Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 272 of 331

testing THE VALUE OF OPTICAL MICROSCOPY IN FORENSICS OF PAINT FAILURES BY RON JOSEPH (1944-2011), PAINT & COATING CONSULTANT, EXPONENT, INC., MENLO PARK, CALIF. Many paint failures, especially in the architectural field, lead to litigation or in- surance claims. Consequently, a paint consultant is often retained, usually by a law firm on behalf of an insurance company, to determine the root cause(s) of the problem(s). Based on the results of the investigation, the strength of the ex- pert report, and other factors, the case will either settle or go to trial. This paper discusses one of the most fundamental tools in solving the paint problem—op- tical microscopy. Microscopes. A stereomicroscope (Figure 1) is one of the most useful instru- ments in a forensics laboratory for paint. Depending on the model, the micro- scope should have the capability of magnifying an image through a range, such as 7X to 64X. How it works: light shines from the top down onto the sample and there is usually sufficient space to allow the operator to tilt the sample and ob- serve it at a wide range of angles. Additionally, the angle of the light source can be changed possibly to see shallow defects (e.g., orange peel, dust, solvent pop- ping, cratering, etc.) when looking at a paint failure. A disadvantage of this instrument is the depth of field is shallow. This means while getting a sharp image at one location, the neighboring area(s) might be out of focus. Fortunately, for those who need excellent focus along the length of the sample, software packages are available that allow images of varying focal lengths to be blended together to form a perfectly focused image of the desired area. For higher magnifications, other types of microscopes might be necessary. A calibrated optical inverted stage microscope capable of typical mag- nifications from 50X to 1000X can be useful (Figure 2). Because the depth of field is more limited than with the stereomicroscope, it is common to mount the sample in an epoxy resin and then polish the mount to an extremely smooth finish using 600 grit or higher abrasive paper, diamond abrasive pastes, or jeweler's rouge. The sam- ple is placed upside down onto the microscope stage and incident light shines from the bottom up onto the sample surface. This dif- fers from the stereomicroscope where the light shines from the top down on to the sample. Many analytical laboratories Figure 1. Typical stereomicroscope with 7X to 64X magnification. 271

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