The beginning of all wisdom is to know the difference between wrought and cast iron. Most people don't. If I hit a piece of cast iron with a hammer it smashes. It is brittle and it won't stand shocks. Hitting a nail, on the other hand, just bends it - that is how wrought iron behaves. It is altogether tougher and more useful than cast iron. Wrought iron is the stuff that blacksmiths want.
All the early iron - in Iron Age and Roman times was this kind of wrought iron. It was made in bloomeries. A bloomery was just a stone bowl or box on the ground. Quite a big one would only be 4 feet by 4 by 4. There was a bellows to urge the fire on. The fuel was charcoal (almost pure carbon) and iron ore. Not to be too fancy, iron ore is a mixture of rust and clay. Rust is iron combined with oxygen. The bloomery's job is to free that iron.
When things get hot the oxygen in the rust has a greater affinity for carbon; it really loves the stuff. So oxygen leaves the iron, and now you have carbon dioxide a gas - which clears off upwards. Unfortunately, the iron now looks around for a little friend - it is very reactive stuff. It hooks up chemically with clay (which is silica and so forth) and forms a slag. About three quarters of the iron is wasted this way. You have maybe 30 lb of iron and an awful lot of slag. The bloomery is not hot enough to melt this; it is a pasty lump. You get this ball of stuff out and hammer it. The slag squeezes out and the iron sticks together. Eventually most of the slag is squeezed out and you end up with an iron bar.
There were many bloomery forges throughout the Middle Ages in this region north of Sheffield. Wortley was a bloomery from at least about 1600 onwards. These bloomeries continued until about 1800. It was excellent iron but it came in small quantities.
A blast furnace was totally different and It made the other sort of iron - cast-iron. Blast furnaces grew up after about 1400 in Europe. (The Chinese had cast iron a lot earlier but that's their history, not ours). Early blast furnaces were tiny, maybe a dozen feet high, but modern ones have grown and grown to hundreds of feet. Again you have bellows, but this time driven by a water wheel - this furnace will be a lot hotter. The charge is different charcoal and iron ore as usual, but there is limestone as well. Notice the boshes, like an upside down cone. Solid lumps in that will not fall through; they will just pack tighter together Only a liquid will find its way down to the bottom.
Now let's get it hot ! As usual the carbon of the charcoal gets up to its tricks with the oxygen in the iron rust. The iron is freed and the carbon dioxide and monoxide gases go on up and out. But the iron doesn't pal up with the clay this time. Instead the clay gets together with the limestone to form a slag, like a greeny, bluey glass that melts and runs down Into the hearth underneath. In fact the iron plays another dirty trick. It dissolves the carbon; a lot of it, and melts and flows down as well So the slag and the carbon rich iron are both down there in the hearth but the slag is lighter and floats on top of the Iron. From time to time you can tap off the iron and run it into moulds to make pigs for remelting; or you can cast things directly. That's how they cast the first Iron bridge at Coalbrookdale.
So there we have the two kinds of Iron - wrought and cast. You could hammer and forge wrought iron, cast was brittle, and broke. You could only shape it by casting so it was no good to blacksmiths. You could weld wrought iron; you can't do that with cast. And cast-is too brittle to make chains - you needed wrought for that. Cast was OK for cannon and fireplaces or hammer-parts, but it was very limited. 'Iron' was really wrought iron.
The big question was COULD YOU MAKE NICE USEFUL WROUGHT IRON OUT OF CHEAP NASTY CAST IRON? There was a way but it was complicated, messy, slow, expensive and didn't give you much wrought iron - this was the finery and chafery method, where you used two furnaces and a lot of hammering. The real answer came late. Various people like the Cranages got part of the answer but the big name is Henry Cort who in 1783 came up with Puddling and Rolling - a double barrelled process. Wortley was the first place In Yorkshire to adopt this and it was extremely useful.
First, the puddling furnace.
Because of the high chimney and the damper, you can vary the heat. The fire and the hearth are separate, so the fire never touches the charge - that way you don't have nasties like sulphur and phosphorus from the coal getting into the iron. The roof of the furnace is that funny shape so that the heat bounces, reflects down into the charge. Anyway the pig iron in the hearth melts. Close the damper and lower the heat. Now the puddler opens that door and he stirs the iron with an iron rake or rabble. As he stirs it, the air gets to it and burns the carbon out. As the carbon burns the iron gets purer - it is turning into wrought.
Now notice a funny thing. Mixtures have a lower melting point than pure things - that's why we put salt on the roads in winter. Cast iron melts at 1200°C; steel, with far less carbon, at 1600°C - and pure iron takes about 1800°C before it melts. The iron can't stay molten as the carbon burns-out, and it goes to dust, like a heap of sand or sawdust. Then "the iron has come to nature". It is ready. The puddler opens the damper, the iron comes up to white, or welding, heat and so it balls together. The puddler gets this ball of iron out of the furnace, and it is given a few blows of a hammer to squeeze it up. Then it is ready for part two of the process - rolling.
