Surface pH tests, now commonly used in preservation, will become increasingly misleading if current trends in the paper industry continue. PH pen customers in the paper industry have already reported to the Editor that papers treated on the outside with common wet-strength agents will give a false positive result--i.e., they will test alkaline even if the paper itself is acidic.
The external use of additives is a way of coping with the tricky chemistry of alkaline papermaking. To keep additives from hydrolyzing, or failing to bond with fibers, when they are added to the stock before it goes to the fourdrinier, they are simply being added to the outer surface of the paper afterwards. However, the surface may then have a pH higher or lower than the paper stock.
Another recent trend, that of making multilayer paper, will result in differences between the surface and core of printing and writing papers. A "sandwich" of three thin layers can be made simultaneously on a single machine, with the more problematic layer on the inside where it will not affect appearance or working properties. The problematic layer may contain recycled or high-lignin fiber.
Consumers who use spot tests to find out whether these inhomogeneous papers will meet the ASTM or ANSI permanence standard will get misleading results, because pH, calcium carbonate, and lignin content will all vary between the top and bottom surfaces. Standard lab tests, which call for slurrying of the samples, will give results that describe the average composition. This will not, however, indicate that all layers of the paper will deteriorate at the same rate, or that all characteristics will change at the same rate as time goes on.
It is possible that unevenly distributed calcium carbonate will buffer all layers against acidic gases or harmful paper components, but there is little reliable evidence to show an effect one way or another. And lignin, which is appearing in white papers now, cannot be kept from a certain degree of yellowing with time and exposure to light, whether it is on the inside layer or not.
All these problems are partly theoretical, because so far no one has found a way to predict the effects of natural aging in this kind of paper on the basis of standard tests. Until this is done, it looks like the best answers for the consumer or purchasing agent will come from accelerated aging of samples. Unfortunately, this takes too much time to do before making a purchase of accepting a shipment, so the customer will probably have to accept the vendor's word on the permanence of the paper. In any case, doing one's own testing is not an option, because accelerated aging cannot be done properly without a special paper-conditioning room, a skilled technician to operate the aging oven, and test equipment to help evaluate the condition of the paper before and after aging.
In the end, if multilayer or inhomogeneous paper becomes common, and it is not identified as such on the package, testing of paper may have to be done centrally or in the institutions that already have the equipment and skills; or the paper companies may have to certify their products somehow; or some central agency may have to test papers on the market and make the results public.
Timestamp: Sunday, 03-Mar-2013 21:39:19 PST
Retrieved: Wednesday, 23-Jan-2019 12:33:20 GMT