Making the Lap
We are now ready for the polishing lap. First, cut a strip of newspaper at least 20" long, and 14" wider than the thickness of the tool; run it through some melted paraffin and set it aside. This is to serve as a collar, to be wrapped around the
tool to retain the hot pitch while it sets.
Now melt the pitch slowly on the stove, in a pot or can. It is highly inflammable, so keep the
flame low, and have on hand a board or other cover that can be immediately placed over the pot to smother any flames. Use a wide,
thin stick for stirring. If a can is being used, it may be gripped with a pair of pliers while pouring. When the pitch is fully melted, pour a large drop onto a piece of scrap glass and let it cool.
Try denting it with your thumbnail. If after about 15 seconds of pressure, there is hardly any indentation, the pitch is too hard; too soft if it has yielded easily. Some mirror makers use the bite test, although this is less determinate than the thumbnail test. Chip the cold drop of pitch free from the glass and place it between the teeth. If it immediately shatters into fragments under pressure, it may be too hard. There should be just the slightest yielding before breaking. The surest way to find out about the temper is to go ahead and make the lap, and see how it works in polishing.
To soften pitch, add just a few drops of turpentine. Do not do this over the stove. It takes very little turpentine for this, so use it cautiously. Stir thoroughly and again try the temper. To harden pitch, cook it for a long period. Adding a quantity of rosin will harden it. No two pitches are alike, so exact proportions cannot be given, but the amount of rosin added ought not to exceed the volume of pitch. Some laps are made entirely of rosin; these are most suitable for hot climates.
part 2 making the lap |
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