In order to render copper plates which are used in printing more
durable, they can be covered with an electrolytic deposit of iron
which possesses an unusual degree of hardness almost superior to
steel. The salt usually employed has been the double sulphate of iron
and ammonia. Professor Boettger, who first invented this process, has
recently devised an improvement in the bath employed. He dissolves 10
parts of ferrocyanide of potassium (yellow prussiate of potash) and
20 parts of the double tartrate of soda and potash (Rochelle salts) in
200 parts of water, and to this he adds 3 parts of persulphate of iron
dissolved in 50 parts of water. A large precipitate of Prussian
blue is formed. To the whole is added, drop by drop, with constant
stirring, a solution of caustic soda until the blue precipitate
entirely disappears, leaving a perfectly clear, light yellow liquid,
which is now ready for use.
Professor Boettger also claims that this solution can be employed with
advantage for dyeing cotton yarn and fabrics a beautiful blue, without
the use of a mordant. For this purpose the goods are put into the
bath, that has previously been slightly warmed, until they are
saturated through and through, and then dried in the air, after which
they are immersed in extremely dilute sulphuric acid (1 to 50), which
neutralizes the alkali, and after washing and drying again they are
permanently dyed a fine blue color.
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