Assessing pitch for mirror making

Bob Neville

I recently received a telephone call from John Vincent, who, with a friend, was attempting to finally figure and polish his first mirror. Their main concern was that after 16 hours the surface, although showing no pits, was ‘rough’ and had a turned edge. Furthermore, they seemed to be unable to control the figure by changes to the polishing stroke. The observation that the pitch squares had needed trimming only once suggested that the material was too hard; but, conversely, conventional chewiness, snapability and thumb-nail penetration tests indicated a soft material. Apparently, samples of this ‘pitch’ had been shown to BAA members at the previous Winchester Weekend, without any firm conclusion being reached. I found this apparently conflicting information intriguing, and offered to test a sample using my Twyman viscometer (described in ‘An upgraded telescope’). A couple of ¼-inch thick squares duly arrived, and I set about subjecting them to the drop test. I also remeasured some of my own pitch which had been used for my last two mirrors. The test involves measuring, over five minutes, the penetration into the pitch surface of the blunt, cone-shaped end of a rod, pushed down by a 1-kg mass. The two samples were clearly considerably different. Firstly, the received sample was slightly flexible – almost rubbery – and not at all brittle, and had a dull surface. By comparison, mine was quite hard, brittle and shiny. However, at 21° C I found that the final penetration distances were almost identical: 0.26 inches for mine compared with 0.27 inches for the received sample. But there was a difference: the tester dropped steadily through my sample, whereas in the other sample it fell very quickly at first and then came almost to a halt (first graph). Similar behaviour was exhibited at 15° C, but my sample hardened up much more, relative to the rubbery material, with drops of 0.035 and 0.15 inches respectively (second graph).

Pitch suitable for mirror making needs to be quite hard in response to short-timescale forces, but should be able to flow steadily under long-period stress. A good pitch will be brittle and shiny, but flow and stick to things if left for long periods (weeks). This allows a polishing lap to be pressed into intimate contact with a slightly warmed mirror over a period of perhaps half an hour, but be strong enough to resist deformation caused by sudden pressures during polishing. The received sample would do neither: it sprang back after pressing, and had too soft and weak a surface to hold the polishing compound in good contact with the moving mirror surface. I had never seen a substance like this before, having obtained some Swedish wood pitch many years before; and the only other lap I had seen was made from a similar, shiny and brittle, but harder pitch.

These results were reported back to John, and subsequently I received a sample of a different pitch that he had obtained. I had said that my own supply was running low, and this sample was thoughtfully sent for me to test with a view to obtaining more suitable pitch for myself. This turned out to be very similar to my own successful mixture, and is labelled ‘BH’ in the first graph. This pitch – perhaps after a short period of being held somewhat above its melting point to expel some of the more volatile components – should make a well-behaved polishing lap, perfectly suitable for polishing and figuring a first mirror.