Forging a bloom into iron bars

Despite the melted tuyere, my first try at smelting gave me 4lb of iron.

When everything behaves, a bloomery furnace makes you a big lump of iron. In my smelt, I made a bucket full of half-smelted iron-rich slag (I will re-smelt this to finish the process) and bowl of fluffy iron metal fragments.

But those fluffy fragments, with a little persuading, are forging up into nice bars of solid metal.

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The fluffy iron bloom bits made during my first smelt (on top of the stump), surrounded by iron-rich slag (on the grass)

Consolidating the bloom

Bloomery smelted iron isn’t ready to use straight out of the furnace. First, it has to be consolidated. Consolidation is a forging process in which the metal is heated, compacted into a tight lump, forge-welded together into a solid bar, and finally drawn out and folded a few times to homogenize its structure.

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A partially consolidated bar of bloomery iron (smelted by Mark Green). I've folded this bar twice, and am about to weld together for a third fold (giving it 8 layers total). By this point, the metal behaves like wrought iron rather than crumbly bloom.

If it helps, you can think of consolidation as mixing and kneading bread dough. The bloom has to be patted and kneaded and worked (wrought, hence “wrought iron”) all over to make it ready for the next step. The goal, as with bread, is to mix the iron and remaining slag together into a consistent, solid structure that can be shaped without crumbling.

You can also think of it like cheese.

Iron bloom is a crumbly cheese, like feta. Bend it, and it will fall apart. Ribbons of slag (glassy impurities, left over from the ore) run through the metal in random directions, like the penicilium mold in blue cheese. Hit the iron while it’s cold, and these ribbons will split apart, giving you a fist-full of crumbles.

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In this lump of barely consolidated bloom, you can see the random lattice of slag-filled cracks where the iron has not yet welded together. As the iron is drawn out and folded, these slaggy cracks are also drawn out along the length of the material, giving it a more consistent, and much stronger, structure.

If however you heat the metal and fold it a few times, those random ribbons of blue-cheese slag stretch out, and the bloom develops a stringy texture. Think mozzarella. Bend a piece of mozzarella string cheese, and it flexes instead of crumbling. That’s because the milk proteins have been stretched into long strings hold it all together. As you fold and hammer bloomery iron, it too develops strings of slag, and these slag strings hold the metal together.

You can see these string-cheese like ribbons of slag when you break a piece of wrought iron in two.

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You can see the string cheese-like texture inside this piece of 19th century wrought iron. Those strings are from the slag, which has been draw out into long fibers inside the metal.

Consolidating the bloom is all about developing that stringy texture in the metal. After a few folds, the metal becomes tough, its tensile strength improves, and you can forge it into tools.

Consolidation: the process

How does this work with hot metal?

First of all, Lee Sauder has a great tutorial for consolidating bloom on his page (in keeping with the food-based analogies above, Lee compares bloom to a slice of pie!). I’ve followed his directions while forging a piece of his bloom that I bought from him, and it’s great advice.

My own iron was a little too fluffy to treat as a slice of pie, though. I took it to my friend Chris Maybury’s shop this weekend to figure out a plan.

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The lump of fluffy bloom, ready to be forged down into a solid bar

Chris Maybury

First, we built a big mound of coal, and hollowed out a cave in its center. This cave was hot enough to weld the metal together, and the carbon burned up any excess oxygen that could have corroded the metal. At forging temperatures, oxygen will react with iron very quickly, turning its surface into magnetite–basically, straight back into iron ore. We don’t want that (we worked very hard to make ore into metal!), so we’re careful to use enough carbon-rich fuel in our fire that all the extra oxygen turns into CO and CO2, keeping our metal safe.

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The coal fire, with the fuel shaped into a mound. The iron bloom is nestled into a "cave" in the side of this heap of fuel.

Chris Maybury

As the blooms heats, the glassy slag that’s mixed up with the iron starts to melt and bubble. The bloom is ready to weld once the slag looks like a runny bubbling honey glaze, or the icing on a cake that’s been left in sun too long–glistening, wilted, and dripping.

I take the bloom to the anvil and gently hammer the fluffy bits of iron in toward the center of the lump. At first, I’m just trying to collapse the air pockets and get rid of the excess slag. With each hammer blow, liquid slag flies off the metal in big, hot, molten droplets. It’s dramatic! Wear old clothes that you don’t mind burning.

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Forging the bloom into a solid lump. At this point, my goal is to collapse the air pockets and drive out excess slag. Once the metal reaches a yellow heat, the slag starts to harden and I have to put it back in the fire.

Chris Maybury

After a few seconds, the bubbly slag cools and starts to solidify back into glass. Once it stops flowing and glowing, and turns a grey color that blocks diffusion of light from inside the glowing metal, I stop. If I hammer it once the slag has cooled, the metal will crack on either side of the cold slag. Slag, remember, is made from silicon, it’s a kind of glass–once it hardens, it’ll shatter instead of forging, and so will the bloom!

I repeat this cycle: back in the fire to liquefy the slag, hammer the iron particles together, reheat and repeat.

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After several passes, the bloom is beginning to look like a bar. At this point, I begin welding the fluffy iron particles together.

Chris Maybury

Slowly, the fluffy sponge of bloom becomes a solid bar. You can hear and feel the difference as the metal welds together. It starts to push back, and the anvil starts to ring. It’s very satisfying!

And so, raw iron bloom becomes a rod of useful wrought iron.

I folded the bar once last night, drew it out, and cut it in half to fold again. That’s where I stopped. When I return, I’ll weld these two pieces toegher and then draw them out and fold them at least one more time. By that point, the metal should have begun to develop the string-cheese texture of wrought iron, and will be resilient enough to forge into a finished tool.

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Two short bars (each folded once), ready to be welded together into a 4-layered bar of iron when I return to the shop.

Lessons I learned

Blacksmiths usually talk about welding in terms of color. You can tell how hot iron and steel are based on the color they radiate, and blacksmiths get good at using color to tell whether or not the metal is hot enough to weld to itself when you hammer it.

With freshly smelted bloomery iron, however (and this is true of all wrought iron, not just new blooms), the metal’s temperature is only one part of the equation. You also need to heat the metal hot enough that the slag will move out of the way when you mash the metal together. If the slag is too cold, it will prevent the iron atoms from joining across the weld. Instead of welding, the iron will skate apart. It might even crack. The slag needs to be liquid.

So when I weld fresh bloom together (and when I forge any wrought iron), I’m not just looking at the color of the metal. I’m also watching the metal to see when the slag starts to bubble out. At the right temperature, the iron perspires. It gets wet. That’s when I know that the slag will move out of the way and allow the iron to join together into a solid lump with I hit it with my hammer.

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A knife blade forged from my bloomery iron, sweating drops of slag in the propane forge. Once the slag seeps out like this, I know it's ready to be hammered.

There’s something very elemental about all of this. I mean that in the sense of premodern elemental theory: air, fire, earth, and water. With my left hand I control the air into the forge, and I watch the fire’s response to ensure that the metal will not burn. The metal itself, made from the earth, is ready to hammer only when its slag flows like water. All four elements are there, meeting in the bloom’s transformation.

At moments like this, medieval ways of thinking about the building blocks of the world make perfect sense.

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Chris Maybury

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