Clickspring – Ancient Tool Technology: The First Hardened Steel
An evening pause: Most of us know that steel in some variety is made by adding carbon to the iron. This video shows how it was done in ancient times.
Hat tip Cotour.
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“The traditional method of applying the carbon to the surface of the iron involved packing the iron in a mixture of ground bone and charcoal or a combination of leather, hooves, salt and urine, all inside a well-sealed box. This carburizing package is then heated to a high temperature but still under the melting point of the iron and left at that temperature for a length of time. The longer the package is held at the high temperature, the deeper the carbon will diffuse into the surface. Different depths of hardening are desirable for different purposes: sharp tools need deep hardening to allow grinding and resharpening without exposing the soft core, while machine parts like gears might need only shallow hardening for increased wear resistance.”
https://en.wikipedia.org/wiki/Case-hardening
The process is instantly recognizable by the rainbow surface colors it gives – an effect particularly admired on firearms
https://www.ammoland.com/wp-content/uploads/2018/08/Henry-Big-Boy-Color-Case-Hardened-Rifle-Receiver.jpg
Many historic and some modern revolver frames were “color” case hardened using bone for the carbon source producing beautiful blue/brown/tan marl patterns.