The pliers are lubricated with clean lubricating and preserving oil. The oil that penetrates the joint finds soot there. This comes from the tempering process, in which the tool steel is quickly cooled in oil after being heated – this optimizes the steel structure and significantly increases its strength. The hardening oil that penetrates the joint space during this "quenching" burns there due to the high temperatures – soot remains. It is not completely washed out in the subsequent washing operations.
If fresh lubricating oil penetrates the joint gap, this soot is dissolved and the very fine particles colour the lubricating oil dark brown.