24 MISC. PUBLICATION 679, U. S. DEPT. OF AGRICULTURE 



mounting sheet entirely free from paper fibers and dust particles, be- 

 cause the thermoplastic cementing resin used for attaching the upper 

 mounting sheet ordinarily renders them invisible. 



Since it is important that both the specimen and the preserving fluid 

 covering it be completely dry before application of the upper Vinylite 

 mounting sheet, the layout should be put into the third-stage drying 

 box again after the cleaning operation and left there for at least 10 hours. 

 This final drying may also be done, if preferred, by heating the layout 

 in an oven at about 55° C. (131° F.) for 4 or 5 hours, or longer 

 if necessary. 



An alternative mode of preparation is permissible where no air bubbles 

 have formed around the specimen during dehydration. In such layouts 

 the cellulose acetate cover film may be left in place over the specimen 

 permanently and the upper Vinylite mounting sheet applied on top of it. 

 When this procedure is followed, however, it is necessary to remove 

 marginal strips of the cover film so that an area of the lower Vinylite 

 mounting sheet, at least 1 inch in width, is exposed all around the 

 specimen. This insures the sealing of the upper Vinylite sheet to the 

 specimen when the thermoplastic cementing resin is applied during the 

 mounting operation. This may be done by scoring the cover film lightly 

 with a sharp scalpel or razor blade and then peeling the strips off, using 

 a straight-edge as a guide. After the marginal strips have been re- 

 moved the exposed area of the Vinylite sheet should be cleaned with 

 moistened wads of soft paper and then wiped dry as explained above. 

 If this temporary application of moisture causes the edges of the remain- 

 ing portion of the cover film to curl, they should be held down with glass 

 strips until they have dried sufficiently to remain in place again. After 

 this cleaning, final dehydration in a drying box containing fresh desic- 

 cant, or in a moderate-temperature oven, is necessary to make sure that 

 the moisture has all been eliminated again. 



Storage of dehydrated specimen layouts. — When the mounting 

 operations are not to be carried out at once, specimen layouts may be 

 kept indefinitely after dehydration, provided they are stored in air- 

 tight cases or cabinets that are kept supplied with effective desiccants 

 such as anhydrous calcium chloride. Layouts stored in this manner 

 may be removed from time to time for temporary study or inspection, 

 and under relative humidities of 40 percent or lower may be left exposed 

 to room conditions for several hours without appreciable absorption of 

 moisture and consequent deterioration. At higher relative humidities 

 it is advisable to return them more quickly to their desiccant-containing 

 storage cases. 



MOUNTING BETWEEN PLASTIC SHEETS 



After the final dehydration, the specimen layout is ready for mount- 

 ing. The mounting should preferably be done in air having a relative 

 humidity below 50 percent, and care must be exercised throughout to 

 keep moisture out of the mount. 



Labeling. — The specimen can be labeled, if desired, by writing with 

 water-proof drawing ink on the lower Vinylite mounting sheet, or plac- 

 ing a typed or hand-lettered paper label between the mounting sheets. 



Applying overlay film with lubricant. — The top mounting sheet 

 (Vinylite) is cut to the same size as the lower mounting sheet, and two 



