Journal of Criminal Law and Criminology Volume 51 Article 10 Issue 4 November-December Winter 1960 The seU of Hydrogen Fluoride in the Development of Latent Fingerprints Found on Glass Surfaces Herbert L. MacDonell Follow this and additional works at: https://scholarlycommons.law.northwestern.edu/jclc Part of the Criminal Law Commons, Criminology Commons, and the Criminology and Criminal Justice Commons Recommended Citation Herbert L. MacDonell, The sU e of Hydrogen Fluoride in the Development of Latent Fingerprints Found on Glass Surfaces, 51 J. Crim. L. Criminology & Police Sci. 465 (1960-1961) This Criminology is brought to you for free and open access by Northwestern University School of Law Scholarly Commons. It has been accepted for inclusion in Journal of Criminal Law and Criminology by an authorized editor of Northwestern University School of Law Scholarly Commons. POLICE SCIENCE THE USE OF HYDROGEN FLUORIDE IN THE DEVELOPMENT OF LATENT FINGERPRINTS FOUND ON GLASS SURFACES HERBERT L. MAcDONELL The author is a technical consultant in the field of scientific investigation and is a senior chemist with the Coming Glass Works, Corning, New York. Prior to his present association Mr. MacDonell has served as a professor of chemistry at Milton College, Wisconsin and has taught both chemistry and criminalistics at the University of Rhode Island. Recently he organized and participated in a seminar on scientific crime detection at Alfred University.-EDITOR. The use of hydrofluoric acid for the development normal composition (9), it has been found that the of latent fingerprints on glass surfaces has been use of hydrofluoric acid vapor will yield detail su- suggested by several investigators (1). In each perior to that which is obtainable by other develop- instance, however, the use of hydrofluoric acid, an ment procedures. In addition, the development aqueous solution of the gas hydrogen fluoride, is process renders the fingerprint absolutely perma- suggested rather than the use of hydrogen fluoride nent by all reasonable standards of physical and itself. This investigation was conducted using hy- chemical durability. Fingerprints thus developed drofluoric acid vapor, a mixture df hydrogen flu- actually become a part of the glass surface by being oride and water vapor, rather than liquid hydro- etched into the glass and will withstand any degree fluoric acid. The choice of the acid vapor is based of weathering that could have been tolerated by upon the well established fact that the destruction the original piece of glass before treatment A piece of fingerprint detail is greatly increased with an of glass processed in this manner may be filed with increase in the density of the developing medium. confidence as it will never become smudged and For example, a gas or vapor technique such as io- will definitely not deteriorate with age. dine or osmium tetroxide fuming should be one of DEvI)Fw NpmEMEHANIsms the first methods considered for the development of latent fingerprints (1) (2) (3) (4) (5). If these The reactions between hydrofluoric acid and sili- methods can not effectively be employed in a given cates are well understood (10). Etching or dissolv- situation, then liquid or solution development pro- ing glass in hydrofluoric acid may be expressed by cedures should be considered (1) (2) (4) (6). Fi- the chemical equation: nally, the use of solid powders when dusting, roll- 6HF + Na2SiOs- 1 SiF4 + 2NaF + 3H20 ing, or spraying should be considered as a last resort (1) (2) (4) (7). Even though they receive the wherein sodium silicate represents a Na2SiO -SiO2 last consideration, solid methods will still find the glass network in the reaction. According to Fam- most frequent application as vapor and liquid comb (11) the additional reaction: techniques tend to be highly selective for paper, 3SiF4 + 3H2O- H2SiO + T 2H2SiFs wood, and other rough or porous surfaces. The use of liquids which may be cured to semi-solid elasto- also takes place as the decomposition occurs in the mers has recently been introduced for lifting finger- water solution. The presence of CaO, MgO, K0, prints (8). It might be well to consider the use of BaO, B20 3, A1203 and other constituents found in these new materials before resorting to solid pro- glass compositions (9) is not taken into considera- cedures; however, once such a method has been at- tion in the above reaction as the destruction of the tempted it is doubtful that subsequent procedures silica network is fundamental to the solution of will yield usable results. these other metal oxides. Furthermore, while the The medium upon which a latent fingerprint is investigation did establish the adaptability of the deposited will, of course, be a determining factor method to many glasses of unusual or uncommon in the selection of a development method. If such a composition, it is unlikely that glass objects other fingerprint were left on a piece of window glass of a than windows, mirrors, bottles, or drinking