100 cc.


25 cc.


20 gm.


5 gm.


1.5 gm

The advantage of the improved electrolytic process lies in the fact that much time is saved, the end result being obtained in a few minutes whereas in the older process it often took many hours. Since there is the danger of etching too far, so that the numbers disappear forever, it is advantageous to reduce the time of watching for the numbers to appear. The end result is no better, but time is saved. The disadvantages are apparent-the process requires more equipment and somewhat more skill, but neither of these is serious.*

*It should be pointed out that while the assembly of apparatus such as has been described is doubtless preferable, quite satisfactory restorations have reportedly been made by the electrolytic method by simply using one, two, or three dry cells with no adjustable resistance or measuring instruments.

Serial numbers are often removed from objects other than guns and, since it is a matter of common knowledge among police officers that serial numbers can be restored, the firearms examiner is often given the problem of restoration on these other objects. Among such are: automobile engine blocks, typewriters, adding machines, I.B.M. machines of various kinds, tape recorders, microscopes, watches, sewing machines, washing machines, and many other items of commerce any one of which may have its serial number removed to prevent the tracing of original ownership.

The author has had occasion to restore the number on an automobile engine block. The block was placed in a horizontal position, with the side on which the number is usually found being uppermost, and a „retaining wall" of suitable wax was placed around the area where the number should appear. The little „pond" was then filled with a 5% aqueous solution of ammonium persulphate. The next morning the entire serial number was legible. To bring the number out in a shorter time a 10% solution would be preferable.

Many cases, because of the nature of the metal, require special solutions as already indicated for the different kinds of steels and iron. Brass requires a very different solution. The author once had occasion to restore serial numbers which had been removed from an oxyacetylene welding outfit. This outfit had been used to remove the hinges from a vault door in a small town bank by melting them. Upon completion of the job the robbers left the welding outfit behind. Knowing that certain brass fittings should show serial numbers, which quite obviously had been removed, the investigating detectives brought these fittings to the laboratory. The job was not difficult. The following solution is an excellent one for brass and consequently was the one used: sodium sulfate (anhydrous), 1.5 gm.; chromic acid (CrO3), 20 gm.; water (distilled), 100 cc.

In some cases the criminal first removes the serial number, then with a sharp instrument and a hammer he pounds a large number of new marks into the metal and, since these are superposed on the original digit marks, the process of restoration is made more difficult and often impossible.

Nevertheless, despite the difficulties and the frequent failures even in the limited number of cases he may meet, the firearms examiner must be prepared to undertake this process whenever it offers help in the solution of a problem presented to him.

Fig. 101. Restoration of serial numbers. Diagram showing an ideal setup for the electrolytic process of etching to restore obliterated serial numbers.

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