Determination of Hairs by Impressions

Abstract

Obscurity due to the transparency of the scalelike structure of hair has been for many years a considerable handicap for their recognized value in identification. Several methods were devised to overcome this handicap, but they proved too crude and involved to be practicable. However, it was later found that impressions of hair rather than hair itself would reveal the scale structure and thus eliminate visual interference from the pigments and cells below. Experiments with various media followed, most of them ending in negative impressions. It was at this point that a joint effort on the part of J. I. Hardy and Thora M. Plitt (U. S. Dept. Int., Fish and Wildlife Service, Wildlife Circular no. 7, 1940) devised an improved method for revealing scale structure with positive impressions. Their imprints were made on a thermoplastic film that softens at a temperature slightly below 100?C. Their technique provides clear, accurate impressions quickly. The method described in this paper does not improve on the results obtained by Hardy and Plitt, but presents another plastic medium (Gelva) and the technique for its application. Gelva has a decided advantage over other media in that the results are equally as good, yet require much less time and effort. Gelva is a cold set type of plastic that reaches the proper consistency for hair impressions in about thirty minutes and remains soft enough for impressions over a period of ten to twelve hours. It is easily applied and does not require heating. Positive or negative impressions can be obtained with Gelva and since there is so little to be gained by positive impressions for use in food habits work, the method and key that follow concern negative impressions. Gelva is the simplest resin derived from the polymerization of vinyl acetate and is a thermoplastic, colorless, granular powder. It is manufactured by the Shawinigan Chemicals Limited in Canada and by the Carbide and Carbon Chemicals Corporation in the United States. (Known as Polyvinyl Acetate in the U.S.) It is supplied in a number of different grades. The viscosity in solution is the distinguishing point between the various grades. Gelva is readily soluble in organic solvents and sets by the evaporation of the solvent. When spread over an area the surface becomes dry to the touch very rapidly; this has a retarding effect on the evaporation of a small percentage of the solvent underneath, promoting a slow rate of drying. This characteristic is the chief advantage of Gelva as a medium for hair impressions. A large quantity of Gelva solution can be prepared at one time and will keep indefinitely if stored in sealers to prevent the evaporation of the solvent. The proper solution is readily prepared by adding equal parts by measure of Gelva (V.25) and ethyl alcohol (95 per cent) together in a double boiler and heating over a low flame until all the crystals have dissolved. The outstanding features of Gelva for the purpose of making hair impressions are: Its slow rate of drying, clearness of impressions, ease and speed in handling, and the ability to store a quantity of prepared solution for an indefinite period 80 Vol. 32, No. I