Hello. I was recently reorganizing my little shop in the basement and was looking at some of my inks. Some seem “stiffer” than others (rubine red seem less “stiff” than pantone red 032) and my opaque white has a hard but flexible surface, something like what you would get with oil based ink, however all of my inks are rubber based. I am just curious if this a because of different pigments used or if something is wrong with the ink. All of them were sealed well to the best of my knowledge.

Thanks for any information!

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Our inks can be formulated to dry in 2 different ways. Oil base ink dries principally by reacting with the oxygen in the air to polymerize and form a skin. Rubber base ink dries principally by absorption into the sheet. However, to a small degree, both types of ink dry in both of these ways. So, oil base ink dries to a small degree by absorption, and rubber base ink dries to a small degree by oxidation-polymerization.

Some inks become stiffer just by standing in the can, but if you put them on the press and let the rollers work them up for a while, they may be fine. This is because the pigment particles “agglomerate,” which means that the particles stick together into bigger (but still very small) lumps. These lumps can be broken up to some degree by working the ink in the press. If you think that the ink is still too stiff after being worked, one thing you could try is to add a drop or two of soybean oil. Soybean oil is a drying oil (it forms a film after exposure to air). However, this may slow the drying of the ink. (I have not had to use soybean oil personally, but if you try it, I would do a test print first, to be sure the ink dries and performs OK with the soybean oil in it).

Your opaque white may have a hard surface because even this ink may have dried to a small degree on the surface like an oil base ink. If you take that hard surface off, it may be fine. Or, what I do, since I usually only need a small amount of ink, is to take a small ink knife and push it down through the hard surface, at the side of the can. Then I fold the skin back, and pull out some ink with the knife. Then I put the skin back and push it down against the ink, to get the air out from under it.

I never thought of using the skinned surface to “seal” the ink back after use, i have always just scraped it off and tossed it. Thanks for the info.