"Cyanide of Potassium", Potassium Cyanide also commonly called "cyanide" is the same material used to create lethal cyanide gas.
Contact with acids release the gas. While still used in industry it is no longer used in the home shop.
A product called "Kasenit" or Casenite is used to surface harden steel.
The old product had cyanide in it and the new is currently cyanide free.
However, this does not mean it does not contain toxic substances.
Refer to the product MSDS (Material Safety Data Sheet).
See Case Hardening
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