Consonni Srl makes burnishing treatments of metals.
The easiest and cheapest way to give brass objects a black colour is to soak them in a cold or moderately hot solution of basic copper carbonate (azurite) and ammonia. Basic copper carbonate can be prepared by adding a hot solution of sodium carbonate to a hot solution of copper sulphate, until the precipitate is formed. It should not be heated further. As it is hard to filter, the precipitate must be separated by decantation, washed with water by stirring, and decanted again. The solution must be stored in closed containers in a cool place; some add graphite to it. After being degreased and slightly etched, the objects to be coloured are immersed in the solution, cold or heated at 30-40 °C (no more than that), and stirred continuously. In the cold solution, the colouring takes place in about 1-5 minutes; in the lukewarm solution, in about half a minute. The colouring mechanism is the following: the dark blue solution contains ammoniacal copper oxide that, due to the presence of oxygen by dissolution of zinc, (hence the need for stirring the pieces), separates black copper oxide, which binds in the pores left free by the zinc on the attacked brass surface. Obviously, the dissolved zinc initially facilitates the process, but when the content is too high, it causes defective coatings. The ammonia should be as concentrated as possible and the copper carbonate should be prepared fresh. The colouring comes out well on brass 60-65; if the copper content exceeds 65%, the resulting colour is brown or reddish¬-brown. The pieces should not be etched in brilliant etching solutions for too long or too often, otherwise the material becomes passive and too much zinc is removed from the surface. In the end, it is advisable to protect the colouring with nitrocellulose or similar transparent varnishes.
A glossy black colour can be obtained on previously prepared surfaces by soaking the pieces for 25-30 min at 55-60 °C, in a bath of antimonyl chloride, that is prepared by adding approx. 20 cm3 of concentrated hydrochloric acid to a solution containing 34 g/l of tartar emetic. After decanting, the water is syphoned and the precipitate (paste-like) is employed for the preparation of the actual bath, operating as follows: to 150 cm3 of paste are added, first 500 cm3 of a solution of 40 g/l of thiourea and 2 g/1 of nickel sulphate, and then 200 cm3 of solution containing 50 g/l of Rochelle salt, as well as enough water to increase the volume to 1 litre; if the pH is not 3.5-4, fix with 1 N sodium hydroxide (if it is lower) or with 1 N hydrochloric ¬acid (if it is higher.) The colouring can be finally protected with epoxy paint.
Steel-grey colouring is achieved with a solution composed of 500 g of hydrochloric acid and 500 g of water, to which are added 150 g of iron chips and 150 g of pulverized antimony sulphide. A solution of antimony chloride can also be used.
A yellow film is obtained with the following bath:, dissolve 3.6 parts sodium hydroxide and 3.6 parts lactose in 90 parts water; boil for 15 minutes, add a solution of 3.6 parts copper sulphate in 10 parts hot water. Operate by immersion at 80 °C.
Antique-bronze colouring is obtained with a solution containing 62 g/l of arsenious anhydride, 50 g/l of sodium cyanide, and 62 g/l of sodium hydroxide. Or: 124 g/l of arsenious anhydride, 62 g/l of copper sulphate and hydrochloric acid, so as to bring it to a litre.
Brown colouring is obtained by immersion in a bath of 40 g/l of potassium chlorate, 40 g/l of nickel sulphate, and 5 g/l of potassium permanganate. A solution of 3 g/l of ammonium sulphide can also be used.