Further Polishing
One may well be a little slow in comprehending all this, but no matter; get the mirror back on the lap and continue the polishing. You can dwell on the whys and wherefores of shapes and shadows while walking around the barrel.
The immediate concern is to polish out the mirror completely, which may take from two to six hours, depending on the thoroughness of the fine grinding, and on the
polishing agent used. The polishing mixture should be thinned so that on looking through the overhanging part of the mirror at the end of a stroke, only a faint coloration is visible. Renew the charge only as often as is necessary to keep a supply of liquid in the channels. With the molded lap, a single watery application will last almost indefinitely.
After working for a half-hour, give the lap a rest. Center the mirror on it, lay the board on top, and place a weight of about 20 pounds on top of that; then allow to press for about 10 minutes. This operation is known as cold-pressing, and should be indulged in frequently. If polishing spells are too long, the lap may soften and cause trouble. On account of the friction drag, the soft lap tends to follow the mirror as it passes back and forth, causing an excessive deepening of the renter. This results in an apparent crater or hole there, or possibly a hyperboloidal figure.
A turned-down edge may also result from plowing into the soft pitch. Following numerous periods of polishing and pressing, the pitch may sink a perceptible amount, flowing into the channels and spreading out so that the lap has a larger diameter than the mirror. Therefore an occasional trimming is in order. The channeled lap may be kept as much as 1/16" less in diameter than the mirror as a safeguard against turning the edge. Don't trim it too small or a turned-up edge may be the result. The harder pitch of the molded lap should stand up long enough to see the mirror through to its completion.
When replacing the mirror on the lap after it has been on the testing stand, you may observe that the polishing drag is not as even as before. This is because the central and more heated area of the lap has sunk more than has the area around the edge, and the mirror will then be riding on a high outer ring of pitch. If polishing were resumed immediately, a turned-down edge and a central hill would result from the mirror being plowed into this ring of pitch. So it is essential, after the mirror has been off the lap for more than a few seconds, that a sufficient amount of time to restore contact (15 minutes or more) be allowed for cold-pressing.
Next- examining for pits
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