§ 128 The Dipping Bath.
An acid dipping bath is one of the most useful adjuncts to the laboratory, not only for cleansing metals for electroplating, but for cleaning up apparatus made out of bits of brass tube and sheet, and particularly for quickly cleaning binding screws, etc., where it is necessary to ensure good electrical contact.
The cheapest and most satisfactory way in the end is to make up two or three rather large baths to begin with. The glass boxes of storage batteries do very nicely for the purpose, and being generally ground pretty flat at the top, they may be covered by sheets of patent plate glass, and thus preserved from the action of the air.
First Bath. — A 30 or 40 per cent solution of commercial caustic soda. Objects may be cleansed from grease in this bath by heating them as hot as is consistent with individual circumstances, and plunging them into it.
It is a considerable advantage to begin by removing grease from articles subsequently to be dipped in an acid bath, both because it saves time and acid, and because more uniform results are obtainable when this is done than when it is omitted. It is a great advantage to have the caustic soda solution hot. This is always done in factories where nickel-plating is carried on, but it is inconvenient in the laboratory. The articles after dipping in the alkali are swilled with water, and may even be scrubbed with a brush, so as to remove greasy matters that have been softened but not entirely removed.
Acid Bath. — A convenient bath for laboratory purposes is made by mixing two volumes of strong commercial nitric acid with one of strong sulphuric acid in a cell measuring, say, 12 X 10 X 15 inches.
Copper or brass articles are dipped in this bath for a few seconds, then rinsed with water, then dipped again for a second or two, or until they appear equally white all over, and then withdrawn as rapidly as possible and plunged into a large quantity of clean water. Care must be taken to transfer the articles from the bath to the water as quickly as possible, for if time be allowed for gas to be evolved, the surfaces become mat instead of bright.
In order to save acid it is advisable to make up a third bath, using those odds and ends of acids which gradually accumulate in the laboratory. Sulphuric acid from the balance cases, for instance, mixed with its own volume of commercial nitric acid, does very well.
The objects to be dipped receive a preliminary cleansing by a dip in this bath, the strong bath being reserved for the final dip. Sheet brass and drawn tube, as it comes from the makers, possesses a really fine surface, though this is generally obscured by grease and oxide. Work executed in these materials, cleaned in alkali, and dipped in really strong acid, will be found to present a much better appearance than work which has been filed, unless the latter be afterwards elaborately polished.
On no account must paraffin be allowed to get into any of the baths. When the final bath gets weak it must be relegated to a subordinate position and a new bath set up. A weak acid bath leaves an ugly mottled surface on brass work.
§ 129. A metallic surface which it is intended to electroplate must, as has been mentioned, be scrupulously clean. If the metal is not too valuable or delicate, cleaning by dipping is easy and effectual. The following notes will be found to apply to special cases which often occur.
(1) Silver Surfaces intended to be gilt. — These are first washed clean with soap and hot water, and polished with whitening. They are then dipped for a moment in a boiling solution of potassium cyanide. A 20 per cent solution of common commercial cyanide does well, but the exact strength is quite immaterial. The cyanide is washed away in a large volume of soft water, and the articles are kept under water till they are scratch-brushed.
Mat surfaces are readily produced on standard silver by dipping in hot strong sulphuric acid. The appearance of new silver coins, which is familiar to everybody, is obtained by this process.
(2) Finely turned and finished Brass Work. — If it is intended to nickel-plate such work, and if it is desirable to obtain brightly polished nickel surfaces, the work must be perfectly polished to begin with. Full details as to polishing may be found in workshop books or treatises on watch-making. It will suffice here to say that the brass work is first smoothed by the application of successive grades of emery and oil, or by very fine “dead” smooth files covered with chalk. Polishing is carried out by means of rotten stone and oil applied on leather.
In polishing turned work care must be taken to move the file, emery, or rotten stone to and fro over the work with great regularity, or the surface will end by looking scratchy and irregular. The first process of cleaning is, of course, to remove grease, and this is accomplished best by dipping in a bath of strong hot caustic soda solution, and less perfectly by heating the work and dipping it in the cold caustic soda bath.
During this process a certain amount of chemical action often occurs leading to the brass surface exhibiting some discoloration. The best way of remedying this is to dip the brass into a hot bath of cyanide of potassium solution. If it is inconvenient to employ hot baths or to heat the brass work, good results may be obtained by rubbing the articles over with a large rough cork plentifully lubricated with a strong solution of an alkali.
If the surfaces are very soiled or dirty, a paste of alkali and fine slaked lime may be applied on a cork rubber, and this in my experience has always been most effective and satisfactory in every way, except that it is difficult to get into crevices. If the alkali stains the work, a little cyanide of potassium may be rubbed over the surface in a similar manner.
Brass work treated by either of these methods is to be washed in clean water till the alkali is entirely removed, and may then be nickel-plated without any preliminary scratch-brushing. The treatment in hot baths of alkali and cyanide is the method generally employed in American factories as a preliminary to the nickelling of small brass work for sewing machines, etc.
(3) Copper either for use as the kathode in electrolysis calibration experiments or otherwise is most conveniently prepared by dipping in the acid bath, rinsing quickly in cold water, scratch-brushing under cold water, and transferring at once to the plating bath. In the case where the copper plates require to be weighed they are dipped into very hot distilled water after scratch-brushing, and then dried at once by means of a clean glass cloth.
(4) Aluminium (which, however, does not readily lend itself to plating operations 1) is best treated by alkali rubbed on with a cork, or by a hot alkaline carbonate where rubbing is inexpedient. The clean aluminium is scratch-brushed under water, and at once transferred to the plating bath.
(5) Iron for Nickel-plating. — According to Dr. Gore (Electra-metallurgy, p. 319) the best bath for cleaning iron is made as follows: “One gallon of water and one pound of sulphuric acid are mixed with one or two ounces of zinc (which of course dissolves); to this is added half a pound of nitric acid.” The writer has been accustomed to clean iron by mechanical means, to deprive it of grease by caustic alkali, and to finish it off by, means of a hard scratch brush. This process has always worked satisfactorily.
(6) Articles soldered with soft solder containing lead and tin do not readily lend themselves to electrolytic processes, the solder generally becoming black and refusing to be coated with the electro-deposit. Moreover, if soldered articles are boiled for any length of time in caustic alkali during the preliminary cleansing, enough tin will dissolve to form a solution of stannate of potash or soda — strong enough to deposit tin on brass or copper. A method of coppering soldered articles will be described later on.
Notes
1 This difficulty has now been overcome. See note, section 138.