Although iron-gall inks used for writing and drawing before the Second World War have been problematic because they are so corrosive that they sometimes eat through the paper, we rarely hear anything about printing inks. They are not known to have caused serious problems. But exactly how stable and non-damaging are they? There are two relevant studies that give us a hint.
The W. J. Barrow Research Laboratory found that the printed areas of 19th century book pages were weaker than the unprinted areas (Strength and Other Characteristics of Book Papers 1800-1899 [Permanence and Durability of the Book, Publication Number 5], Richmond, VA, 1967). The authors summed up their findings on p. 37: "While writing ink used in the nineteenth century is known to be injurious to paper because of its sulfuric acid content, conclusions from this study indicate that printer's ink did no visible damage to the nineteenth century papers tested. There was, however, a loss of 30% in folding strength due to encrustation of the ink and some loss in 13% of the papers due to injurious ingredients (other than carbon and oil) in the ink." Much or all of the raw data is published in the booklet. Table 13, for instance, gives number of folds (at 1/2 kg tension) of inked and uninked areas of 48 papers, 1800-49, that had a reasonable strength and showed the same pH in the inked and uninked areas.
That low folding strength in the printed area may not all have been due to encrustation, however. Helen Burgess reported in a 1991 paper that the degree of polymerization in two books, printed in 1854 and about 1901, was lower in the printed areas by 6% and 12%. This would not have been affected by any ink encrustations. ("Evaluation and Comparison of Commercial Mass-Deacidification Processes: Part 1 - Project Planning and Selection of Materials," by Helen D. Burgess and Elzbieta Kaminska, AIC Book & Paper Group Annual, 1991, p. 22-42. The data cited is on p. 42.)
There must be thousands of printing inks sold today, and ink manufacturers can be expected to change the formula whenever they believe this would improve the product. So it would be risky to generalize about the stability and permanence of inks now on the market, except to say that there have been no reports of really destructive printing inks lately (if ever).
Luis Nadeau, author of the Encyclopedia of Printing, Photographic, and Photomechanical Processes (Abbey Newsletter, Nov. 1991, p. 119) answered a query about durability of printing inks recently on the Cons DistList. He said, in part, "CEI (Comite europeen des associations des fabriquants de peintures et d'encres d'imprimerie) recommendations 12-66 and 13-67 are for high quality pigments for letterpress and offset process printing that can be used to print three and four-color posters that will resist the effect of sunlight and pollution for several years." Mr. Nadeau's e-mail address is firstname.lastname@example.org. He lives in Fredericton, New Brunswick, and his fax is 506/450-2718--or it was, three years ago.