ForMatter/Finishes/patina/Niter blue (molten-salt blue on small steel parts)
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Niter blue (molten-salt blue on small steel parts)

patina · colored · niter bluing, molten-salt blue, watch-screw blue, potassium nitrate blue

Small steel parts (watch screws, gun front-sights, tool detailing) plunged briefly into a pot of molten potassium nitrate at ~310 °C — the surface oxidizes to a rich peacock blue. Thinner than gun-blue, deeper-color than open-air heat tinting, the traditional finishing of haute-horlogerie steel components. Reads as small-scale, archival, jeweler-grade.

Crucible of pure potassium nitrate (KNO₃, saltpeter / niter) heated to molten ~310 °C; clean degreased small steel parts suspended in the bath for 20–60 sec. The molten salt provides oxygen via thermal decomposition (KNO₃ → KNO₂ + O), oxidizing the steel surface to a thin blue-tinted magnetite (Fe₃O₄) layer ~100 nm thick. Color is precise and reproducible — bluer than open-air heat tinting at the same nominal temperature because the molten salt holds temperature stable across the part. Removed, rinsed, oiled. Used historically on watch movements (blued screws are a haute-horlogerie convention dating to A.-L. Breguet), gun front-sights, surgical-tool details. Restricted to small parts (a tablespoon's worth at a time) due to thermal-mass requirements.

character — deep peacock blue, thin oxide layer, archival, jeweler-grade detail register.

Finish properties

  • levelcolored
  • subcategorymolten-salt thermal oxide
  • applies tometal

Incompatibilities

  • stainless steel (chrome blocks the oxide reaction)
  • large parts (thermal mass exceeds typical bath capacity)
Produced by processesHeat Treatment of Steel

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.