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From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Instructions for creating a dead black finish using a liver of sulphur bath, with precautions for soldered articles.

Identifier  ExFiles\Box 38\6\  Scan150
Date  1st December 1908
  
A.W.BEECH/1.12.08 (F.H.R.) & 2.W.B

Contd.
-2-

bristled brush, leaving a deep brown or semi-black deposit.
This may be further protected by a coat of lacquer.

Should a deep dead black finish be desired, further treatment (the lacquer being, of course ommitted)-must be given as follows:-

A liver of sulphur bath is made by dissolving 2 ozs of liver of sulphur in one gallon of hot water. When the solution is cold the article is immersed in this, and after some five minutes removed, and, without rinsing, it is reheated until the surface becomes dead black. After a further treatment, as regards brushing the surface, a finish of lacquer should be given. This process gives a most durable finish and is much used by one of the largest telephone manufacturers.

Some soft solders melt at about 250° Fhr. and therefore care sgould be taken in heating soldered articlesm such as horns, etc. - Orient.
  
  


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