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About the Author: livescience

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  1. I am amazed at the amount of incorrect information in this thread. I am going to attempt to perform a public service here with some debunking. But I know there are plenty of skeptics here, so please everyone look up how metal oxidation works, how solid solutions work, and also carburization is an interesting topic!

    First: Scale

    The layer of “crust” being knocked off at the start is called [SCALE](https://en.wikipedia.org/wiki/Mill_scale), not SLAG. These are two different technical terms! Scale is the iron oxide layer that forms when hot steel (or more correctly hot iron [Fe]) makes contact with the O2 in the atmosphere and forms iron oxides like wustite (FeO), hematite (Fe2O3), and magnetite (Fe3O4). [SLAG](https://en.wikipedia.org/wiki/Slag) is a product of refining liquid metal and often a very complex ceramic type material (not my area of expertise). What you are seeing here is SCALE.

    Second: Carbon

    In no way is the carbon in the steel leaving the steel to combust. Carbon is crazy soluble in high temperature Iron (>1% by mass!), the typical forging process will not see enough [Decarburization](https://en.wikipedia.org/wiki/Decarburization) to create actual combusting carbon. That process just takes way to long for the diffusion too take place during the forging you see here.

    Third: Electrical Spark

    Electrical sparks….. just no. Nothing here is electrified, why would it be? Where is the circuit?

    Finally: My take

    Of course there is no way to know for 100%, but this phenomenon is most likely sparking of the iron it self as it is forged. Or more precisely the rapid oxidation of the iron into iron oxides 4Fe + 3O2 -> 2Fe2O3 for example.

    Source: Degree in Metallurgical engineering and 4+ years working in an integrated steel mill

    Edit: grammar/spelling

  2. When you pull red hot steel out of a forge it is covered in slag; deposits of carbon originating from within the steel itself. When you begin to deform the steel the slag flakes off. In this case the slag is braking off finely enough that it burns in the presence of oxygen and heat radiating off this absolute unit of red hot metal.

    TLDR high carbon steel has crusty carbon on it. Carbon burns when hot

  3. I’m not qualified to answer, but here’s my take on it.

    I think the sparkles are more oxide generating and being expelled. The first time it goes up in flames because there’s a lot of built up oxide, and the press is breaking it up into tiny pieces. At a high temperature, this causes a dust explosion. But the other times, there’s barely any oxide on the surface, and so its expulsion merely results in a tiny spark that goes nowhere because there’s not a cloud of ignitable particles nearby that’s dense enough for another chain reaction to occur. At the same time, I believe that not all of the oxide is expelled during the first pressing, and that’s why it’s being done multiple times. That, or the sparks are actually due to impurities in the metal.

    I base these assumptions on the fact that you can observe a decreasing amount of sparkles after each pressing.

  4. The sparkles are electrons coming off the material as force is applied. Which is separate to the material falling off.

    The video was on a few months ago, and I got a wonderful explanation. Which I frustratingly can’t find now, I’ve posted too much twaddle in the meantime.

  5. As a smith I’ve always enjoyed scale coming off and sparking and stuff…but this…this is sooooooooooo satisfying.

  6. My guess. The external microscope layer solidifies when loosing heat to the air, and is more brittle. Then this external layer shatters, when bent, exposing iron particles which are quick to oxidized with the oxygen in the air(burning), an exothermic reaction.

  7. Squeeze a wet sponge. Things that aren’t “sponge” come out. Water, dirt, etc.

    Impurities in this chunk of hot metal are getting squished out. I bet a lot of it is gas pockets.

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