Technology: Hazardous compound helps to preserve crumblingbooks

日期:2019-02-28 08:07:06 作者:还旋 阅读:

By JOHN EMSLEY LIBRARIANS may be able to save millions of books from slowly crumbling with a new chemical process that uses a hazardous flammable compound, diethyl zinc (DEZ). Chemists in the US have successfully completed an 18-month trial of the technique, which neutralises the acids in paper which causes books to decay. The method was developed by the Dutch chemical giant, Akzo, in collaboration with the US Library of Congress. It can treat 1000 books at a time at a fraction of the cost of microfilming. The world’s libraries and archives are today stocked mainly with books that are destroying themselves because of a new way of making paper that was introduced in the middle of the last century. In this process, wood pulp became the main source of the cellulose from which paper was made, replacing the cotton or linen rags used previously. Unfortunately, book publishers were unaware that wood pulp’s slight acidity would eventually threaten their work. The acid attacks the cellulose polymer of paper, breaking it down into shorter and shorter pieces until the paper’s structure collapses. The only answer is to neutralise the acids in the paper by chemical means. This has generally been done by unbinding the book, treating it page by page with a carbonate solution, and then rebinding it. The cost can be as much as Pounds sterling 200 per volume. Akzo’s method can be done without taking the binding off the book. On the face of it, DEZ would seem the last chemical that should be brought in contact with paper. This volatile liquid bursts into flames when it comes in contact with air. However, it is not DEZ’s sensitivity to oxidation which is the key to its use as a preserving agent, but its ability to neutralise acids by forming zinc salts with them. Because DEZ is volatile it permeates the pores in paper. When it meets an acid molecule, such as sulphuric acid, it reacts to form zinc sulphate and ethane gas. DEZ is such a strong base that it will react with any acid, including the weaker organic ones. It will also react with any residual water in the paper to form zinc oxide. This is an added bonus for the book conservators, since it buffers the paper against future permeation by acidic gases from the atmsophere such as sulphur dioxide. Not only will DEZ protect against acid attack but it is also capable of neutralising alkalis, which threaten some kinds of paper. It can do this because zinc oxide is amphoteric – capable of reacting with either acids or alkalis. The Akzo method treats closed books and protects every page. It adds about 2 per cent of zinc oxide to the weight of the book. Much of this is deposited near the edges of the pages, the parts which are most affected by the acid from readers’ fingers or environmental pollution. The only risk in the Akzo process comes from the DEZ itself; this caused a fire at NASA’s Goddard Space Flight Center where earlier tests on the method were carried out. For the process, the books are gently heated under vacuum for a day to remove residual traces of moisture. The chamber is then flushed with dry nitrogen gas for five hours to remove the remaining air before DEZ is introduced at a low pressure into the gas stream. DEZ is passed through for about eight hours. Unreacted DEZ is trapped out of the exit gases and recycled, while the ethane is burned off. When the process is complete, the chamber is purged with nitrogen to remove residual DEZ. The whole process takes about three days. The cost per book is about Pounds sterling 2, considerably less even than the Pounds sterling 40 for microfilming. This work was originally funded by the US Library of Congress, which has over 10 million books now at risk. According to Dick Miller, Akzo’s director for book preservation, tests have shown that the method can deal with hundreds of books at a time. A million books a year could be rescued by the new process, for which Akzo has been granted exclusive rights. The treated books should then survive for hundreds of years. Another national institution, the British Library, launched an adopt-a-book scheme to help it to meet the costs of processing books. The BL has so far raised over Pounds sterling 80 000. But if the traditional method is used, this will barely cover a twentieth of 1 per cent of the 2 million books the BL needs to treat. Edmund King of the BL’s preservation service says that the BL has developed another method which coats the individual fibres of the paper with ethyl acrylate polymer, protecting the books not only against acid attack but actually making them stronger (Technology,