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In 1993 the International Standard, ISO 9706, was approved but the dispute now is whether lignin is of harm or not. Our opinion is that as long it is not proved that that it is harmless we can not take the risk to continue to jeopardise our cultural heritage and allow material that for some reason lacks scientific certitude to be used as permanent paper.
It is important that all interested parties participate in the work through their national standardisation organisations.
The amount of raw material for paper was limited and when Scheele discovered chlorine in 1774, dyed rag could be bleached and used for paper. The bleaching with chlorine caused, however, oxidation of the cellulose and slightly lower permanence.
The increased demand of paper needed more efficient manufacturing processes and in 1806 a new sizing procedure was developed. Rosin size was precipitated on the fibres with alum. Alum is acidic and a slow hydrolysis of the cellulose could take place in the paper. Alum also took away the calcium carbonate left from the washing procedure. This caused again decerased permanence of the paper.
New printing techniques increased the demand of paper even more and large efforts to find other raw materials were necessary. In 1858 the groundwood process was developed and later on in 1874 the sulphite cooking process. Now wood could be used and there were no limitations for paper production. These early procedures to defibrillate wood fibres by grinding and the acid sizing procedure caused the real low permanence.
It is likely that the hydrolytic deteriorating processes dominate for papers made from pure cellulose, e.g. cotton, thus giving this good correlation between accelerated and natural ageing. Modern papers contain, however, also other substances, lignin and hemicelluloses from wood and papermaking additives, this causes different climate, temperatures and relative humidities, to give different results. It might be that oxidative and crosslinking reactions play a greater roll and thus make it more difficult to predict ageing. Natural ageing is also dependent of the conditions under which it takes place. There is no standard climate for natural ageing.
For many years cotton linters and addition of small amounts of calcium carbonate was used to make papers with good permanence. This was the situation in the 1980s when the International Standardisation organisation through its Technical Committee for Information and Documentation/ Physical keeping of Documents; ISO/TC46/SC10 started its work on a standard for permanent paper.
As we see it, the proof is the largest problem. During the drafting of the standard several methods for accelerated ageing was studied. A temperature of 80oC and a relative humidity of 65 % up to 24 days was used in an interlaboratory test. Some acid and lignin containing papers gave very good strength after accelerated ageing, even better than neutral papers made of chemical pulps and that we knew for sure is not the case in natural ageing. The decision became to exclude accelerated ageing from the requirements. An Informative Annex in the standard explains why. There were three reasons
Accelerated ageing is still used for studies of permanence and is often claimed to prove that papers containing lignin show very little deterioration. And that is right, they show little deterioration in accelerated ageing if the conditions, temperature and humidity, are the right ones. But, as they did in the beginning of this century, we have to correlate with natural ageing and that takes time. Another possibility is to know what reactions take place and how harmful they are.
During the time when the working group was in action, experts from archives and libraries and papermaking came to solutions that were reached due to understanding of each other's problems. We based our decisions on mutual respect and as papermakers we found that the fear for lignin containing papers was so strong among the experts from archives and libraries. They had the experience of too many deteriorated papers that had to be restored at high costs. We had also to take our responsibility for the cultural heritage. Now that we had the know-how to produce paper with high permanence there were all reasons to take every precaution and exclude material that we knew too little about.
However, ISO 9706 does not state any limit for lignin content. Instead, the standard has a limit in Kappa number, a figure that expresses the material's sensitivity to oxidation. The logic was; if the paper is sensitive to oxidation, it is likely to get oxidised with time and thus unstable over long time periods.
There are also other papers that contain easily oxidised substances, for instance some coated papers. Soft latex binders with a high amount of double bonds is also easily oxidised and give high Kappa number depending on the composition of the coating layer and the amount of coating on the paper. Some of these papers do not fulfil the requirements of ISO 9706 either, though they may have a high degree of permanence they are not suitable for library and archival purposes.
In the meantime different parameters influencing ageing must be studied. An ISO standard ISO/CD 15659 Information and documentation - Archival board - Migration test is under development by ISO/TC46/SC10/WG1. It is intended for testing of boards to be used to protect archival material. It has been found that some papers of too low quality might discolour adjacent papers. We have made tests with this method and found that some papers containing about 20 percent of CTMP causes a considerable discoloration of the blotting paper close to it, while others do not. What is the difference between these materials? In this test only the change in brightness is measured. Some experts claim that this discoloration might also decrease strength
ASTM, American Society of Testing and Materials and its Institute for Standards Research, is engaged in a multi-year research programme to create scientifically sound methods for the prediction of the life expectancy of printing and writing papers. The goal is expanding understanding of the fundamental mechanisms of paper ageing and developing accelerated ageing test methods that correlate well with natural ageing results. To ensure maximum reproducibility, papers were specially made. They cover a range of different compositions. This is interesting. This will give us indications what papers deteriorate less than others will. But will it also give better knowledge of the reactions taking place in ageing papers? As we said before what do we mean with natural ageing?