I have some large format negs from 1930-40s. In order to crop them down in size for printing, the original photographer, 70 years ago or so, applied a black, paper tape to mask off the part of the neg which he intended to print. O, just a minute. I'm gonna get a snap shot for you to see... ok, got it!
Anyway, how can I remove this tape in order to see details hidden behind it? The tape is on the emulsion side.

I was going to recommend medical alcohol - isopropyl alcohol (70 percent).

I've heard of people using vegetable oil or mineral oil to remove the gum from taped areas, but I can't vouch for this being used on negatives.

It would be preferable if you called a local expert and inquired about techniques and possible damage before you attempt anything.

My advice is to also get a scanned copy of the negative first for a backup.
Don't use any liquids that can mix with the dirt, and adhesive and cause more harm.
Such as water or any kind of window cleaner.Use the isopropyl alcohol like Mahvin was saying.
Also you may want to buy a can of keyboard air.

NEVER use water on the emulsion side as it can damage it permanently.
First blow off the area you are cleaning to remove all debris, or most of it.

Use a lint free cloth , and try an area where its not very important first just to be sure.
Wrap the cloth around your finger and moisten it with a small amount of alcohol.

If this doesn't work in a small area, get a pro to do it for you.

Is the idea that the alcohol will soak through the tape, loosen the glue and the tape should either pull off or dissolve? I would be more comfortable if the tape would come off, even in pieces and then I would remove excess tape and glue. But I want to be clear that this tape is really stuck. In one spot on the edge I ran a razor under the tape to try to lift it and when you start to pull the loosened edge up, the the just rips. It is some really good tape.
I will try the isopropyl alcohol in a small spot and see what happens.

Vegetable oil (the edible type) will not harm either the emulsion or the acetate.
I have done B&W photography ~50 yeaars ago.
You could use an oven tray with some oil in it and soak the tape.
It might take maybe a day or so for the gum to soften.
Solvents are dangerous to the acetate or whatever the material is.
The emulsion is gelatine with silver salts.
Gerard.

I would get negative cleaner. It is a special solvent that I used to use in my darkroom, that is safe for negatives.

Thank you all. I will let you know the results. I will try veggie oil first to be safe.

Good luck with it, and yes please post your results.