Brilliant Ground Glass

[N Dhananjay]

Sorry Geert polishing a glass surface and making a ground-glass are two different things. If not a highly polished glass would be the best ground-glass. As mentioned before the brightness of a ground-glass depends on the form and size of the scattering elements on the surface.

Peter K

My understanding is that the coarser the glass, the more difficult it is to focus on fine detail. Also, I would imagine that coarser glass would be less bright - think of it as small facets that are at some angle to the light. Fine grinds increase transmittance while coarse grings increase diffusion - so I see it as a tradeoff. The diffusion is what allows us to see the complete image. At the extreme, without a ground glass, you can focus on an aeriel image but you will only be able to see one small section at a time.
Cheers, DJ

[CP Goerz]

'What kind of wax are you using? Parrafin, paste furniture wax, etc?'



Any wax will do, I just used some paraffin type wax but have seen beeswax and petroleum work too.



CP Goerz/dagor77

[Peter K]

My understanding is that the coarser the glass, the more difficult it is to focus on fine detail. Also, I would imagine that coarser glass would be less bright - think of it as small facets that are at some angle to the light. Fine grinds increase transmittance while coarse grings increase diffusion - so I see it as a tradeoff. The diffusion is what allows us to see the complete image. At the extreme, without a ground glass, you can focus on an aeriel image but you will only be able to see one small section at a time.

It's not so easy at all. One had to use a different ground-glass for different focal-lenghts and also for different bellows-extensions. And for different wave-lenghts too, because diffusion depends on the size of the diffusing elements and it's angle in direction to the optical axis. So every ground-glass is a compromise.

Peter K

[Nathan Potter]

I've made hundreds of these plates for a variety of purposes and have some additional observations as well as some confirmation of what was said above.

I think image diffusion is a misnomer in this context; this is really image scatter. The ground surface refracts each circle of confusion in all directions from each of the small craters formed by the grinding process. The observers eye, from one point, thus can see the entire screen at nominally equal brightness. At exact focus on the GG the refracted image is that of an airy disk (1.22 lambda in dimension) for a perfect lens. Details in the grinding procedure will affect the uniformity of the screen brightness across its extent as well as the quality of the image as viewed by a loupe.

As the grit size gets smaller the glass approaches a less fractured aspect and so is less effective in scattering the image. This results in increased "hot spots" as mentioned above. But also results in a finer fracture and so higher resolution when viewed through a loupe, also as alluded to above.

Scratches generally result from foreign particles larger than the abrasive becoming entrapped between the glass and the grinding mechanism. They can come from the grinder if it is another piece of glass (say chips from the edge of the grinding glass), contaminated grit or a dirty ambient.

Pressure applied during grinding is an important factor in the ultimate quality of the plate. A light touch is desireable but slows the grind rate. Again, as mentioned above, too high a pressure can cause scoring and scratches in the glass especially when combined with larger particulate contamination.

Glass grinding tools are effective for a glass against glass system but can be prone to chipping and contaminating the slurry with larger glass particles. I generally use a stainless steel plate say 2 inches in diameter which has been previously ground flat and to which a handle of some sort has been epoxied.

It is quite important to make sure that an adequate amount of abrasive is maintained between the grinding surfaces as well as a decent bead of water. If during grinding some stickiness develops add more abrasive and maybe more water. You can hardly have too much abrasive.

Also mentioned above is to cut the glass to size prior to grinding. The reason being that the grinding operation introduces micro cracks of some depth into the glass so subsequent attempts to cut to size can cause the scribe line to wander off break point. It is also wise to bevel the edge of the glass with fine lapping paper to both avoid cutting yourself later and to avoid chipping larger pieces off the edge during lapping. But after edge beveling be sure to get rid of every bit of abrasive that may have come from the edge bevel operation.

The application of a wax is an interesting idea - never fooled around with that.

Nate Potter, Boston MA.

[Paul Kierstead]

I noticed an E-Bay seller in Torrance, CA named Brilliant Ground Glass. Here is a link to his E-Bay store: http://stores.ebay.com/Brilliant-Ground-Glass.

I get a "Sorry, this store does not exist."?

I have never looked through a "premium" ground glass. How visible improvement would these be over my stock wisner gg or stock calumet gg? Particularly with wide angles (80mm)? I find focusing in even moderately low light to be a serious battle.

PK

[jb7]

This is the way I do it. I'm sure there are many ways and probably better ways but this works for me. The thing to keep in mind all the while you read this is to take your time. This process is meant to create uniform little conchoidal chips in the glass. If it's done right, they will essentially act like little surfaces for the image but also pass a maximum of light. If you get in a rush or try to apply too much pressure, you will end up with an uneven glass or a ground glass with bigger or uneven little chips. It will work but won't be as bright as one done with care.

