Ground glass and light seals

[alexmuir]

I'm looking for some advice about maintaining the screen on my 4x5 camera. It is a Horseman screen with a fresnel lens. The screen has grid lines, format markings and a clear centre spot. I've noticed some 'clear' areas across the ground glass, so I took it apart to see what these are. At the same time, I found foam which appears similar to light seal material along each edge of the frame. This has deteriorated and left black residue on the edges of the glass. You can't see this when the frame is assembled. Does anyone know what causes the 'clear' patches on the ground glass? Should I bother cleaning and replacing the foam? I can use the screen as it is, and I don't want to risk damage, but would clean it if it can be done easily.
Alex

[DaveP]

Ground glass tends to look clear if you get moisture or grease/oil onto the rough side. Even grease from fingertips or nose will do it. Could be that?

[alexmuir]

Thanks Dave. I did wonder about that as I've washed a plain ground glass before, and it looked clear when wet. On this one, the ground surface isn't exposed as it faces the fresnel screen. It could have been exposed to moisture at some point before I bought it, although the camera is generally in good condition. I might try washing in the area of the marks to see what happens.
Alex

[paulc]

Originally Posted by alexmuir

I might try washing in the area of the marks to see what happens.

Be very careful if the grid lines are on the glass - I have a GG from an MPP that I wiped with a damp cloth, and the lines started to come off.

[alexmuir]

Thanks Paul. I would prefer to keep the markings if there is a risk of damage with cleaning.
Alex

[JOReynolds]

The clear patches are probably just greasy. The Fresnel lens is machined or moulded from acrylic, which can easily be degraded by unsuitable solvents. A weak solution of washing-up liquid will remove grease, as will Isopropyl Alcohol (IPA). Foil-packed spectacle-cleaning cloths are probably safe. Do not use anything abrasive.
Ground glass can be cleaned with window cleaner but it may be difficult to remove it from the etched pits. Soapy water is safer.

[alexmuir]

Thanks Jonathan. I will leave the fresnel lens aside. I have some isopropyl to try on the glass. I'm guessing the foam along the edges is simply to seat the screen securely in its frame, rather than a light seal. There doesn't seem any need for a light seal, other than perhaps to prevent flare on the screen?
Alex

[DaveP]

Seal might just be to stop your breath condensing between the layers on cold days?

[taulen]

Did you ever figure it out ? I love to get a finish to a thread like this