PRESERVATION OF SPECIMENS IN PLASTICS 27 



■ uncovered areas are left. For spreading the resin an 8-inch glass rod 

 bent at an oblique angle 3 inches from the end will be found convenient 

 (fig. 8, B). 



Applying upper mounting sheet. — The upper mounting sheet with 

 overlay film attached with Span-Tween lubricant should now be applied 

 and kept as nearly in register with the lower sheet as practicable. 



Reheating. — The cover tray should be placed once more over the 

 mount and left for 2 or 3 minutes to allow the cementing resin to become 

 workably fluid. 



Working out excess resin and air bubbles. — After removing the tray, 

 the mount should first be worked over to eliminate the larger air bubbles. 

 This can best be done with the fingers, which should be protected from 

 the heat by gloves or pads. This preliminary working over should be 

 followed by a careful "ironing" with an electrically heated squeegee — 

 a thermostatically-controlled photoprint sealer is suitable — until all 

 remaining bubbles are worked out and the excess resin has been extruded 

 into the marginal area formed by the over-lay sheets. When using the 

 heated squeegee, a transparent worksheet capable of withstanding the 

 heat satisfactorily should be interposed between the squeegee and the 

 mount to protect the upper surface of the mount. A 10- by 16-inch 

 sheet of 0.015-inch rigid type cellulose acetate film will be found suitable 

 for this purpose. 



In working out the bubbles with the heated squeegee, the cementing 

 resin should first be pushed in around the specimen and the excess forced 

 out into the marginal area so as to carry the bubbles with it. Finally, 

 starting on the right-hand side of the mount, "iron" out as much of the 

 remaining resin as can readily be removed. Continue to do so all around 

 the mount until its upper surface becomes smooth and level everywhere 

 except over the specimen. While this is being done, the mounting 

 sheets and the overlay sheets should be kept in approximate register by 

 holding them in position with the left hand. 



Cooling and flattening. — After the ironing operation has been com- 

 pleted, the mount is removed from the mounting table and placed 

 on a flat surface where it is allowed to remain a few minutes until it has 

 cooled. During the cooling period it should be held down until it 

 becomes rigid again in order to prevent it from curling. This can be 

 done by weighting it down with a sand bag large enough to cover it, or 

 by placing upon it a stack of newspapers half an inch or more thick and 

 applying enough pressure with the hands or a weight to keep the mount 

 flat while cooling. 



Finishing the Mount 



Trimming. — After the finished mount has cooled, it should be squared 

 on a paper trimmer. While this is being done the overlapping edges of 

 the overlay sheet should also be cut away together with the excess 

 mounting resin that has been extruded into them. These may be dis- 

 carded or the resin may be reclaimed by heating the trimmed pieces in 

 a suitable container at 120° C. (approximately 250° F.). 



Removing overlay film and cleaning the mount. — After the mount 

 has been trimmed, the overlay film should be stripped off and the mount 

 rinsed in cool water until all of the lubricant has been washed away. 

 In case the cementing resin may have run over the back or the front of 



