognise them, you may also want to remove the poorer bits of ore. These might be told apart by the a different colour or density. Naturally, it is always easiest if you have access to some kind of proper chemical analysis. Nevertheless, if some of the ore is of a lighter hue (e. g. yellow instead of red), and feels lighter, there is a very good chance that it is of a poorer quality.
One of the big differences between the smelters of old and us, is that they were probably much better at sorting the ore.
Some also like to to take a magnet to their ore, and only take that which sticks. This ensures that you are taking only the richest bits. This method might enrich even very poor ores. In the case of magnetite, this can be done before roasting. All other ores need to be well roasted before they will stick to a magnet.
Therefore when not working with magnetite sand, magnetic sorting should only be done after roasting and crushing.
This in turn brings us to the next step.
Roasting removes chemically bound water,turns non-oxide ores into iron oxide, and prepares the ore for crushing.
Most iron ores fall into two broad categories:
A) Ores that need to be roasted in order to be usable in the bloomery furnace.
B) Ores that may still benefit from roasting, although it might not be completely necessary.
There are few examples where the act of roasting is completely redundant. Such a case would be sandy bog ores. These are found beneath the turf as a layer of red, iron rich soil and may be used as found. Although a bit of drying might make it easier to estimate how much you are charging into the furnace.
Ores which absolutely need to be roasted are:
Iron Carbonates (Siderite)
Iron Sulphates (Iron Pyrite)
Dense Rock Ores (e.g. Haematite, lump Magnetite)
In the case of the first two, the roasting turns them into an iron oxide. Since iron oxides are what we smelt (reduce) in the bloomery process, this is a necessary prerequisite.
In the case of the hard rock ores, roasting makes them more porous and easier to crush, which in turn makes reduction more effective.
Roasting is still recommended for those limonites and bog ores, which come in larger lumps, since this makes crushing a lot easier.
So… How do you roast ore?
It is quite simple… All you will is some firewood and a place where you can safely make a fire.
First you start a campfire in a safe spot. You can even make it a bonfire if you are feeling particularly ambitious with your ore-roasting project.
Once the fire is going well, you lay a horizontal layer of firewood on it.
Spread some ore on the firewood.
Follow this up with another horizontal layer of firewood, laid perpendicularly to the previous layer.
Put another layer of ore on the fire.
Repeat until you run out of ore – creating burning layer-cake, composed of interchanging strata of firewood and iron ore.
Should the pyre start collapsing on one side too quickly, just lay on a bit more wood there. The whole thing will eventually collapse on itself, forming a pile of glowing coals and red rock.
Leave to cool overnight.
That is it. Your ore is now roasted and ready for crushing. As you can see, this is not exactly rocket science.
A Note on Safety: Putting large rocks into a hot fire can often result in them bursting apart. In case that you will be roasting pieces of ore larger than about fist-size, then I would advise you not too spend too much time roasting marshmallows and gazing into the fire, lest you want to risk being hit by some stony shrapnel.
From Mountains to Dust. With force and determination we turn rock to sand.
The last step in preparing the ore for smelting consists of breaking up the lumps to the consistency of gravel.
You might be wondering why this is necessary.
The first reason would be that reducing the ore to a uniform small size makes it easier to move through the furnace shaft without things getting stuck.
The second, and more important, reason is that it increases the surface area of the ore. Let me explain…
In the first part of this series, I told you that bloomery smelting is a thermochemical reaction between iron ore and hot carbon monoxide gas. Chemical reactions can only happen on the surface, where the reagents actually meet and react to each other. Therefore a greater surface area means more places where the reaction can take place, making the whole thing more reduction process more effective.
You of course cannot take it too far. Grinding the ore to a fine dust would theoretically maximise the surface area, but ore so fine will mostly just get blown back out of the furnace shaft as you try to smelt it.
In practice something with the consistency of sandy gravel will be just about right. Ore crushed to a consistency where the largest bits are roughly pea-sized. This will naturally also contain a lot of smaller grains, and even some dust, but the quantity of these should remain manageable.
