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  1. #11
    Neanderman's Avatar
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    Quote Originally Posted by Photo Engineer View Post
    HF is a strong acid by virtue of the fact that you get nearly 100% disassociation into H and F ions... etc.

    I have not put the charges in as it would be confusing.
    Even without the charges, a clear and lucid explanation for people without a lot of chemical knowledge.

    Thanks.

    Ed

  2. #12
    Murray@uptowngallery's Avatar
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    Thanks, PE, et al. (letters I can probably use correctly).

    Murray
    Murray

  3. #13

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    HF acid, Is just too dangerous. Heck in less than a hour with 600 grid, water, and using 4x5 and 2x3 glass tools, I had a 11 by 14 inch piece of ground glass form float window glass.
    It's not the camera......

  4. #14
    Ole
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    There's yet another way...

    Calcium fluoride, CaF2, is insoluble. Yet a fine powder in contact with glass will dissociate enough in strong hydrochloric acid to etch the glass.

    I discovered this when cleaning calcite off an otherwise nice mineral sample with quartz and fluorite - the calcite disappeared as it should, but the quartz was etched too. I hadn't seen the frosting of fluorite...
    -- Ole Tjugen, Luddite Elitist
    Norway

  5. #15
    Struan Gray's Avatar
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    Ole, are you sure you didn't just dissolve the flourite? That's how they make HF industrially.

  6. #16
    Ole
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    Struan, the (formerly) nice bright shiny top of the quartz crystal was definitely etched. Some small fluorite crystals which had been embedded in the quartz were now sitting on top of little "humps" of quartz.
    -- Ole Tjugen, Luddite Elitist
    Norway

  7. #17
    Struan Gray's Avatar
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    Sorry Ole, I just had a brainfart and forgot that fluorite *is* CaF2.

    I'll go and get that second cup of coffee now....

  8. #18

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    I used to use HF to dissolve silicate rocks for analysis as well as etch them for certain processes. For etching we used to use a plastic dish with the HF (very little) and suspend the object for etching over it and let the vapour do the job. The surface had to be grease-free, including fingerprints. Dunking the sample generated too much local gas pressure and disturbed the crystals on the surface.

    All this was in an acid-rated washable fume cupboard, with neutralizing solution, barrier gel, double gloves, face masks, and lots of calcium glauconate on hand. It was also an organic-free lab. The local hospital had an advisory that we used it.

    The geologists used to scare the chemists with this stuff. Even 'cut' to 10% or so I would be very cautious.
    I feel, therefore I photograph.

  9. #19
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    I'm familiar with HF, know the precautions and so forth. And I wouldn't use it, absolutely not. Not worth the risk.

    YMMV, but something that could kill me or cause intense and irreversible pain is just not something I need to get involved with for the sake of a focusing screen. Especially when granite surface plates and diamond grinding powder are so (relatively) cheap these days.

    However, if etched rather than ground glass is an absolute essential, anyone have thoughts on using warm phosphoric acid? That strikes me as a safer (safer, not 'safe') alternative to hydroflouric acid, and more controlable too.


    Peter

  10. #20
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    To clarify a few of the responses:
    • Yes, I agree this stuff is deadly, dangerous and unpleasant. Take all precautions if you decide to use it.
    • No, I do not agree that it is too much trouble to bother with. A chemically etched ground glass is 2 to 3 stops brighter than surfaces produced using the 600 grit abrasive method. If you're doing medium or small large format work (apologies for oxymoron but 5x7" or less is tiny) this can be a real advantage for focussing.
    • Yes, diluting the mix about 1:10 with H2O will give a slower more controlled etch.
    • No, You will never get dead flat surface when etching glass but this is because glass is amorphous, not crystalline. Note however that IMHO the etchant gives a smoother finish than the abrasive method and I assume this is because you are removing material from the total surface area rather than breaking chunks off where they stick up.


    My intent was to make this useful technique available again to others who were not able to easily source a chemical glass etchant. I acknowledge that the technique is VERY dangerous and uses toxic chemistry.

    For me personally the benefit of a 2 or 3 stop brighter viewing screen offsets the risk involved as I am happy and able to use the appropriate safety measures.

    I would never store this in my own household and recommend that you do not either. I would much prefer to leave it at work (yep! in the tea room ) or dispose of the remainder when I'm finished. It's cheap enough to throw away and buy another if you need to.

    Etching gives a much smoother finish and appears to produce a flatter surface than the abrasive method.
    I have not personally measured the surface roughness / flatness of samples but the etched surface seems flatter and smoother.

    Also, since the initial post I have found that etching an already abraded surface (I generally use valve grinding paste) gives the best result as unabraded glass tends to remain too transparent (or do I mean clear?) for good viewing or focussing. This goes back to my assertion that the surface is flatter / smoother than the abrasive method: If you etch a clear piece of glass it remains clear, so it seems the entire surface is being etched evenly.

    Thanks also to Richard Ide for the response recommending alternative masking materials. When I get some more glass I'll probably give beeswax a go as a mask. I suspect the high sulphuric acid content might eat enamel paint.

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