When last I wrote on this blog, I had decided to store the mirror on top of the lap for 24 hours, the idea being to give an extended cold press to try and fix the 2 facets which weren’t making contact with the mirror surface. Several days passed before I managed to get back to polishing, and I found that, while the contact issue had not been resolved, the gaps between the facets had narrowed noticeable. I put in an additional hour’s worth of polishing and decided to repeat the experiment.
The following evening, there was still no improvement, apart from the gaps between the facets having shrunk even more. They were now only about 1mm deep, and maybe 0.5mm wide. On the advice of the ATM class, I prepared for a bit of lap maintenance, excavating a newer and deeper set of channels. I planned to do another hour’s polishing before making any change, but to my dismay I found that I hadn’t packed the lap/mirror combination level. As a result, the mirror had slid a few centimeters to the side before coming to rest against the side of the container, leaving a pronounced lip pressed into the pitch. Bugger.
So, to remedy this, I decided to try a warm press. Filling a basin with hot water (“hot” meaning “Slightly uncomfortable to keep my hands in there”), I soaked the lap and the mirror together. The heat softened the pitch to the point where I could easily dent it with my fingers. I pressed in some deep channels with a steel ruler, then laid out the mirror on a table and spread a nice thick layer of cerium oxide slurry. Then I put the soft warm lap face-down on the mirror, stacked a few books on top of that lot to build the pressure, and poured myself a nice hot cup of tea.
Extended tea-time over, I removed the books and had a look at the lap surface. Wow… completely smooth – I not only pressed out the ridge, but all signs of faceting. Also, a fair bit of pitch had mushroomed out the side and had to be trimmed away. I could try the process again, but I’d noticed that simply pressing channels in caused the edges of the facet to rise up. Pressing them down again would only tend to close the channels. What to do…
I decided to use one of the other classic methods: Carve the facets in with a sharp utility knife. I used the steel ruler again, just to score the surface and give myself some guidelines, then got to work digging. This turned out to be very slow work, but seemed to be going well until the surface of the pitch suddenly cracked and a huge chip the size of a 5c coin broke away. I was now right back at the beginning, with minimal channeling between facts, and a region of lap that was severely recessed away from the mirror surface. That was when I decided to put everything away, go to bed, and leave it for the experts to fix at the ATM class.
But by the next evening, I’d regained some confidence and had some time to think about the problem.
I decided to go for a warm press again. Same procedure as before, softening the pitch, smearing slurry against the mirror, steel ruler to press in more channels, stacking books… but this time I was careful to only leave it for a minute. The results were a lot better – the newly pressed in channels survived, while the nasty divot was still there but much shallower. I let everything cool down, gave it a cold press and got on with polishing. The lap was very grabby at first, just like when it was first made, but it took only a few minutes to settle. More cold pressing, more polishing, and things seem to be working as they should. The hole is still there, but steadily getting shallower and hopefully will vanish completely as the pitch continues conforming to the work.
Current status: Mirror seems to be polished out to within a few millimeters of the edge. Total time spent polishing (not including lap maintenance, pressing, etc): about 6 hours. Prognosis: Positive!