PAINT SURFACE REPLICA TECHNIQUES 



layer (40A) is objectionable, either platinum 



o 



or palladium (4-8A) may be substituted. 

 The metal is deposited at an angle of from 

 tan~i 1:1 or 1:3 depending on the inherent 

 roughness of the surface. The lower angle is 

 preferable for fine detail (high gloss paint 

 surfaces) whereas the higher angle is used for 

 coarse structures (weathered paint surfaces). 



Positive Replica Technique 



A schematic representation of the steps 

 used in the preparation of the positive replica 

 is given in Figure 5. In this technique (3) 

 an intermediate thick replica is reciuired 

 from which a thinner second replica can be 

 stripped. Careful control of the solution con- 

 centration of the intermediate replicating 

 medium is highly critical since too low a 

 concentration results in stripping difficulties 

 while too high a concentration results in ex- 

 cessive shrinkage of the medium at the con- 

 tact surface of the sample resulting in the 

 development of wrinkles. 



For paint surfaces a 5 % aqueous solution 

 of methylcellulose (Methocel, Dow Chemi- 

 cal Co.) or poly\'inyl alcohol (DuPont) in 

 distilled water is employed as the inter- 

 mediate replicas. The solution is applied uni- 

 formly with the aid of a dropper maintaining 

 the paint surface in a horizontal position. 

 When dry, the film is detached slightly with 

 a sharp razor and then carefully pulled off 

 with tweezers. The stripped film is then at- 

 tached with Scotch tape to a microscope 

 slide and placed in the vacuum evaporator 

 for coating with carbon or silica at normal 

 incidence. 



i/e* oi«. cmeoN kods 



tt.r.:M 



C3 



Fig. 5. Positive replica technique. 



Fig. 6. Carbon evaporation apparatus. 



Carbon Evaporation Procedure 



The method of carbon evaporation consists 

 of passing 60-cycle alternating current of 

 approximate^ 20 to 50 amperes through y^" 

 diameter graphite rods (National Carbon, 

 pure spectrographic grade) in a vertical posi- 

 tion above the specimen with one rod ta- 

 pered to a fine point. The carbon rods are 

 held in close contact by a spring as showai in 

 Figure 6 so they do not separate during the 

 evaporation process. Intense local heating 

 occurs at the regions of contact. 



A satisfactory carbon thickness is main- 

 tained by visually observing the color (light 

 brown) developed on a piece of white porce- 

 lain containing a small drop of oil (Apiezon 

 B) (1, 2). The condensed carbon is clearly 

 visible on the porcelain but not on the oil 

 drop which shows up in sharp contrast. 



When the evaporation is completed the 

 carbon plastic -coated slide is removed; cut 

 into 0.3 cm squares with a sharp razor and 

 placed on the surface of a Petri dish contain- 

 ing distilled water until the methycellulose is 

 completely dissolved. From two to three 

 hours are required for complete solution of 

 the methylcellulose, with frequent replace- 

 ment of distilled water. Specimen screens are 

 placed on top of the floating carbon sections 

 and remo^'ed with forceps. When dry, the 

 carbon-coated screens are placed in the evap- 

 orator and shadowcast with a suitable metal. 



203 



