HELP! Serious YELLOWING PROBLEMS with Letterpress Inks

If anyone out there can shed some light on this issue, or perhaps suggest a solution, I would be oh so very greatful. I am experiencing a considerable amount of ink yellowing on printed items within a matter of a few weeks. I am using Gans letterpress oil based inks and mixing them with a low yellowing tint base. There is yellowing occurring in almost every color, but it is obviously the worst in the lighter colors that require more white or tint base. My light creamy peach colors are turning a dingy dirty taupe that is virtually unrecognizable. Blues are turning green and so on. Has anyone found any great results in using rubber based inks (or any other inks) that may reduce or alleviate the yellowing? Any suggestions would be of help. Sincere Thanks.

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What exactly is a “non yellowing tint base”?
Normal practice for PMS mixing is to use pantone white (a transparent white, what was once called mixing white), though many also use opaque white when adjusting color for letterpress.
If colors are shifting after you print, I would really suspect the tint base. Of course, colors can be affected by the preceding ink on the rollers, but you’d notice that immediately.
There shouldn’t be any difference in regard to color between oil base and rubber base inks.,

Thank you for your response. Well, it’s not “non-yellowing”, but it is a “low yellowing” tint base, which is a mixing transparent white. It can be purchased through Gans. It is exactly what you are talking about, but in a low-yellowing form that can be used in lieu of opaque white as a mixing agent to lighten color. Interestingly enough, I had originally ordered a specific Pastel PMS direct from Pantone because I was trying to achieve an exceptionally light color from the Pastel PMS book. This ink I received has been my worst offender to date. The stuff yellowed within two weeks. I found out that they did not use low-yellowing white to mix the ink, but frankly even after mixing it myself, I had similar (although less intense) yellowing issues. I can’t figure out if some colors are just too light to achieve due to the amount of tint base/trans white that is required.

Call me crazy, I think the “low” yellowing base is causing your yellowing problems. I have printed a number of PMS jobs and NEVER had a yellowing problem. I use oil base inks. Usually Van Son brand. I also use transparent white oil base of no particular brand. Sometimes Van Son, other times whatever I can get my hands on. I mix the PMS color then I cut that with transparent white 50/50. I am usually spot on the color. There may be some minor dry back but not enough to get in a twist over. How about the paper you are using? Look for common denominators in your process and begin to vary one by one until you find the culprit.

if the ink you mix and ink you bought does the same thing it’s not the ink.

What do you clean your rollers and ink disc with?

Are your problems just with PMS pastel inks, or also with the regular PMS mixes?
My PMS mixing experience is just with the standard offset ink formulas. The Pastel formulas are different, and made of component inks from a different numbered series. I’d be interested to know how pastel tint base differs from Pantone white.
The various PMS books show how the ink would look as printed by offset; since letterpress typically lays down a heavier ink film, most of us add white so the final printed color will match the swatch. This wasn’t necessary in earlier times because letterpress inks were made with less pigment in them relative to lithographic inks.
The pastel formula book does say that lighter colors will fade more quickly than darker colors, but fading and yellowing are different problems. I would show the printed samples to the inkmaker and see if they can find the cause.

No, the problem is not just with pastels. It is with greens and blues and the lighter colors that require more of the extender and opaque whites in order to achieve. And, no, there is no difference between the opaque white for PMS pastels, only the base mixing inks. However, you can order them with a low-yellowing extender (which we are not sure really makes a difference.) I have considered the stock, which was the Copperplate 600gm pasted sheet, as the culprit. I recently remixed the ink from scratch yet again and applied it to several different papers, including Somerset Radiant White. Thank you everyone for your helpful comments. We are trying to narrow down the problem and your suggestions and questions have been very much appreciated.