I've just ground my own glass

[vdoak]

The idea for me is even pressure, thus a sandwich (but not necessary). The grinding paste is only ever used on one side of the ground glass that is to be used in the camera. The surfaces one grinds against need to be flat, but can be many.

[keithwms]

The main thing is to randomize the contact between the plates as much as possible. You can get excellent results with two surfaces if you do that. But if you do it the wrong way, you'll wind up with a ground circle in the middle which will of course then be concave.

[vickersdc]

Toffle: I can see your 'problem' - it's not a sandwich! You still only grind one piece against another (i.e. two pieces of glass involved at any one time). It's just that you swap the pieces around - so you could grind A and B, then B and C, then A and C, then B and A, and so on...

Does that help you?

[Toffle]

Toffle: I can see your 'problem' - it's not a sandwich! You still only grind one piece against another (i.e. two pieces of glass involved at any one time). It's just that you swap the pieces around - so you could grind A and B, then B and C, then A and C, then B and A, and so on...

Does that help you?

... so you're saying I'm not insane? Wait till I tell the guys!!!

(that means I finally get it... )

[PeteZ8]

Yes, there is. The traditional method uses hydrofluoric acid, something you do not want at home. There are other methods, though.

Hydrofluoric acid isn't so bad. You only need a very small quantity for a project like this, probably less than an ounce. Just make sure to wear heavy, heavy rubber gloves (not disposables) as hydrofluoric acid does nasty things to bone.

It's commonly used in the "security etching" done on car windows.

[PeteZ8]

I've used an acid etch technique - I think it was hydrofluoric suspended in a manageable paste ...

Bright enough

That's what the window etch stuff is like. Comes in a small tube with a sponge applicator on one end. Some kind of pink stuff with a slightly thicker consistency than Elmers glue.

[BetterSense]

Hydrofluoric acid isn't so bad.

It IS so bad. It's one of the most dangerous substances to handle. Even a small amount on your skin will absorb through your skin and dissolve the calcium out of your bones. I handle many dangerous chemicals daily and out of all of the HF is the most terrifying. It's used in semiconductor processing to selectively dissolve silicon dioxide.

[richard ide]

I agree completely. HF is the only chemical I refuse to have anything to do with. We use it at work and handle it only with a fully protective suit and respirator.

[keithwms]

If you have a 1% body-surface area (~ palm-print sized area) of contact with pure HF then you are likely dead. And you won't feel much pain from it when the first contact begins... nor will any amount of emergency care help you, once you do realize that you've made contact with it. It will simply bind all the calcium in your body and give you a heart attack.

So... it is that bad. And somehow the idea of working with gloves on big pieces of glass with HF doesn't make me feel very comfortable!

There are plenty of other, much safer methods. The grinding method works very well and, again, it works well on plexiglas too... whereas HF and similar etchants do not.