Preparation of Pigment Mounts for Microscopy R. P. ALLEN', The B. F. Goodriol
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OWDERED materials of various chemical compositions
and widely different particle size characteristics are used for many purposes in rubber compositions and in paints, enamels, lacquers, and plastics. I n the rubber industry these materials are called "pigments" in a generic sense, regardless of whether or not they are used for coloring purposes; in paint formulations their true pigment use is more important. I n any case these finely divided materials are evaluated frequently on the basis of their microscopic characteristics. It is essential. therefore. to have available a method of Dreparbu . . .. mounts of katcri:ils ;Anping in size and elivracter from clinnuel black tn aslic.ctos fibers, irnm zinc oxide to ground whiting. In the microscopic cvnluntim oi thrse n1ntrri:ik the preparntion of the mount is nnr of tlar niosi imp(rrt:int ntrpr. ~~~~~~
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Thc technique employed by Grrm conaistrd in rubbing out tlir pnrticlcs inti, :.II 6 xcrrtlingly t h i n lnyer with turpentinr, and mnimtiiig thrm in glycerol (,r s m c othrr suitable medium. I.nter r h i 3 mctli(d w m nir.difird by adopting a "multiplc wrilgt: tlisp:rsion" whicli provided n mount iii wliicli t h pigment layer mricd froin 0 tn 25 purticlvc in drpth. Concerning the mcthinl 1 9 f Crwn, Ilaslnm and Hall (-0) stated that w i t h tl grwt (ltial of prsctire nnd w r c n slidc c m Iw prt~pnrrd that will give fairly sntisfnrtc.ry rrsults. It 1v39 ioiind in the G~~orlricli lnlioratory that the technique developed by Crrrrii possessed two rather serious pitfnlkn crrt:lin : i m ~ u n of t segregntion invnrinhly prraistt 11und some grinding of the Inrgr pnrticlrs nitcn occurred. The larger partirles n'crc swept tr, tlir ?ides nhere thcy wuld he owrlookrd. Even at best the nirtliod iq it:Idnm capable of providing a mount in which any limited area is oompletely r e p resentative of the pigment. Haslam and Hall (2) also recog'nized this disadvantage and overcame the difficulty by gathering all the pigment dispersed in turpentine in the center of the slide. The pigment is first ruhhed nut in doubledistilled turpentine until the lumps have been disintegrated and then, when the turpentine is practically evaporated, the sam le is all pushed t o the center of the slide and the remainder of ,%e tu entine is driven off with heat. It is extremely important that the pigment be gathered together in the center of the slide, for in the rubbingout operation the coarse particles are carried to the edges of the slide and would he lost in the final mount. The pigment is then oovered with dammar which is melted under the cover glass. Haslam and Hall state that this mount is far simpler t o prepare successfully, better dispersion of the pigment is assured, a more permanent mount is secured and, owing t o the rigidity of the dammar, it lends itself to easier manipulation to the high-power objectives on the microscope.
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