16 April: 320 — 3 Comments

  1. Hello Allen,
    I came across your site as I am trying to find information on the progress and problems of grinding my own 6 1/4 inch Pyrex blank/mirror.
    I am grinding with 220 carborundum for what seems like an age. I have maybe 10 pits that seem to change in colour (dark spec, then a pit, or white spec then a pit). It seems no matter how much grinding I do at 220, new pits turn up. I was speculating that maybe my blank has many “micro bubbles” near the surface that I am uncovering and then chasing through. If that is the case, then I will never eliminate pits. Or is it something else, like you mention regarding sub surface fracturing? Any advice?

    • Hi John

      The thing about those tiny bubbles is that you can see them before they reach the surface. If you hold the glass up to a bright light and try to look through it, you should see any bubbles that are in the glass. The thing is that my own glass had bubbles becauseit was low-quality recycled glass made from crushed cola bottles – quality purpose-made mirror blanks are simply not available in South Africa. So without actually seeing your glass, or knowing where it came from, I’d say that bubbles are unlikely.
      It’s more likely that your grit is contaminated with a few grains that are coarser than 220, or that you’re dealing with subsurface fracturing as you suggested. I’d suggest marking every pit and continuing until every one of those are gone. Once you’re sure that you’re only dealing with new pits, and that every trace of the previous grit’s damage has been erased, you’re basically done with this stage.

  2. Hi Allen,
    many thanks for the reply.
    I kept grinding today and the pit count went down but erratically – like 10….6….4…7….3….1…3…5…2…..1, a bit frustrating. as I never can get to “zero”.

    I also use the Sharpie method to identify the pits and then to see them grind out. I mark the pit themselves and also “underneath” them on the back of the mirror blank so I can re- check the location. But I’m convinced there are new ones showing up.

    I got my blank (a 6 1/4 to practice and an 8 1/2 for the end telescope) from the UK. The guy made this video so he’s pretty knowledgeable.

    I guess I will keep at it but if I can get down to 1 pit, I’ll move to 320….


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