glass- HERBERT L. MacDONELL [Vol. 51 ware will often be submitted as evidence bearing development would yield superior results to the latent fingerprints. liquid analog. In addition, it was believed that the There is some confusion as to the physical mech- basic character of the reaction would remain un- anisms involved in the selective etching of glass changed so long as the same reagent was used. surfaces by hydrofluoric acid vapor on glass upon While the first assumption was correct, the second which latent fingerprints have been deposited. At was entirely incorrect producing exactly the oppo- room temperature a piece of glass which has been site from the predicted result. coated with a film of grease or wax will not be APP.4RATUS "wetted" by water or aqueous solutions. This re- pulsion offered by the waxy substances in a latent Initial experimentation was conducted in a fingerprint to the etching action of hydrofluoric rather small polyethylene container having no pro- acid is the basis for the use of this acid in the con- vision for vapor circulation. Fingerprints developed ventional development of fingerprints on glass (1). in this apparatus had a somewhat spotted surface Theoretically, when a piece of glass is submerged with respect to etching. It is possible that the vapor in hydrofluoric acid etching will occur in every area was composed of several gradient layers of different not protected by a film of grease or wax such as density and composition. Overlapping of these would be deposited by the friction ridges of a finger. layers could have been the cause of poor reproduci- Therefore, if a piece of glass containing a latent bility. Some fingerprints developed using the vapor fingerprint were so treated the fingerprint would over hydrofluoric acid were excellent; however, protect specific areas and result in a liquid develop- others were either not etched sufficiently, were un- ment method. In theory it would seem that this evenly etched, or were so badly etched that detail method would be highly successful and therefore was destroyed beyond all usefulness. In every in- desirable. In practice one achieves little success in stance, without regard to the degree, etching al- obtaining reproducible results. In no instance has ways occurred within the area of the fingerprint the author ever felt that the results obtained fol- impression itself. This phenomenon was entirely lowing this procedure were superior to those which contrary to the predicted results described above. could have been obtained had the fingerprints been Apparently, hydrofluoric acid vapor is selectively developed by careful dusting. An explanation for absorbed by the fingerprint deposit to such a de- the shortcomings of the liquid hydrofluoric acid gree that considerable etching takes place in this method may be stated briefly in the following para- area while little or no action occurs on the rest of dox: If the concentration of the acid is too high the glass. Hydrofluoric acid vapor consists of both then the submersion time of the glass must be very hydrogen fluoride and water vapor. It is possible critically controlled. Dilution of the acid will allow that water vapor is initially absorbed by the finger- a longer submersion time; however, this increased print and that hydrogen fluoride is subsequently soaking time permits acid penetration of the grease absorbed into this water thereby forming hydro- or wax barrier to such a degree that considerable fluoric acid in the fingerprint pattern. The acid etching takes place even in the friction ridge areas. would thus be confined to the fingerprint impres- In view of this dilemma only a small effort was sion and would produce etching only in this area. made with liquid hydrofluoric acid etching of glass In an effort to achieve more reproducible results as a practical means of fingerprint development. a larger container was obtained, formed from poly- The possible use of hydrofluoric acid vapor for the ethylene as shown in figure 1. A small paddle was same purpose presented itself as a possible solution fashioned from sheet polyethylene and polyethy- to the problems described above for two reasons. lene tubing fitted over a glass rod for rigidity.' The First, the concentration of the reagent would be 'The large polyethylene container used in this considerably lower in the vapor phase and thus study was manufactured by Tupperware of Orlando, longer exposures could be tolerated without exces- Florida. The small polyethylene container was cut from the bottom of an empty hydrofluoric acid bottle. Like- sive etching and ultimate destruction to the entire wise, the polyethylene used to fabricate the fan was glass surface. This longer exposure time would cut from the side of an empty hydrofluoric acid bottle and worked into shape with a hot glass rod. The poly- allow considerably greater reproducibility. Second, ethylene tubing and hydrofluoric acid were purchased the vapor would not produce a soaking effect on from Arthur H.