PAINT SURFACE REPLICA TECHNIQUES 



ping, this difficulty maj' be eliminated by 

 immersing the screen in dilute solution of an 

 adhesive before placing the screen on the 

 replica. 



The tape is carefully pulle'd away and the 

 plastic replica of the surface removed with it. 

 To remove film-coated screen from the 

 Scotch tape, a dissecting needle which has 

 been sharpened to a fine point is used to cut 

 the film around the edge of screen. Fine- 

 tipped tweezers are then used to remove the 

 screen from the tape. Sticking of the screen 

 to the tape may be eliminated by placing a 

 small circular disk of paper (}^'' diameter) 

 between the tape and screen. The replica is 

 then shadowcast by coating the surface with 

 a suitable metal, but more about the shadow- 

 casting process later. 



Negative Replica (Backed). An alter- 

 nate procedure for preparing a negative rep- 

 lica of a paint surface is to back the negative 

 replica with a heavy coating of plastic, as 

 shown schematically in Figure 2. This pro- 

 cedure is used in cases where stripping an 

 unsupported film is difficult, as in rough 

 paint surfaces, and the resultant replica 

 would be distorted or torn b}^ stresses ap- 

 plied in stripping. Plastic # 1 is applied to 

 the surface with a dropper and allowed to 

 drain vertically. When dry, plastic # 2, in a 

 heavy concentration which is not miscible 

 with plastic ^1, is applied similarly and 

 dried. Some typical plastic combinations 

 which have been applied successfully to the 

 study of selected paint surfaces are given 

 below. 



(A) 1-2% solution of 'Tormvar" in ethyl- 

 ene dichloride followed with 2-3% solution 

 of "Parlodion" in amyl acetate. 



(B) 1-2% solution of "Parlodion" in amyl 

 acetate followed with 4-5% of polyvinyl 

 alcohol in water. 



The double ffim is then stripped and 

 brought in contact with an appropriate sol- 

 vent to remove the backing film. In the case 

 of (A) amyl acetate would be used to remove 

 the "Parlodion" while in (B) warm water 



^tc^- 



Fig. 1. Negative replica technique (unbacked). 



^LC^- 



Fig. 2. Negative replica technique (backed). 



Fig. 3. Intermediate shadowed negative rep- 

 lica. 



would be used to solubilize the poly\dnyl al- 

 cohol. In both cases, plastic # 1 should be 

 uppermost and once solution is complete the 

 replica is mounted on grids (200 mesh stain- 

 less steel wire disks) brought from below and 

 shadowcast with an appropriate metal. 



Intermediate Shadowed Negative 

 Replica. Another variation of the negative 

 replica techniciue requires the use of an in- 

 termediate as shown in Figure 3. In this 

 case, a thick plastic is applied to the paint 

 surface, dry stripped with Scotch tape, in- 

 verted and shadowed obliquely with a suit- 

 able metal and then coated at normal inci- 



o __ o 



dence with 200 A of silica or 100 A of carbon 

 in the vacuum evaporator. The plastic rep- 

 lica is removed by washing in a suitable 



201 



