Pitch laps.
The secret of making an excellent mirror is making a good lap. this method of making a lap was published in sky and telescope over 25 years ago.
1. Melting the pitch. This is best done on an old electric hob. The fumes produced are very strong smelling it will impregnate clothing and linger for days , I would recommend melting the pitch out side, also avoid breathing in the fumes .
2. Cover the surface the mirror with a small amount of polishing powder this will prevent the pitch sticking to it. Warm up the tool in hot water and dry. Place a ring of masking tape around the warm tool and pour the melted pitch on to it. Try to form a uniform layer over the whole surface.
3. After purring the pitch on to the tool, quickly place the mat on top of the tool and press using the mirror. Carefully remove the mat from the tool when the pitch has cooled, try not to damage any of the pitch squares left behind.
4 To get good contact between the tool and the mirror cold pressing is essential placing a heavy weight on top helps the process. clean up the squares with a sharp knife as required and then repress.
5. When good contact has been made over the hole of the tool with the mirror the lap is ready for use. when completed the finished lap should look like this. the laps in the picture have been used for polishing and are covered in optical rouge.
Another way to make a pitch lap is to cast the squares in a wooden mold, and stick them on to the mirror.
1. First draw one inch squares on to the surface of the tool. make sure that the square in the middle of the mirror is off set. this is so that it doesn't cause a central zone during polishing.
2 . Cast strips of pitch 1 inch wide in a strip. remove the pitch and break into one inch squares.
3. melt one side of the pitch squares with a candle and stick onto the tool.
4. when the tool is covered in squares, cold press to ensure good contact.
Polishing
Add a small amount of water and polishing powder to the mirror, place the mirror on top of the tool and start to polish slowly and smoothly using a w stroke of 1/3 the amplitude of the mirror diameter, listen for any unusual noises if heard stop straight away and clean off the mirror. It is possible a small piece of grit has been trapped that may scratch the mirror
Completely polishing a 6 inch mirror by hand can take more than 8 hours! I have found that breaking the polishing time into 2 hour sessions works well.
It takes about 1 hour of polishing for the tool and pitch to warm up properly, if polishing is going well there is a firm but smooth resistance to the mirror as it is pushed across the lap.
The mirror will start to polish out from the centre
