Nationaal Archief, The Netherlands
At the National Archives, The Netherlands, a survey of the personal archive of an important Dutch statesman, Johan Rudolf Thorbecke (1798-1872) was performed. J. R. Thorbecke was an important person, he was the prime minister of the Netherlands three times and a very influential political personality of the 19th-century Netherlands.
His archive contains all his personal correspondence and documentation and is from an era, which was extremely interesting also from the papermaking perspective. Namely, in the 1840s and 50s, the technology of papermaking changed drastically. While approximately before 1850, most of the paper was produced manually and using long fibres derived from annual plants (e.g. hemp, linen and cotton), short wood-derived fibres were used afterwards. A change in the technology of sizing had even greater long-term consequences. Namely, acidity was introduced into paper in the form of alum (aluminium sulphate), which led to the relatively fast process of acid-catalysed hydrolysis. Due to this, paper lifetime was suddenly reduced for a factor of 10. The folders to be surveyed and five randomised lists with subsets of the collection were prepared in advance. The folder was selected, paper dividers were randomly inserted and the sheet where a divider was inserted was characterised if it contained text. If not, the following sheet was characterised. This is a general principle devised and applied to object selection during surveys at this institution.
After a certain paper sheet was selected, it was measured using the SurveNIR instrument. The results were calculated and surveys of the five subsets were compared.
The distribution of gelatine content demonstrates the variability of the collection – while there are very rare objects from the time before 1840s with gelatine content close to zero, their number increases afterwards. This can be linked to the change in papermaking. On the other hand, there is still a high number of gelatine-sized papers after this period, which could indicate that gelatine-sized continued to be produced and used after the year of 1850 by Thorbecke. One should take into account that good quality paper was perhaps preferred for personal communication.
The next issue of particular interest was the distribution of pH of paper in the collection. The average pH of pre-1940 papers was 7.2 ± 1.4, while this number decreases to 5.4 ± 1.2 after 1940. There is still a good number of good quality mildly alkaline papers among the post-1940 papers, though, which confirms that Thorbecke used good quality paper even after acid-sized paper was introduced in the second half of the 19th century.
To understand the values of pH obtained, one needs to take into account that the uncertainty of determination of pH using the SurveNIR instrument is in average 0.8 unit, and one should also take into account the method used for determination of pH. Namely, we used cold micro-extraction, which gives higher values for gelatine-sized papers than determination of surface pH.
Finally, we were interested in the condition of a particularly important document, the Surinam declaration of independence. Using the SurveNIR instrument, we were able to determine the pH and degree of polymerisation (DP) of the document. The pH is 6.7 ± 0.6, while the DP is 1600 ± 80. These data assured us that the document is safe and should survive many centuries to come under favourable environmental conditions.