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  1. #1
    Ian Grant's Avatar
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    Making a Ground Glass Focus Screen

    This is a relatively simple task and the requirements are minimal.
    The blank glass cut to size (2mm thick is ideal)
    A sheet of Wet & Dry #200 to #400 grit
    The grinding grit (one or two grades) #400 & #600 Silicon Carbide
    A piece of glass as the grinding tool (blank) approx 2"x2" to 3"x3" (5x5 to 7.5x7.5 cm)
    (I use 1/4" - 6mm thick glass as the grinding blank as it's easy to handle.)





    SAFETY FIRST:

    The first task is to remove all sharp edges from the glass used as the grinding blank.
    Place a few drops of water on the wet and dry and smooth off all edges & corners.

    The same may be required with the blank screen, it's simple & quick.



    Place the blank screen on some newspaper or similar on a flat surface, wet the back first - it stops the glass sliding.
    Put a little #400 grit on the blank screen (about 20% of what's shown above) and add a few drops of water - this is a 5"x4" Crown graphic screen..
    Now place the grinding blank on top and begin randomly grinding the screen.



    After a while (2-3 minutes) add some fresh grit, a few drops more water, and start again.
    Make sure that all parts of the screen are being ground and after the second grinding wash the screen well, dry and inspect.

    Modern glass is very flat and not prone to hot-spots, but if you used an old lass plate to start with then you have to keep grinding until it's flat.



    Now you have to decide on the final required fineness, for larger screens a final grind with #400 grit is probably all you require.
    However for smaller screens a couple of further grinds with #600 grit gives a much smoother finer finish.




    It's possible to grind a screen with just #600 grit or finer but the process is very much slower.
    It's faster to grind with a coarser grit first then finely grind after.

    Finally wash carefully to remove all traces of grinding past etc and dry, then inspect.
    Re-grind if necessary.



    The final screen should be as good as any commercially available glass screen, and much better than many older OEM screens.

    Older screens can be improved easily by re-grinding them.

    The economics.

    The total cost to make 10 screens, 1 10x8, 1 Whole Plate, 1 Half plate, 4 5x4's (Crown/Sped Graphic etc), 2 9x12's and a Quarter plate plus 3 grinding blanks, was £10 ($14) for the glass, cut to size and then £20 ($28) for the 2 grits.

    As the there's more than enough grit left over for at least 20x more screens it's easy to see just how little each screen costs in real terms.

    If I cut my own glass the total costs would drop even further.

    __________________________________________________ ____________
    Here's a 1898 take on the subject:


    From The British Journal Photographic Almanac 1898

    Ground Glass. --Rev. Arthur East says that there often appear in photo-graphic papers articles on substitutes for ground glass, it being little understood, probably, what an exceedingly easy process it is to make the very thing itself; and that starch, arrow-root, and suchlike things are almost, if not, quite as much trouble to utilise (and of not one-tenth the beauty and durability) as ground glass at home.

    The following plan may, therefore, be acceptable :—

    Take a clean negative glass of any size, and lay it on a flat, hard surface, such as a board or stone slab and sprinkle on this a pinch of emery powder, No. 1 (flour of emery will do, and do well, but the next quality, coarser, works more ‘ quickly). Lay on this a piece of broken glass about an inch square or there-abuts, and moisten the emery with a little water (do not use much so as to let it get 'sloppy,' and work all about).

    Now work round and round all over the negative glass with the moderate pressure of two or three fingers until the gritty sound begins to go, which means that your emery is getting ground too fine (this will be in about a minute or two). Put on another pinch of emery, and work as before; in about ten minutes rinse your plate under the tap, and you will find probably with your finely ground surface a few ‘islands’ looking shiny, return the plate to the board, and work these patches out, and you will find a surface ground as finely as any you can buy, and there is no difficulty whatever in getting a perfectly even surface free from any scratches or defect whatever.

    Any ironmonger will supply the emery, a pennyworth by post if you live ‘beyond the region of lamp-posts,’ and even the ordinary domestic knife powder will do, but it is too fine and works too slowly, and is inclined to be gritty and make deep scores in the surface of the glass.

    If an extra finely ground surface is required as for a focussing glass, it only means rather longer grinding, and perhaps a little flour of emery to finish with; but the whole process can easily be done with one quality of emery, of which the best is probably the No. 1. If the surface of negative glasses were perfectly flat, it would be possible to grind two together, but the surface is never flat, and a small piece of glass is best to grind with. Some samples of glass are harder than others, an take rather longer to finish.

