Oil v. Rubber based Inks

I have two simple questions…
1. Other than reading the can or tube, how cano ne find out if an ink is rubber or oil based?
2. I inadvertently mixed an oil and a rubber based ink and got the resutls I wanted both in color and performance. These two should not be mixable, right? Thanks.

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Oil based tend to be more workable at room temp. Also, the rubber based takes longer to dry out.

We mix them often (not on purpose) and the results are great. It doesn’t seem to effect performance.

Oil base ink forms a skin on the surface in the can. It dries by evaporation. Rubber base won’t form a skin, but it will get thick and tar-like if it’s old enough. It dries by absorption, which is why it doesn’t work well on coated paper. I have never had a problem mixing them.

Cool and thank you. Neil

Oil base ink dries predominantly by what is called oxidation-polymerization, which means it reacts with the oxygen in the air to form a skin (whether it is on the paper or in the can). Rubber base ink dries predominantly by absorption into the sheet. However, to some degree, both types dry by both methods.

Even though oil ink dries mostly by reacting with the air, it also has components which absorb into the sheet. Immediately after printing, some of the more fluid oils, etc. in the ink, go into the sheet, leaving the less fluid ones on the surface. This makes the ink start to get hard soon after it is printed, and helps to avoid smearing if you touch it, and offsetting (where the ink from one sheet transfers to the sheet above it in the pile).

Even though rubber base ink dries mostly by absorbing into the sheet, it does have a small amount of material which dries by reacting with the air to help complete the drying process. There isn’t enough of this to make the ink dry on the press overnight, however.

When you mix rubber base and oil base inks, you are just combining the two drying methods. For instance, the more rubber base ink you put into oil base ink, the more the ink will dry by absorption and the less it will dry by reacting with the air. The more you put oil base ink into rubber base ink, the more it will dry by reacting with the air and the less it will dry by absorption into the paper.

In my opinion (and you can take it for what it’s worth), there are only a few ways we can get into trouble by mixing rubber and oil base inks. 1) If you use rubber base ink, or a mixture with a lot of rubber base ink in it, on non-absorbent stocks like coated paper or plastic, since the ink can’t absorb into the substrate to dry, it may dry VERY slowly if at all. 2) If you mix more and more oil base ink into rubber base ink, eventually you will reach a point where the ink WILL dry on the press overnight, whereas rubber base ink by itself will not. (But probably most of us don’t leave ink on the press overnight anyway). 3) The ink companies say never to mix different brands together (or probably different types of ink of the same brand either). However, I can’t remember ever having a problem with letterpress ink by mixing oil and rubber base inks. Doing this would void any responsibility the ink company might have if a job was spoiled, but most of us are not going to try to make a claim against an ink company anyway.

Thanks for the thorough explanation.

I’ve been mixing them lately with no negative results. I find the oil based gives me a crisper lines compared to the VanSon Rubber based. They also clean up faster and the ones I have a old printer gable me - they are probably over 10 years old - no skins. All my rubber based have tons of skins on them so not sure who’s don’t. You can buy a spray that helps the lids not stick (advertised to make no skin form also if sprayed on sidewalls of ink) which I advise. My lids come off easy.