First, I cut the glass to size and clip the corners. I've been told you can cut the glass after grinding but I ruined 2 glasses that way and never tried a third time. I believe the people who say it can be done, but I can't figure it out. The cut keeps wandering. Cut 2 pieces of equal size. One will be the one you concentrate on grinding. The other will be the tool you grind with. In practice, you should end up with both of them ground to satisfaction so you can have a spare. If you do this, make sure to clip the corners on both pieces and test fit them into your camera before expending the effort.

Second, you will want to get your work area prepared. Lay a couple of sheets of newspaper out on the table. They will keep everything clean and help the glass to not slide around. You will want a bowl of water handy, with a spoon in it to dispense the water, as well as the grits you will use. I like aluminum oxide in 5 micron and 3 micron sizes. I got a pound of each at a lapidary shop and barely used any to grind a half-dozen glasses for myself and a couple of other people.

With your spoon, put a few drops of water on the newspaper and set one piece of glass down. The water just sticks it down to the paper. Put a couple pinches of 5 micron alox on the glass and then pour a spoonful of water over it and stir it a bit with the spoon. It should make a thin slurry with enough grit to more than cover the glass. You'll get the feel of how thick or thin it needs to be pretty quick. Lay the other piece of glass on top and with what I can only describe as "a medium finger pressure" start grinding it in a circle. When the glass doesn't want to move against the other piece anymore, the slurry is drying up and you need to add more water. If the slurry is looking too thin, add a pinch of grit. It's kind of a go by eye thing.

Grind grind grind...Check the pieces from time to time. They will start by grinding unevenly in patches. That is the stage where you are flattening the waves out of the glass. At some point, you will check the pieces of glass and they will be evenly frosted from edge to edge. At that point, take both pieces of glass to the kitchen sink and rinse all the grit off. Rinse some more. You want to get ALL of it off or it will just put scratches in the 3 micron grind. After rinsing wash them with dish detergent and set them aside to dry.

Now go clean up the work area. Replace the newspaper and the bowl of water. Wash the bowl and spoon, too. Everything should be new to make sure you have no 5 micron grit hanging around.

For the second stage, do everything as you did before but with the 3 micron grit. Apply less pressure, only enough to be able to move the top glass in relation to the bottom. More will create scratches or tiny chips that will degrade the quality of the glass. As you go with this second stage, you will be able to see the finer grind of the 3 micron in contrast to the grind of the 5 micron. It is subtle but able to be seen. Once both pieces of glass show this fine grind evenly, you are done.

I line my glasses with a .5mm mechanical pencil. It is easy to see while also being unobtrusive to viewing the image as a whole. If you want a bolder grid, .3mm drafting pens work well. Draw the line quickly or the ink can bleed and make an uneven line.

I've just followed Christopher's method and now have two lovely pieces of ground glass-
I used automotive valve grinding paste- coarse followed by fine- no idea what the grits are, but it went exactly according to plan...

Many thanks-

j

[Michael Graves]

I think he shut down his store in response to some of the fee increases. However, I'm pretty sure he'll still make you a glass. He made mine for me after his store disappeared and it's gorgeous. Every bit as good as the old Satin Snow. I found focusing my old Wollensak f9.5 to be much easier. Email him at hopfsteve (at) gmail.com and ask him.

As far as making your own, that was never the point of this thread. Somehow, it got hijacked. And aside from that, if you've never done it before, it's not quite as easy to get a smooth glass as some make it seem, and you only have to waste the time and money on one failure to make his prices look like a bargain.

I get a "Sorry, this store does not exist."?

I have never looked through a "premium" ground glass. How visible improvement would these be over my stock wisner gg or stock calumet gg? Particularly with wide angles (80mm)? I find focusing in even moderately low light to be a serious battle.

PK

[jb7]

Actually, it wasn't at all difficult-
€4.40 on 2 pieces of glass, cut, clipped,
and €10 on paste- 2 grades-

And the results, to me, are brilliant-

Wouldn't be the first tangential thread-

j

[Bob Crowe]

Try stores.ebay.com/Steve-Hopfs-Ground-Glass-Store I've got one on order but it's not here yet so I can't personally comment. I think this is the same guy.

Bob

I get a "Sorry, this store does not exist."?

I have never looked through a "premium" ground glass. How visible improvement would these be over my stock wisner gg or stock calumet gg? Particularly with wide angles (80mm)? I find focusing in even moderately low light to be a serious battle.

PK

[Paddy McKay]

[QUOTE=David Karp;401222]Here is a link to his E-Bay store: http://stores.ebay.com/Brilliant-Ground-Glass

I tried but couldn't connect via the above link. This is the active link to Steve's Ebay store that I found:
http://stores.ebay.com/Steve-Hopfs-Ground-Glass-Store