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Removing an old lap from the tool
Put it in the freezer overnight.
When you take it out, it will start to make cracking sounds as the glass
and pitch warm and expand at different rates. At this point, the pitch
will almost fall off.
Mats for pressing patterns into pitch laps
Plastic laundry basket material! It comes in various sizes and designs and can be purchased
for a couple of dollars (pounds, marks, yen, whatever...). Both a square and herringbone design have been
used successfully. It is reusable and pitch does not readily adhere to
it, at least the first few times it is used. It generally has a taper,
so be sure to place the narrow side to the pitch. In order to ensure
the mirror is in full contact at the edge, the facets should have the
same appearance all over. If not in full contact, you'll notice that
the polishing agent has a different appearance (milkiness if you use
Cerium Oxide) over the lower facets. Be sure that the edge facets are in
contact with the mirror before polishing begins and cold or hot press as
needed. If for some reason the mat sticks while pulling free, run warm
water over it as you pull and it will come free without chipping the
pitch excessively. For faster mirrors, because of the deep curve, if
you imprint from a flat, freshly poured lap, you should make sure to
mold the curve first without the mat or it won't fully emboss the center
on the first attempt.
Tropical Lap
This was developed in the 1960's in Australia by Dr. Tenukest. As the
name implies, it was intended to be used in
extreme conditions of temeperature and/or humidity.
- 5 parts rosin
- 2 parts woodflower (very fine softwood sawdust)
- 1 part castor oil
(all quantities by volume)
- Melt the rosin by slowly heating it.
- Add castor oil and mix well.
- Add woodflour slowly, constantly mixing until the mixture is evenly
saturated.
- Grab a small bead, cool it and test for hardness. If too hard, add
castor oil, if too soft, add more rosin.
- Allow it to cool a little, and pour it on the tool. While hot
it will flow significantly better than normal pitch, so be sure to make
a dam around the edge of the tool.
Work with the lap in the same way as a normal pitch lap. It will take much
less heat to soften it - submerging the tool in hot water for only few
minutes will soften it enough to press channels. Also, cutting is a
bit easier than with normal pitch.
The thermal stability is excellent but since it is harder, it will
produces surfaces somewhat inferior to softer pitch - a tad more
ripples.
Portions contributed by:
Bratislav Curcic
Melbourne, Australia
More tips wanted!
Submit your tips and tricks for working with pitch laps here.
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