(4% in acet. anhyd.) 30 cc.
The final conclusions were that solutions Nos. 2, 8, 9, 10, and 11 can be used successfully on cold rolled steel. Solutions 1, 2, 3, and 5 will give good results on cast steel. When No. 3 is used the metal has to be immersed in the solution and the solution boiled to get good etching. This often cannot be done, so No. 3 is not recommended. Solutions Nos. 2, 3, 4, and 10 can be used successfully on malleable cast steel. No solution found to date is equally good for all of the different kinds of steel encountered in gun manufacture.
In general, the technique used is to swab the filed surface with a cotton swab wet with the reagent, which may or may not be heated. Etching naturally proceeds faster with hot solutions. Sometimes care must be taken not to etch too fast, particularly if the filing has been deep. The numbers may come up and then vanish-and once gone they cannot be restored. Another technique is to build a little „dam" or retaining wall of modeling clay or a suitable wax around the part to be etched and to fill this little „pond" with the reagent (cold) and wait for development of the numbers. As the numbers appear they should be photographed (or at least recorded) promptly for fear that they may disappear-to be forever lost. They are best photographed while wet, as the numbers show up better then. To prevent too-rapid evaporation they may be wetted with glycerine while being photographed. The best results are highly dependent on proper illumination. Some experimentation will be necessary to determine the best angle for this.
Some study was made of „anodic" or „electrolytic" etching. In this process the metal to be etched is made the anode in an electrolytic bath of some dilute acid or etching solution and a current of electricity is then passed through the bath.
While it was found possible to restore numbers in this way, the process seemed to offer no advantages over the use of reagents alone and is more difficult to carry out.
Professor Ralph Turner of the State University of Michigan, who worked with the author on the problem of restoration of serial numbers while a student (and who is largely responsible for the results obtained), has more recently made a study of the electrolytic process, basing his procedure on suggestions made to him in a personal communication from Mr. Shigeo Arai of the Scientific Crime Detection Laboratory of Tokyo. The results of this study were presented at the 5th Annual meeting of the American Academy of Forensic Sciences in 1953 in a paper by Turner and Burgess.
The electric circuit used by Arai and by Turner is shown in the accompanying diagram (Fig. 101). The gun (or other object) is made the anode and the cathode consists of a loop of wire (or a battery clip) which holds a cotton swab kept wet with the solution. The metal surface, which has been smoothed and polished and treated with acetone or other suitable solvent to remove grease or oil, is gently swabbed with the wet cotton cathode over the area to be etched. Then, starting with a value below the decomposition potential of the solution, a voltage is applied across the electrodes and gradually increased until a point is reached at which the current will flow. If the voltage is raised much beyond this point the flow of current will increase rapidly. This threshold value at which the current starts to flow will be different for different solutions. It should not be exceeded more than a few tenths of a volt. (While the purpose of this electrolytic method is to speed up the process of etching it must not be speeded up too much since pitting will then occur, and the procedure may get out of control and the numbers will be irretrievably lost. There will be an optimum voltage (and amperage) which will give the best results. This optimum must be determined by experiment.) After a few minutes the numbers should begin to appear and care must be taken that the etching process does not proceed too far. In but few cases will more than ten minutes be required and it may take as little as three or four. As soon as the numbers can be read the process is stopped and the electrolyte is washed off completely-using a neutralizing solution, followed by water. If the results appear to be satisfactory a photograph should be taken, after removing the water with acetone and applying a thin coating of oil. The author prefers to use glycerine rather than oil to keep the surface wet while photographing.
Turner tried the solution suggested by Arai (H2O-500 cc.; cone. H2SO4-15 cc.; gelatin-1 gram; and CuSO4-1 gram) but after considerable experimentation found combinations of ingredients that produced better results. The results for eight solutions were reported. Some were based on Arai's suggestions and others on the solutions previously used in the author's laboratory but later modified as to concentrations. The eight solutions had compositions as shown in Table 11.
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