How to Etch Fingerprints on Metal

(This article is Chapter XXV from Chapel's book "Fingerprint: A Manual of Identification," 1941, pp 287 - 289. Thanks to Jim Edmonston, Riverside CAL-ID.)

CHARLES EDWARD CHAPEL

The identification tags worn during the World War by officers and enlisted men of the Navy and Marine 
Corps saved many of the bodies of the dead from being consigned to unmarked graves and also resulted in the restoration to their families of those who lost their memories from shell shock. These tags were plates of monel metal, perforated at one end and suspended from the neck by a monel wire incased in a cotton sleeve. On one side was the etched fingerprint of the right index finger; on the other side were etched the individual's initials and surname, the month, day and year of his enlistment or commission, expressed in numerals (e.g., 1- 5-1916), and the month, day and year of birth, similarly expressed. This side also bore letters appropriate to his branch of the armed services, such as U.S.N., or U.S.M.C. The process for etching fingerprints on metal was developed by Mr. J.H. Taylor chief of the identification section, Bureau of Navigation, Navy Department, Washington, D.C., who obtained the adoption of these identification tags by the Navy. Since 
the same process can be used by anyone for marking metal objects with indisputable signs of his ownership, the etching method is presented here in detail.

 The materials needed are: an outfit for taking fingerprints on paper, as described in the chapter about "How to Take Good Fingerprints"; a supply of printer's ink thinned to a consistency suitable for its easy use with an ordinary steel pen (diluted with gasoline, turpentine, or other "thinner"); gilsonite, or powdered asphaltum; nitric acid, diluted with 1 part nitric acid to 2 parts water volume, and kept in a glass dish; an alcohol lamp, electric stove, gas jet, or some other heating device; and some means of holding the metal without touching the surfaces to be etched, such as a wooden clamp. 

The actual process is as follows: 

Step 1. Take an ordinary, rolled fingerprint on paper as a test to see if the finger is clean, the ink of the proper consistency, etc. (Use the regular, undiluted printer's ink for this.) 

Step 2. Record all the facts that are to be etched on the metal on a piece of paper for easy and quick reference during the etching process. 

Step 3. Make an ordinary rolled fingerprint on the metal after inking the finger with the regular, undiluted printer's ink. (The instructions for taking fingerprints given in the chapter entitled "How to Take Good Fingerprints" apply with full force to this step of the etching process.) 

Step 4. Use the diluted printer's ink and a clean, blunt steel pen to print or write the desired facts on the metal. Either turpentine or gasoline may be used as a thinner, but turpentine is  preferred because it does not dry so quickly. The ink must be barely  thin enough to write with; if it spreads on the metal it is too thin;  if it does not flow from the pen easily it is too thick.  

Step 5. While the ink is still fresh on the fingerprint  and writing, sprinkle the surface with finely powdered asphaltum or  gilsonite. This must be fine enough to pass through a sieve having  100 meshes to the square inch. Sprinkle it very thickly over the  inked areas but do not neglect any of the surface. Some of the powder  will mix with and adhere to the ink; the excess must be removed by  tapping the edge of the metal and blowing on the powder. 

Step 6. The metal is now held with forceps over a flame  or stove until the ink and asphaltum (or gilsonite) have melted together,  forming sharp, glossy black lines. If not enough heat is applied  to melt completely the asphaltum the action of the acid will be too  powerful. The complete melting is indicated when the lines are glossy. If  too much heat is applied the lines run together and are obliterated. The  degree of heat which is best is slightly above the boiling point of  water. 

Step 7. When the metal has cooled, it is placed in  a solution of 1 part nitric acid to 2 parts water by volume, and left  there for about one hour, being taken out from time to time to observe  the progress of the etching by the acid. If the acid action is too  lively, water may be added; if the action is too slow, nitric acid  may be added; to speed up the action concentrated muriatic acid in  the proportion of 1 part muriatic acid to 30 parts nitric acid may  be added, but this is seldom necessary. In the case of identification  tags, where both sides of a monel metal disc are being etched, the  name side is left up since this results in a deeper etching which  will not wear off quickly. The acid solution is kept in glass, china,  or enameled containers because they are impervious to the action of  the acid.  

Step 8. When the acid has eaten the fingerprint and  lettering deeply enough into the metal, usually in about one hour,  the metal is removed, rinsed thoroughly in water, and dried.  Metal spoiled in the inking process can be cleaned with  gasoline, re-inked, and then etched, but if any ink is left from the  previous attempt it may result in an indistinct etching. If a metal  other than monel metal is etched the time required for the action  of the acid may be more or less than that required for monel metal. If  an object of value is to be etched and it is made of some other metal,  a trial etching should be made first on a less expensive object made  of the same metal.  

(Editor -- In submitting this article, Jim Edmonston commented  that at a previous SCAFO meeting, Bill Leo circulated a GI dog tag  from WWI. Jim commented that the detail was "unbelievable, the pores  along the ridges jumped out, it was that clear." This article is  reprinted for historical purposes and is presented with a warning  against attempting to replicate this process. A thorough understanding  of the chemistry and hazards involved must precede any such process.)