The rolls can be water or steam powered. They work on the metal, compressing and elongating it as if it were being hammered. So once again, you get that beautiful fibre we find in wrought iron. But there is another big advantage. You can make the grooves in the rollers any shape-you like. They can be L or T or square or round. You can make long bars of lovely wrought iron to any section an engineer might need. You could never have had the railways without it. They tried making cast iron rails, but they were too brittle - they just would not stand up to a bump from the truck. Wrought iron might bend under impact but it doesn't break - it is tough. This rolled iron also meant that you could build iron ships like the Great Britain and Great Eastern with rolled sheets and frame members. Suspension bridges too, are a consequence of puddling and rolling. Those great chains the deck hangs from have to be wrought iron.' You could never use cast for that.
Puddling and rolling went on at Bottom Forge, a few hundred yards away from the other side of the bridge. In those days you couldn't build a huge factory - there wasn't enough power in a little river. Wortley belongs to the age when man-still had to co-operate with nature. Nowadays we have so much steel and concrete and power that we impose our will upon her.
The bars from Bottom Forge came up here [Top Forge], and they were fitted together with an iron ring to hold them. Then they were heated up to welding temperature in reheating furnaces. The cranes swing them nicely across to the hammer, which the water wheel turns and the noggins will lift and drop the hammer as they turn. you are forming the bars of wrought iron into one consolidated mass, a tough single piece of metal that will stand up to the impacts and strains that a railway axle is subjected to. Cast iron would never do for this. They said that Wortley axles never broke. To test one, they would drop a two ton weight on it. It bent, but it did not snap.
Lastly, we come to steel. Steel comes between cast iron and wrought cast iron has too much carbon - 2% or more; wrought iron has practically none. Steel has less than 2%; even at that it is 'hard and brittle, though they made razors from it. But there were many grades of carbon steel, each with its own task. To make it, they used big stone chests set in a beehive shaped furnace. Here at Wortley, we found some odd looking remains a few years ago at the other end of the foundry, and Ken Barraclough - the steel historian - twigged that they were like the drawings of a cementation furnace that a Swedish visitor to England had drawn. Anyway, this is how it worked.
A layer of wrought. iron bars is placed in a stone chest and then packed round with charcoal. There were all sorts of magic mixtures, but the essential thing was the Carbon. Then another layer of bars, more charcoal, more bars and so on. Top in all off with a layer of stone dust from the grinding wheel. Then cook it for seven days or a fortnight. The iron never melted but it got hot enough for the carbon to soak in - and turn, it in to steel. They left a hole to pull one of the bars out now and again to see how it was doing. When it was ready the gases made lumps - blisters - hence the name, blister steel. It wasn't very good - all hard on the outside 'and softer inside where the carbon hadn't penetrated so they heated it and folded it over, hammered, heated and folded, welding it together till it was in fine layers like puff pastry. It wasn't good for very little things like watch springs - it still had layers of slag in it but it was fine for cartspring, swords and big stuff like that.
That was blister steel. For the next part of that story, you should go to Abbeydale. There they broke up bars of blister steel and melted them in crucibles. It took 1600°C to do it - that was the difficult part - but they could manage it with specially mixed clay for crucibles, high chimneys and a good draught. Then they poured the melted steel into ingot molds and all the muck floated to the top. This left beautiful, well mixed, pure steel at the bottom. When it cooled, they broke off the bad bit and used the good stuff.
This story of the different irons and steel lies behind most of the development in engineering. It is no coincidence that suspension bridges with their great iron chains, like Telford's masterpiece at Menai, or Brunel's great bridge at Clifton, came after 1800 - made possible by Henry Cort's puddling and rolling . The cast iron arches had come earlier, thanks to the Darbys and their use of coke to make cast iron.
The significance of metals is always there if you look for it. Even in the ancient poem Beowulf, or the Song of Roland, or the King Arthur legends it is to be found in those magic swords made by the fairy smiths, swords with names like Excalibur, Dumndel and Hrunting. Good steel was rare and the process was not easy to repeat. If a blade was strong and passed down from generation to generation then a name and legend began to grow up round it.
Plainly, I have simplified the story to reduce it to manageable compass, but I hope that it is not distorted in this telling. I hope it will serve as a Christmas tree to hang the more detailed bits on. A lot of questions remain unanswered here. There were many kinds of ore and fuel, each with their own problems. I have not mentioned coke, or Bessemer, or the new alloy steels which kept Sheffield famous. But, after all, the story begins with wrought iron and that astonishing difference between the metal that smiths loved and the brittle, intractable material from the blast furnaces. In that story, Wortley more than plays its part. History has handed us a document written in soil, stone and iron which is of inestimable value. If we can read - and preserve - that document, we shall be better, wiser men.
Editors note - This article was first delivered as a talk to the general public at Wortley Top Forge.
The Cutting Edge - No.2 - Autumn 1985
Return to Magazine & Journal Articles Contents Page