I have reprinted this from the Stones Crayons website, with permission. Craig Cornwall, a Tamarind Master Printer has written one of the most definitive pieces on modifying litho inks that I've seen....
One of the most often asked questions in lithography is how do I know when to modify my ink? The question right behind it is, how do I modify my ink?
How do I know when to modify my inks?
Lithographic inks should be designed so that they do not require modification to print. They should print right out of the can about 80-90% of the time. The other 10% of the time you should only change that property of the ink that is not performing to your expectations.
There are 4 basic properties of ink:
Length - Can be measured by putting an ink knife in the ink and then drawing the knife straight up. The thread of ink that is generated will indicate the length. Long or short.
Tack - This is how sticky the ink is. Usually long inks are very tacky
Body - This is the stiffness of the ink. Does it stand up like putty or run like honey?
Grease content - This is the amount of grease, natural or otherwise, in the ink.
There are 4 basic Ink modifiers
Magnesium carbonate - A white powder that stiffens, reduces greasiness, and reduces tack and length. It also has the property of holding the ink together.
Varnishes - 00 Reduces tack and length, increases greasiness, reduces body
#3 Reduces tack and length, increases greasiness, reduces body
#5 Increases tack and length, reduces body
#7 Increases tack and length
Reducers - Reduces length and tack
Properties of ink are interrelated. You cannot change one property without to some extent affecting another. Stiffening ink with a stiff varnish also increases tack. Let moderation be your guide. The more you modify an ink the less likely it will be to print without problems.
General image clean up procedures.
Put a couple of passes of ink on the image, then clean off the unwanted ink. (If caught early, they may come up simply by rubbing your finger over them with a little water.) If it is an extreme case, you may have to resort to Q-tips and a solvent like acetone. (A solvent that flashes off quickly will allow you to get into tight spots without jeopardizing the image around the problem.) In really stubborn cases with color inks, you may have to put the image back in black so you can see what you are doing and to help stabilize the image. Put a light etch over the printing surface and smooth evenly with Kim wipes or a couple of paper towels. (Don't talc the image or you will have to wash it out with solvent.) This light etch will allow you to have time to modify the ink and will re-establish your gum film.
Clean the ink slab and roller by scraping up the ink slab with a razor blade paint scraper. Then roll out the roller. Then scrape up the slab again. Do this a couple of times till there is little ink being laid down. You do not have to clean the roller or slab!
Let's look at some real examples to make it more applicable.
You have just begun printing and everything is going fine. After a several prints you notice tiny dots of ink are showing up in non-image areas. The image was etched properly and was stable when you began printing but now the image is developing measles. (The earlier you catch the problem the easier it will be to correct. Don't pretend they will go away or that the won't get any worse. It is not true!)
This problem I find most easily solved by adding Mag to the ink. This will stiffen the ink slightly and absorb some of the greasiness of the ink so it will print clean. Mag can be added in large amounts as the ink will absorb much more than you would think. However, it does not require huge amounts. Try about 1/4 of the ink volume. Mix in the mag thoroughly to a portion of your ink. (Do not mix it into the entire ink mass because once it is in you cannot take it out.) Lay out your ink again on the ink slab. Wash off the gum with water and paper towels and resume printing. This should correct the problem.
Your image is made up of crayon and tusche washes. You are printing in a colored ink and the ink is quite thin without much body. You are concerned that the washes will fill-in because the ink is so soft. Add Mag to your ink in larger quantities. You can make your ink too stiff with this modification so don't over do it.
Your printing and small pieces of the paper are being pulled off from the ink. The ink probably has too much tack and is pulling the paper fibers apart. Reduce the tack by adding a little Setswell or other reducing medium. (You can use Vaseline as a tack reducer.) It will not take a lot and too much will cause other problems or turn your ink to slime. Put in just enough so the paper stops picking.
As you roll up your image for printing you get scum wherever you change roller directions. The ink in this case needs to be stiffened. Mag might do the trick but you probably also need to increase the tack to help pick up the scum. Mag will stiffen the ink but it will reduce the tack. Add a little stiff litho varnish #7 or #8. This will increase both the tack and the body at the same time.
As you are printing you notice that some of the delicate areas of your drawing are disappearing. This could be caused by the ink not having enough grease to keep the image printing and the water is burning the image. By making the ink greasier you can encourage the image to come back. Add just a little #0 or #00 varnish to the ink. These varnishes add grease and loosen the body to encourage a fuller print. Caution: This can get away from you very quickly and it may bring back those delicate areas it may also cause your darks to drop into solid oblivion. Add small amounts and consider going back to the original ink if needed.