Colouring Metals

It is sometimes desirable to provide metal objects with a coloured surface without painting. Below are some recipes for chemical baths to produce different colour finishes on different metals.

Brass

Black on brass

  • Copper carbonate 0.45kg
  • Ammonia 1 litre
  • Water 2.5 litres
  • Temperature 80°C (175°F)
Mix copper carbonate and ammonia thoroughly before adding water. An excess of copper must be present. The colour may be fixed by a subsequent dip in 2.5% solution of caustic soda.
 

Blue on brass

  • Lead acetate 85 grams
  • Sodium thiosulphate 230 grams
  • Acetic acid 115 grams
  • Water 4 litres
  • Temperature 82°C (180°F)
This will produce a blue colour, and also on polished high carbon steel. The colour will change if not lacquered.
 

Green on brass

  • Ferric nitrate 30 grams
  • Sodium thiosulphate 170 grams
  • Water 4 litres
  • Temperature 70°C (160°F)
 

Dark brown on brass

  • Potassium permanganate 30 grams
  • Copper sulphate 230 grams
  • Water 4 litres
  • Temperature 95°C (203°F)
Immerse for 2 to 3 minutes.
 

 

Steel

Black on steel (#1)

  • Caustic soda 3.6kg
  • Sodium nitrate 45 grams
  • Sodium dichromate 45 grams
  • Water 4 litres

Black on steel (#2)

  • Caustic soda 2.3kg
  • Potassium nitrite 850 grams
  • Potassium nitrate 850 grams
  • Water 4 litres
Both solutions are used at their boiling point approx 145°C, and temperature maintained by adding water or caustic.
 

Browning steel (Gun barrels)

  • Copper sulphate 80 grams
  • Mercuric chloride 20 grams
  • Ferric chloride 120 grams
  • Nitric acid 600 grams
  • Denatured alcohol 2800 cc
  • Water 4 litres
This will give a durable finish.
Method:-
  1. Dip in solution or brush on.
  2. Place in a hot box for 30 minutes at 80°C.
  3. Transfer to a humidity box. This may be a box into which live steam is introduced. When the temperature reaches about 65°C shut off the steam and allow to stand in the box until a uniform film of red rust forms.
  4. Immerse in boiling water until the red rust is converted into black oxide.
  5. Dry and scratch brush with a steel wire brush.
  6. Repeat the previous operations 3 times.
  7. Oil with oxidising oil such as linseed.