ForMatter/Finishes/patina/Gun blue (cold blue / hot blue on steel)
finish_patina_gun_blue

Gun blue (cold blue / hot blue on steel)

patina · patinated · bluing, rust blue, cold blue, hot caustic blue

The deep blue-black on the steel of a fine revolver, a Mauser barrel, an antique knife. A controlled rust — cold-blue solution gives a thin black film on contact; hot-blue dunks the part in boiling caustic nitrate for a thicker, harder, more durable layer. Doubles as light corrosion protection.

Cold blue: selenium dioxide / phosphoric acid solution wiped onto degreased carbon steel, builds a black ferric-selenide film instantly but soft (~1 µm, scratches with a fingernail). Hot blue: 135 °C bath of sodium hydroxide + sodium nitrate + sodium nitrite for 15–30 min, grows a uniform magnetite (Fe₃O₄) layer 2–5 µm. Hot blue is the gunsmith standard; cold blue for touch-ups. Both need oil immediately after to seal — the porous oxide will rust if left bare.

character — deep blue-black, slight luster, traditional firearm-finish, oils hold the color.

Finish properties

  • levelpatinated
  • subcategoryselenium / nitrate chemical bluing
  • applies tometal

Incompatibilities

  • stainless steel (chrome oxide blocks the blueing reaction)
  • aluminum, brass, copper (no iron to oxidize)

Second life

reversibilitymoderate — patinas can be stripped (acid, mechanical polishing) and re-applied; the substrate metal is preserved through the process. The historical patina cannot be exactly reproduced after stripping.
blocks substrate recyclingno
renewabilityfield-renewable — a patina can be refreshed or applied to a stripped piece by a metalsmith with the right chemistry. Conservation-grade re-patination is a specialty (Sculpture Conservation Studio practice).

Hughes & Rowe *The Colouring, Bronzing and Patination of Metals* (Crafts Council, 1991, Watson-Guptill ed. 1995); American Institute for Conservation patina-conservation guidelines.

Citations