A Note on Silicone Oil as a Mounting Medium for Fossil and Modern Pollen

Recently Andersen (1960) has critically evaluated the characteristics of a number of mounting media for fossil and modern pollen and has suggested silicone oil as the most favorable. As this represents an important contribution to pollen analytical technique, it seems appropriate to call the attention of North American workers to Andersen's paper and to mention a few facts regarding the use of silicone oil. As Andersen points out, the advantages of this medium over those previously employed are several: its refractive index is lower (1.403), it is available in a wide range of viscosities, it is nonvolatile, it is chemically pure and stable, and pollen grains mounted in it do not swell as in glycerine and glycerine-jelly (a fact that is of profound importance in size-frequency studies of fossil and modern material). Furthermore, the medium can be obtained readily. Andersen has suggested that the optimum viscosity for general use is AK 2000 on the scale employed by Wacker Chemie in Munich. Silicone oil is available in the United States from the Dow Corning Corporation, Midland, Michigan. The oils are marketed under the name of "Dow Corning 200 Fluids" and are available in viscosities ranging from 0.65-2,500,000 centistokes (cs.). AK 2000 is equivalent to 2000 cs. The standard viscosities sold by Dow Corning do not include 2000 cs, but this can be obtained by blending the 1000 and 12,500 cs oils. It is worth emphasizing that critical grains can be relocated even after considerable time, vertical storage of slides, and moving about of slide boxes, provided that, as Andersen notes, certain precautions are taken. The pollen should not be too dense in the preparation (an undesirable characteristic in any preparation regardless of the medium used) and the drop of material used should be just sufficient for the oil to reach the edges of the cover slip. I recorded the stage coordinates of a number of critical grains which were mounted in AK 2000 oil while working at the Paleobotanical laboratories of the Geological Survey of Denmark in 1958-59. Upon completion of counts the slides were not ringed, but a drop of nail polish was placed on each corner of the cover slip to prevent movement. Ten months later, after the slides had been shipped back to the United States, the grains could be relocated at exactly the same coordinates. Hence, properly handled, silicone oil has great potential for pre-Pleistocene as well as Pleistocene material. Movement of grains can be minimized further by employing oils of higher viscosity than Andersen has recommended. This is important in photomicrographic work in which one might wish to turn a grain to different positions and hold it while making several exposures. For example, Mrs. Clisby (personal communication) uses oil of 60,000 cs viscosity for modern material. The writer has tried this, as well as several less viscous oils, and has found silicone oil having a viscosity of 12,500 to be ideal. The best chemical procedures for use with silicone oil are carefully described by Andersen. However, Mrs. Clisby suggests that the benzene employed must be U.S.P., thiophene free, in order to prevent the formation of a fine precipitate in the medium. My experience corroborates this.