Paper

Since ancient times, before the discovery of paper, people have been writing on different materials: stone, tree bark, papyrus plants (Cyperus papyrus), parchment (vellum), etc. Parchment was produced from the dermis of animal skin. Papyrus is composed mainly of cellulose (around 53–62%) and lignin (22–33%), and the main component of parchment is collagen [1,2].

However, probably the most important writing material is paper. It was invented in China 2-1 century BCE and papermaking was introduced to Europe (Spain) by Arabs around the 12^th century [3,4]. By definition, paper is composed of matted or felted sheets usually made of cellulose fibres, formed on a wire screen from water suspension, followed by pressing and drying [5]. Paper is a multi-component material and its behaviour (chemical and mechanical properties, stability,  resistance to degradation, etc.) depends upon the nature and characteristics of the paper components and upon their interactions [6,7]. Fig. 1 generally summarizes the main components of the paper [6-8].

Early European papers (before the mid-19^th century) were made from old rags (linen, cotton, hemp) that consist mostly of pure cellulose. In order to make the paper surface less absorptive gelatine or starch flour was added as a sizing agent. Sometimes no sizing was used at all (often with hand-made paper) [3,7]. Lime (calcium hydroxide) was usually added to the fibrous slurry before the beating process. It reacts with CO₂ from the air and forms calcium carbonate, which afterwards provides the alkaline reserve for the paper. The alkaline reserve is an important property of paper, defining its long-term ability to withstand acidic degradation (see below). Due to that, old papers can show alkaline reaction even today [6].

In the middle of the 19^th century, signficant changes took place in the technology of papermaking: wood-derived fibres and alum (Al₂(SO₄)₃·nH₂O))-rosin sizing were taken into use [7,9]. Due to the hemicellulose and lignin in the wood cellulose pulp, the quality of paper decreased considerably. Due to the influence of oxygen from the air, especially in the presence of even a small amount of transition metal ions, lignin oxidises easily to compounds containing chromophores and carboxyl groups. This process leads to yellowing of paper and degradation of its mechanical strength because the formed acid contributes to the hydrolysis of cellulose. Also, lignin absorbs UV radiation and makes the paper sensitive to sunlight [6,10–12]  Fig. 2a and Fig. 2b presents paper made from old rags and paper from wood-pulp.

Requirements for the permanence of papers

The resistance of paper to ageing is important both for the documents stored in the memory institutions, as well as for different enclosures made of paper. The international EN/ISO 9706 standard “Information and documentation – Paper for documents – Requirements for permanence”  defines the specifications for permanent, durable paper, which should be made of quality bleached chemical pulp with the absence of mechanical pulp (Kappa number should attain values lower than 5, which means that the paper may contain only a small amount of easily oxidized material (e.g. lignin)) and neutrally sized. In addition, it should contain as minimum 2% by mass of calcium carbonate filler, acting as an alkaline reserve (alkali reserve at least corresponding to 0.4 mol of acid per kg of paper). The pH should be in the range of 7.0 to 10.0 and tearing resistance of at least 350 mN. Optical bleaching agents are not allowed.

There are also standards for archival paper (ISO 11108) and for boxes, covers and other enclosures, made from cellulosic materials for storage of paper and parchment documents (ISO 16245).

In conclusion, the significance of any paper item should not be underestimated: everything, from works of art to the ephemera clutter of everyday life, adds to the broader picture of the past lives of our ancestors. 

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