You can take many approaches to crushing ore. You can pound it in a large mortar. Alternatively you may lay a tarp or some sack-cloth on the ground, and crack it with hammer or stone on some kind of anvil. You may also put a large steel plate on the ground and hammer your way through the ore pile.
The density and type of ore used will determine how much work you will have to put into this. Dense rock ores might require quite a some pounding. Some of the lump bog ores and limonites, on the other hand, will practically fall apart in your hands once roasted. Putting these in a bucket and mashing them with a stick might be enough.
Once you have crushed the ore, you can sieve it through some fine chicken wire to make sure that there are no overly large lumps left, but I have never bothered with such details. The odd larger nugget of ore will not ruin a smelt, and you can always pick it out when you are charging the ore.
In the end, weigh your crushed ore, put it buckets and store it out of harms way until the smelt.
We are almost there now.
The only thing that is keeping you from smelting now is an ample supply of appropriate fuel.
III. Charcoal – The Metalworker’s Fuel of Choice
So far you have learned about handling clay and ore. Now let us get the charcoal ready.
I will not be going here into the methods of making your own charcoal, which would be of course the most authentic and engaging way of doing this.
Should you happen to have the required space, time, supply of surplus and wood, and the passion for making charcoal, then you probably already know much more about charcoal than I do.
For everybody else, I shall be presuming that you will be using store-bought charcoal.
You will want lump hardwood charcoal. None of that charcoal briquettes nonsense.
What sort of hardwood, you might ask? It does not actually matter that much. In different parts of the world, different woods might be considered ‘The Best’. But these regional preferences mostly stem from what sort of wood is available in the region in the first place. So, beech, birch, oak, hazel, funny sounding exotic woods – go with what is available. In the end, they are just lumps of carbon.
You will also need A LOT of the stuff. You can easily use up 30 kg to charcoal for a small smelt, which consumes only 15 kg of ore.
So procure whatever can be got in your area at reasonable price. Ideally, you should look for something dense and not too crumbly, since that will help ensure an even burn.
One last tip on buying charcoal:
Try to get it in the biggest bags possible. Those 2-3 kg bags that you get at the gas station can be half-filled with useless crumbly bits and charcoal dust. I have found that restaurant size bags generally contain proportionally more usable charcoal.
Which brings me to the next point…
You cannot just open a bag of store-bought charcoal and start pouring it a furnace shaft. Down that road lies much wailing and gnashing of teeth.
If you fill a furnace shaft a bunch of oddly sized charcoal lumps and charcoal dust, then you run a very high chance of everything getting stuck. And a stuck smelt is annoyingly frustrating experience, which can be easily prevented by chopping and sieving the charcoal.
Therefore to prepare the charcoal for smelting you will need to spend some time with a chopping block and a cleaver or hatchet. Chop the large lumps into thumbnail- to walnut-sized pieces. Some like to go even finer, but this has proven to be fine enough so far.
This is another time-consuming task. You should now be noticing why I mentioned assistants as a necessary requirement of iron smelting in the previous instalment of this series.
Once your charcoal is neatly hacked to pieces, you will still need to remove most of the charcoal dust.
I usually do this by keeping my chopped charcoal in a large wicker basket with two sturdy handles. This way I can just shake and twist the basket until the dust is ejected in a dramatic black cloud.
You can try to chop all of your charcoal ahead of the smelt. Alternatively you can prepare enough to get started, and then somebody has to be on charcoal duty during the smelt.
It bears repeating that you will need a lot of processed charcoal for the smelt. And should anything be left afterwards, it will come in good use during processing of the iron bloom.
Thus we have come to the to the end of this sermon on the various materials used in smelting.
The main lessons learned here could be summarised as:
I. Clay matters. Mix a lot of temper into your clay. Think sand glued together with clay. Keep the mixture on the dry side. It easier to add water than to take it away.
II. Not all ores need to be roasted, but almost all benefit from it. So chuck your ore in a fire and crush it to gravel once it cools down.
III. Do not forget to get enough lump charcoal. Chop it nice and even. Then sieve out the dust.
That is it for now. You are ready to build a furnace and run a smelt.
In the next episode I will tell you how a bloomery furnace is built.
Did I miss something? Do you do something different or do you have a tip to share? Comment bellow!
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