ELECTRON iMICROSCOPY 



microscope iu the study of crystallized 

 media," Bvll. Soc. Franc. Mineral, et Crist. 

 77, 833-839 (1954). 



68. DwoRNiK, E. AND Ross, M., "Application of 



electron microscope to niineralogic studies," 

 Am. Mineral. 40, 261-274 (1955). 



69. Fahn, R., "Applications of the electron micro- 



scope for the investigation of rocks and 

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Thomas F. Bates 



PAINT SURFACE REPLICA TECHNIQUES 



Since it is impractical to view paint sur- 

 faces directly because of difficulties in elec- 

 tron transmission it was necessary to develop 

 indirect methods. These procedures classi- 

 fied as replicathig methods are based upon a 

 reproduction of the surface. The reproduc- 

 tion may be in the form of a negative or 

 positive replica. In the negative replica the 

 heights and hollows are reversed with re- 

 spect to the original surface. The positive 

 replica reciuires an intermediate replica from 

 which the final replica is made and the 

 heights and hollows are in the same relative 

 order as on the original surface. As a rule 

 both methods employ metal shadow casting 

 to enhance the contrast of the replica and to 

 emphasize the dimension normal to the sur- 

 face. 



Both negative and positive replication 

 techniques may be employed in the study of 

 paint surfaces. However, the replica tech- 

 nique employed is governed strictly by the 

 nature of the surface; that is, the solvent 

 system used in replication should not attack 

 the paint surface and introduce artifacts. A 

 detailed description of various replica tech- 

 niques which may be used in the examina- 

 tion of paint surfaces will be presented as well 

 as examples illustrating paint films in var- 

 ious stages of degradation. Xo attempt will 

 be made to discuss the mechanism of paint 

 film formation since this paper will be con- 

 fined strictly to techniques. 



Replicating Systems 



The organic nature of most paint sur- 

 faces presents a problem in replication since 

 the solvents used may attack the organic 

 phase and cause some uncertainty in the rep- 

 lication process. However, this problem may 

 be circumvented by using water-soluble in- 

 termediate plastic replicas or by carefully 

 selecting replicating media the solvent sys- 

 tems of which are known not to be miscible 

 with the paint surface under study. 



Perhaps the simplest method of replica- 

 ting paint films is by making unbacked 

 negative replicas; this method will be dis- 

 cussed first. 



Negative Replica (Unbacked). In the 

 unbacked negative replication technique the 

 surface is reproduced in such a manner that 

 the replica represents a surface in which the 

 heights and depressions are reversed with re- 

 spect to the original surface. This method 

 employs only one replicating medium. A 

 schematic representation of the steps used 

 in the preparation of an unbacked negative 

 replica of a paint surface is given in Figure 1 . 



A plastic solution from a dropper is flowed 

 over a small portion of the paint surface and 

 the excess solution drained off by drying the 

 film at room temperature on the paint panel 

 in a vertical position. Some typical plastic 

 solutions are given below: 



(1) 1-2% parlodion or collodion in amyl 

 acetate. 



(2) Freshly prepared 1 % solution of Form- 

 var in ethylene dichloride or dioxane. 



(3) 1-2 % ethyl cellulose in ethylene di- 

 chloride. 



One should remember that the chemical 

 nature of the sample should be such that it 

 is not attacked by the replicating solution. 

 To remove the replica from the paint sur- 

 face, moisture from the breath is condensed 

 onto the surface and then removed with 

 Scotch tape; first, however, several specimen 

 screens about %" apart are placed on the 

 replica. If there is some tendency for the 

 replica to wrinkle on the screen after strip- 



200 