    __________________________________________________ ________

    Substitute #400 grit Silicon Carbide for the coarse emery and #600 grit Silicon Carbide for the "Flour of Emery" finer grade, and everything else is the same


    Ian
    Last edited by Ian Grant; 06-24-2010 at 07:05 PM. Click to view previous post history. Reason: tidy up

  2. #2

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    Very good article. Thanks Ian. Now just need to find a silicon carbide supplier.
    van Huyck Photo
    "Progress is only a direction, and it's often the wrong direction"

  3. #3
    Christopher Walrath's Avatar
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    Ian, have you ever used automotive grade valve lapping compound and do you know of its effectiveness, pros, cons?
    Thank you.
    CWalrath

    "Wubba, wubba, wubba. Bing, bang, bong. Yuck, yuck, yuck and a fiddle-dee-dee." - The Yeti

  4. #4

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    Thanks Ian - I'll have to have a go at this for my Speed Graphic!

    Have you tried doing two at once instead of using a separate grinding piece? As you say, modern glass is very flat so the reason given in 1898 for not doing it shouldn't stand any more...

  5. #5

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    Quote Originally Posted by David Grenet View Post
    Thanks Ian - I'll have to have a go at this for my Speed Graphic!

    Have you tried doing two at once instead of using a separate grinding piece? As you say, modern glass is very flat so the reason given in 1898 for not doing it shouldn't stand any more...
    I thought that too but it does not work so well, principlely because it is difficult to get all parts of the GG surface grinding equally. You will need to concentrate on a few areas where your random movements are not so random usually the corners. To get the corners of two gg's going together is hard and you will end up doing an uneven job on one or both.
    Regards
    Bill
    "There are a great many things I am in doubt about at the moment, and I should consider myself favoured if you would kindly enlighten me. Signed, Doubtful, off to Canada." (BJP 1914).

    Regards
    Bill

  6. #6

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    Quote Originally Posted by Christopher Walrath View Post
    Ian, have you ever used automotive grade valve lapping compound and do you know of its effectiveness, pros, cons?
    Automotive compond is coarser and faster. The coarser the grind the brighter the image but the more a center spot dominates.
    Regards
    Bill
    "There are a great many things I am in doubt about at the moment, and I should consider myself favoured if you would kindly enlighten me. Signed, Doubtful, off to Canada." (BJP 1914).

    Regards
    Bill

  7. #7
    Ian Grant's Avatar
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    Quote Originally Posted by Christopher Walrath View Post
    Ian, have you ever used automotive grade valve lapping compound and do you know of its effectiveness, pros, cons?
    Yes, I've used fine lapping paste in the past to make a temporary screen, but it's to coarse, it's not really any faster than using #400 grit either. It takes more cleaning off after as well.

    Quote Originally Posted by David Grenet View Post
    Have you tried doing two at once instead of using a separate grinding piece? As you say, modern glass is very flat so the reason given in 1898 for not doing it shouldn't stand any more...
    You can grind 2 smaller screens together, I've done it but it's not as easy to maintain even pressure etc while you're grinding - there's too large a grinding area, also more difficult as it's harder to hold, this is where a smaller 1/4" - 6mm thick piece of glass really comes into it's own, even with larger screens.

    Quote Originally Posted by doughowk View Post
    Very good article. Thanks Ian. Now just need to find a silicon carbide supplier.
    You might try here they should stock grit, if not they'll certainly tell you who does

    Gem Lapidary Jewelers
    Gem Lapidary Equipment Inc*
    4206 Herschel St
    Jacksonville, FL
    32210-2208


    Ian

  8. #8

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    ive used fine automotive grinding paste - no problems my set up is to place a layer of news paper on a true flat surface and keep a bucket of warm water with detergent in it to one side to do the wash downs with and wipe down at the end with alcohol at the end to remove any residual oil from the grinding paste

    DCM

  9. #9

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    for those looking for silicon carbide try those dealing in amateure telescope making they usually have supplies of the stuff.

  10. #10
    Ian Grant's Avatar
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    Quote Originally Posted by david_mizen View Post
    for those looking for silicon carbide try those dealing in amateure telescope making they usually have supplies of the stuff.
    Lapidiary suppliers (stone polishing) are rather more common and carry Silicon Carbide, as do some jewellery suppliers.

    Ian

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