A process of treating steel or iron with chemicals to create a protective blue oxide layer, preventing rust and corrosion.
From the verb 'blue' (to make blue, or to treat with bluing chemicals) + '-ing' (gerund suffix); the technique developed in the 18th-19th centuries to protect metal surfaces.
Blueing is based on a controlled oxidation process—gunsmiths perfected this technique because weapons needed to resist corrosion, and the blue color became a sign of quality craftsmanship.
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