This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
(See also Plating.)
First, clean the articles to be plated. To remove grease, warm the pieces before a slow fire of charcoal or coke, or in a dull red stove. Delicate or soldered articles should be boiled in a solution of caustic potash, the latter being dissolved in 10 times its weight of water.
The scouring bath is composed of 100 parts of water to from 5 to 20 parts of sulphuric acid. The articles may be put in hot and should be left in the bath till the surface turns to an ocher red tint.
The articles, after having been cleansed of grease by the potash solution, must be washed in water and rinsed before being scoured. Copper or glass tongs must then be used for moving the articles, as they must not afterwards be handled. For small pieces, suitable earthenware . or porcelain strainers may be used.
The next stage is the spent nitric acid bath. This consists of nitric acid weakened by previous use. The articles are left in until the red color disappears, so that after rinsing they show a uniform metallic tint. The rinsing should be thoroughly carried out.
Having been well shaken and drained, the articles are next subjected to the strong nitric acid bath, which is made up as follows:
Nitric acid of 36° Bé.. 100 volumes
Chloride of sodium (common salt)..... 1 volume
Calcined soot (lamp-black)............ 1 volume
The articles must be immersed in this bath for only a few seconds. Avoid overheating or using too cold a bath. They are next rinsed thoroughly with cold water and are again subjected to a strong nitric acid bath to give them a bright or dull appearance as required.
To produce a bright finish, plunge them for a few seconds (moving them about rapidly at. the same time) in a cold bath of the following composition:
Nitric acid..........100 volumes
Sulphuric acid.......100 volumes
Chloride of sodium... 1 volume
Again rinse thoroughly in cold water. The corresponding bath giving a dull or matt appearance is composed of:
Nitric acid........ 200 volumes
Sulphuric acid..... 100 volumes
Sea salt........... 1 volume
Sulphate of zinc. . .1 to 5 volumes
The duration of immersion in this bath varies from 5 to 20 minutes, according to the dullness required. Wash with plenty of water. The articles will then have an unpleasant appearance, which will disappear on plunging them for a moment into the brightening bath and rinsing quickly.
The pieces are next treated with the nitrate of mercury bath for a few seconds.
Plain water.......10,000 parts
Nitrate of mercury 10 parts
Sulphuric acid..... 20 parts
It is necessary to stir this bath before using it. For large articles the proportion of mercury should be greater. An article badly cleaned will come out in various shades and lacking its metallic brightness. It is better to throw a spent' bath away than attempt to strengthen it.
The various pieces, after having passed through these several processes, are then ready for the plating bath.
A few words on the subject of gilding may not be amiss. Small articles are gilded hot, large ones cold. The cold cyanide of gold and potassium bath is composed as follows:
Distilled water.....10,000 parts
Pure cyanide of potassium......... 200 parts
Pure gold......... 100 parts
The gold, transformed into chloride, is dissolved in 2,000 parts of water and the cyanide in 8,000 parts. The two solutions are then mixed and boiled for half an hour.
The anode must be entirely submerged in the bath, suspended from platinum wires and withdrawn immediately the bath is out of action.
Zinc, tin, lead, antimony and the alloys of these metals are better if previously covered with copper.
The following are the formulas for the other metals per 10,000 parts of distilled water:
Crystallized phosphate of soda, 000 parts; alloys rich in copper castings, 500 parts.
Bisulphide of soda, 100 parts; alloys rich in copper, 125 parts.
Pure cyanide of potassium, 10 parts; alloys rich in copper, 5 parts. Pure gold transformed into chloride, 10 parts; alloys rich in copper, 10 parts.
Dissolve the phosphate of soda hot in 8,000 parts water, let the chloride of gold cool in 1,000 parts water; mix little by little the second solution with the first; dissolve the cyanide and bisulphide in 1,000 parts water and mix this last solution with the other two. The temperature of the bath may vary between 122° and 175° F.