Best Cleaning Method for LF lenses??

[mentalcrisis00]

I got my Schneider 90mm super angulon in today. I took it apart to clean and noticed a bunch of creeping crud on the inside glass of the rear element. I bought the lens from ebay and the seller said it had been put on a toyo studio camera and never taken off. I imagine it hadn't been cleaned in a long while.

I assumed that the mechanics of these lenses are the same as most lenses today so I used my usually cleaning method for glass. Folded Pec Pad with 4 drops of Eclipse Optic Cleaner, lightly massage the glass in a circular motion from middle to edge. I did this and I was shocked that the pec pad turned black! Is this something I should be worried about or was it just a huge amount of grime? This only applied to the rear element, the glass on the front element didn't make the pad black, I have a picture below.

I stopped cleaning after one swipe because I was worried maybe there was some sort of coating that I was rubbing off. Should I keep using this method for glass cleaning on these old optics? I'm theorizing that seeming the lens was never taken off the camera that the previous owner only cleaned the front element, leaving all the other exposed elements to collect dirt.

[John Koehrer]

If there were smokers in the shop it could be tobacco tar. Even inside the camera smoke could have done the deed or possibly out gassing from A Plastic bellows.

[Jim Noel]

I do not use PEC pads or liquid cleaner on any lens. I use only activated charcoal powder on a Q-tip in a circular motion from center out. The lenses sparkle and never a scratch.

[Peter De Smidt]

You have probably dissolved some of the black paint used around the periphery of the back side of the cell. I've had this happen on an older Schneider 90mm. You want to make sure that there isn't now a haze of this paint on a part of the glass in the image path, all the while doing your best not to remove anymore of it from where it's supposed to be.

[Frank Petronio]

It's probably a good thing to remove.

[Curt]

One thing for sure is to make one pass then change the pad, use as many as it takes but don't rub with a dirty pad.

[mentalcrisis00]

As far as the paint on the metal is concerned I didn't really touch that part at all. I may have grazed the painted parts a couple times but didn't rub it enough to get that much paint. That lends me to believe that it's dirt or something. I use the pad and eclipse method because it was the method that Nikon recommended to me for their optics and CCD cleaning on digitals.

Curt thanks for the word of warning, I changed the pad after every pass and used like 5 of them lol. I just wanted to make sure that the new chemicals weren't bad for the old lenses... Even though eclipse is made for optics I wasn't sure. Thanks all

Update: Just tried a pad with some eclipse on a small part of the black metal ring of the lens and it didn't take the paint off. Eclipse is supposed to be pretty safe on paint and plastics seeming it's made for electronics cleaning, at least so they say. I've seen a lot of old LF lenses where the paint was flaking off, I'm sure if this was the case on this lens the black stuff would be the paint Thankfully it was just dirt and now my glass sparkles.