Case hardening

In North Rashini, people observed that if you put the low quality iron back in a bloomery furnace, you get harder steel. They suspected that it was from the charcoal so they did an experiment. They put soft iron in a pot of charcoal and burnt it. The noticed that it got harder and understood from it that adding charcoal to iron made it harder. After a few hundred years the technique for case hardening was perfected and it started to enter widespread use. It became popular for engraving and warfare only by the 4th century B.B.   In the East Hu-Balai the notion that adding carbon to iron or steel made it harder was well known for a long time. For them, if a piece of steel was too soft they would melt it back down and add carbon or add iron to it if it was too soft or too hard. The East Hu-Balaiins came up with case hardening at around the 6th century B.B by trying to find a cheaper way to make hardened steel points for engraving metal to make artistic creations or to make harder arrowheads that caused more damage. They even developed a way to case harden and then quench and temper steel armor plates at around 190 A.B (that technology took a few centuries to spread from Alda, where it was invented).

You take the bits of metal and put them in a pot of powdered charcoal. The pot is baked in a very high heat for about eight hours before the case hardened metal is taken out.

Kiln, clay pot, charcoal, powdered charcoal and iron/steel.

It's usually done by blacksmiths.