Hi Beach

In my experience, I have found it best to see what solvent loosens or lifts the pigment from the ribbons, then find an oil that will readily mix with that solvent. Mix the solvent and oil to a 10:1 to a 20:1 solution of solvent to oil and wet the ribbon with this solution. The solvent dissolves the pigment and brings it into suspension in the mixture. The solvent then evaporates leaving the pigment suspended in the oil and ready to transfer onto the paper.

For ribbons from the 1940's to the 1960's, acetone and castor oil seem to work. For ribbons from the 1970's to the 1990's, naphtha and mineral oil seem to work well and if today's new ribbons need a little extra moisture, Varsol and non detergent SAE-30 oil appears to be the most effective. I know that other members here have tried other oil and solvent combinations with varying success and I also believe this topic was covered in an earlier thread quite a few years ago. All the best,

Sky

Here are a few anecdotal descriptions of the ribbon revival process:

In 1994 Louise Milano of Des Plaines, Illinois claimed that she had been using the same typewriter ribbon since 1957. There are crucial details missing, such as how often she actually used that same ribbon over those 37 years, or how many times she had to revitalize it, but here’s her shared technique:

“Hand roll all of the ribbon on one spool. Take a can of 3-in-1 Oil and oil drop by drop all around the ribbon. Wrap the ribbon in plastic cling wrap and let it sit for a couple of days. You'll be surprised at how much black is left on the ribbon."

 
And here’s another technique that has been used over the years:

“After the ribbon has been wound up on the spool tightly, remove it from the machine and lay it flat on a heavy piece of foil. Get pure glycerin at your drugstore, put some in an eye dropper and squeeze twelve drops or more (depending on the size of the spool) through the openings in the spool where the typewriter ribbon shows through. The glycerin will soak down through the ribbon, loosening all the old dry ink (this is why your typewriter didn't print darker) and rejuvenate the rib­bon. Wrap the spool of ribbon in the foil and leave it in the exact position it was in when you applied the glycerin. Let it soak for at least three days.”

 
One more home handyman pointer:

“Typewriter ribbon may be renewed by using machine oil. Squirt from the can onto the ribbon wound on the spool. Be sure to go all around it, even through the holes in the top of the spool. Fasten up in a plastic bag and leave until the oil penetrates…

For what it's worth, the glycerin method is the one that I see most often mentioned in older sources. I've never bothered to try any of the techniques, mostly because ribbon is inexpensive. I'm normally not a wasteful person, but part of my logic is that by buying ribbon it helps to ensure that those making it will continue to do so. When those resources have disappeared I'll defininitely begin to rejuvinate what's left.

Uwe wrote:

For what it's worth, the glycerin method is the one that I see most often mentioned in older sources. I've never bothered to try any of the techniques, mostly because ribbon is inexpensive.

I had never heard of the glycerin method for typewriter ribbons until now.  Coincidentally, I tried using glycerin just last week to rejuvenate a ribbon.  I thought I was being original!   Anyway, I had used glycerin to rehydrate dried out stamp pads in the past, and reasoned that it should work similarly for typewriter ribbons.  It worked well.  I used a 50/50 mixture of vegetable glycerin and distilled water.  The water is only to help the glycerin spread more evenly and will soon evaporate.  After a couple of days, the ribbon was leaving dark and well-defined impressions.  Much better than the WD-40 method which left me with greasy, blotchy impressions on the paper, as well as an odor.  I imagine that glycerin could be mixed with other carriers like alcohol or mineral spirits, but water seemed to work fine for me.