ELECTRON MICROSCOPY 



Silira Kvaporation Procedure 



In the process of silica evaporation (4, 5) 

 the intermediate repUca of the paint film is 

 placed about 7 cm below a conical tungsten 

 (O.o mm) wire basket containing about a 1 

 mgm chip of ciuartz. The evaporator is evac- 

 uated to about 10-* mm of Hg and a current 

 of 20 to 30 amps passed through the basket 

 for 20 to 30 seconds. If some difficulty is 

 experienced in evaporating the (quartz) sil- 

 ica, a colloidal suspension of colloidal silica 

 (Ludox) may be substituted and micro- 

 pipetted on a comparable tungsten filament, 

 dried and e^'aporated. Another alternative is 

 to use SiO which evaporates more readily 

 than (quartz and does not attack the tung- 

 sten. 



As in the carbon technique, the plastic- 

 silica combination is immersed in water to 

 solubilize the plastic. Once solution is com- 

 plete the silica replica is picked up on speci- 

 men screens, shadowcast and examined as 

 normally. Sometimes if the silica films are 



extremely thin, special lighting conditions 

 may have to be employed to enhance the 

 visibility. 



Photof;ra|>hic Procedure 



An RCA-type EMU electron microscope 

 equipped with self -biased electron gun and 

 binocular viewer was employed. A platinum 

 objective aperture with an opening of 50 

 microns was used to increase the contrast in 

 the final image. Negatives were taken at an 

 electronic magnification of 1500 X and en- 

 larged optically to 8000 X. Kodak Medium 

 Plates were used and developed in Kodak 

 Versatol Developer. Electron micrographs of 

 the paint surfaces prepared by the negative 

 replica technique were processed to the posi- 

 tive print stage, while micrographs prepared 

 by the positive replica technique were proc- 

 essed to the negative print stage. This was 

 done to make heights and depressions more 

 readily appreciated. 





(a) Cellular pattern (X 8000) (b) Pigment flocculation near air interface (X 8000) 



Fig. 7. Negative replicas of paint surfaces; a. Unbacked technique; b. Backed technique. 



204 



