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GALVANIC TREATMENTS > Burnish
 
The most simple and cheap method to realize in black colour on brass is to dip the objects into a cold solution or moderately hot of carbonate basic copper ( azurite) and ammonia.

Copper basic carbonate can be prepared adding a copper sulfate hot solution, sodium carbonate hot solution until the precipitated formation. After we have not heat it. The precipitated have to be separated for decantation , washed shaking with water and decanted again. The solution must be kept in closed beakers and in fresh places; someone add graphite. The objects to colour , degreased and lightly pickled, are dipped into the cold solution or heated to 30-40 °C ( don't beyond), taking care to continuously shake them. Into the cold solution they colour in about 1-5 minuts; in that tepid in about 0,5. The colouring mechanism is the following: dark blue solution contains ammoniac copper oxide which,to the oxygen presence under zinc dissolution, separate black copper oxide , that fasten into the pores left free by zinc on attacked brass area. At the beginning zinc that goes in solution and helps the process, instead when it is in strong contained, it provokes an defective blooms. The coloration prospers on brass 60-65; if the copper contained exceeds the 65% the final colour will be brown or liverish. It needs to avoid to pickle too long pieces and continually too into brilliant pickling solutions, otherwise the material becomes passive and a good part of zinc goes away from the area. At the end is advisable to protect the coloration with nitrocellulose or varnish similar to it.

A polish black colour, on a well praparated areas, can be along dipping the pieces for 25-30 mins, to 55-60°C, in an antimonyl chloride base bath, adding it into a solution contening 34 g/l of emetic argol ca. 20 cm3 of chloridric acid concentrated. Let it decant , let the mother water trap and the precipitated ( pasty) is used for the bath preparation and operating like the following: to 150 cm3 of batter we add before 500 cm3 of solution of 40 g/l of thiourea and 2 g/1 of nickel sulfate, then 200 cm3 of solution containing 50 g/l of Seignette sal and, at the end, many water to bring the volume to 1 liter; if the pH was not 3,5-4, you'd correct it with caustic soda 1 N (if inferior) or with chloridric acid 1 N ( if superior). The colour can be final protected with epoxy paint.

The grey steel bears with a solution composed by 500 g of chloridric acid and 500 g of water, to which we add 150 g of iron beating and 150 g of atomised antimony sulfide. We can also use an antimony chloride solution.

To obtain a yellow bloom with the following bath: release in 90 parts of water 3,6 parts of caustic soda and 3,6 parts of lactose; to boil for 15v minutes. To add a 3,6 copper sulfate parts solution in 10 parts of hot water. We can operate for immersion to 80°C.

Ancient bronze coloration bears with solutions containing 62 g/l of arsenic trioxide. 50 g/l of sodic cyanid c 62 g/l of caustic soda. Or: 124 g/l of arsenic trioxide, 62 g/l of copper sulfate and much chloridric acid, arriving to the liter.

Brunish coloration bears for immersion in a bath of 40 g/l of potassium chlorate, 40 g/l of nickel sulfate and 5 g/l of potassium permanganate. Alternatively also with a solution of 3 g/l of ammonium sulfide.


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