On pitch

Brian Manning

I was most interested in Bob Neville’s assessment of pitch, as it confirms what I have been thinking for more than ten years now – and I had noticed even before then that laps would not always stay in really good contact after warm pressing. In 1989 I made a triplet lens and also did a little touching-up of a 3-inch Cassella objective, and had quite a bit of difficulty. I began to think that some pitch could be slightly elastic, and wondered if the problem might have arisen due to my practise of adding beeswax to make it easy to cut channels. Around 1990 I purchased some Gugolz grade 55 pitch from HV Skan – and at £15 for about 2 kg it was quite expensive. I have used it only twice – the first time on a 7-inch mirror, and just recently on a 13.25-inch mirror. It is terrible stuff to make into a lap and quite impossible to cut channels with a knife, and the only way I found was to press them in. I resisted the temptation to add beeswax, and when made, getting the lap into contact was very easy. With the 13.25-inch mirror I worked face up with a 9.5-inch lap, which took about three attempts. When attempting to press channels with a steel bar about 1/8 inch thick and 1 inch deep, it suddenly occurred to me that if I made small facets less than 1 inch wide, then the bulges that spread from each channel would meet, and I would not produce the hollow facets that occur with wide ones. This worked a treat, and produced a very nice lap. There are some problems, including the trimming off of the overhanging pitch, and if it was not too bad I left it. I polished for twelve hours, and re-pressed only once. The facets could be trimmed with Texereau’s chisel method, using Fairy Liquid and by rocking the chisel. After two hours the figure was quite close to a sphere, but not so close at the end, because I was trying to get the edge to polish up. As I was short of time (retired – ‘but surely not’, you will say) I opted for leaving a trace of pitting in the outer zone, and there is slight TDE (turned-down edge ). Has anybody ever produced a mirror that will show a complete diffraction ring under the knife edge? The only time I have ever produced surfaces with virtually no TDE was when I made some small flats and pressed the larger lap on a flat with rouge, and then washed the rouge off before use – the idea being that perhaps as the rouge flows under the edge of the glass it takes a moment to be pressed into the pitch.