Examining for Pits
Pits are apparent in the early stages of polishing by the grayish appearance of the surface, especially at the edges. This grayness is the diffused reflection from the myriads of tiny pits left by the emery.
As polishing progresses it disappears, until the surface has the appearance of being fully polished. But do not be deceived by this first impression. On the silvered or aluminized mirror, the all-but-invisible pits that may still be present would stand out in disastrous prominence. (Some expect the silver or aluminum coating to fill up tiny voids, such as pits and scratches, but as it is only a fraction of a wave length of light in thickness, the coating merely "lines" them and renders them more visible.)
With a reading glass, focus the image of the sun or any bright light on the mirror's surface. A small patch of wet paper stuck to the surface will guide the lens to the position of sharp focus. Where any pits are present, the "burning spot" will show up clearly. It can barely be seen on a fully polished surface. Compare its appearance at the center, which area will have the best polish, with that near the edge of the mirror.
Or hold the mirror so that the reflection of an overhead light is seen on its surface, near the edge. Draw it up close to the eye, keeping the reflection in view, and place a magnifier before the eye. Scan the surface of the mirror at the edge in the vicinity of the light reflection, and the pits can be seen, if there are any, looking like a. very fine powdering of dust particles.
If you have difficulty in seeing the pits this way, hold the mirror with a frosted lamp back of it and examine the surface with the aid of the magnifier. Any pits should be visible against the luminous background. Polishing should continue until they are no longer visible. If necessary, the stroke should be shortened to facilitate polishing of the edge.
Next- Correcting |
|
