2023-05-20 01:18:10
The first “paper” in history was papyrus, a support that was made in Ancient Egypt from the plant that gave it its name: Cyperus papyrus. In China, around the 2nd century BC. from C., a different support began to be manufactured, it was made from the remains of cotton, silk and hemp, a technique that would reach Europe in the Middle Ages.
The Chinese paper-making method basically consisted of generating an interweaving of cellulose fibers from a paste obtained after grinding some raw material -from chopping up natural fibers of vegetable origin- and mixing it with water. Subsequently, the paste was pressed and dried, thus obtaining paper.
It is white thanks to cellulose
In the beginning, everything that had cellulose was used as raw material, such as fabric waste, hemp, cotton… It would be the demand for that invention that would force the use of wood from woody plants, such as trees.
The paper is white due to the cellulose, although sometimes it can be of another shade due to the impurities or dyes that accompanies it. It was taste, fashion and the need to highlight individuality that led to the appearance of leaves with different shades of color with the help of dyes.
In any case, and despite the fact that the chemical industry is capable of offering us an infinite palette of colors, the paper that continues to triumph is white, since it allows for a greater contrast in brightness between what is written and the background.
Lignin is responsible for aging
Time has left a recognizable mark on all the supports that have been used to write throughout history. We all know that, in the case of paper, it takes on a yellow hue, becomes brittle, and also has a characteristic odor.
The fault lies not with the cellulose but with lignin, a polymer that holds the cellulose fibers together, giving the wood enough rigidity so that the trunks of the trees stand upright.
The paper-making process consists of eliminating the lignin, so that the result is as white as possible, however, it cannot be done in its entirety, for this it is necessary to add alkaline substances and increase the “bleaching” process by means of of chlorine, peroxides and sulphites.
When lignin is exposed to air, it oxidizes and changes occur in the chemical structure of the polymer, forming molecules –chromophores- that reflect certain wavelengths that correspond to yellow and brown.
For this reason, the lesser the amount of lignin in a paper, the longer it will take to yellow. This helps explain why newsprint, which is the cheapest, yellows so quickly.
Rejuvenating the manuscripts
A few years ago, scientists took a giant step forward by discovering the cellulose chromophore. It is an aldehyde, a group formed by a carbon atom that is attached to an oxygen atom through a double bond, and to a hydrogen atom by a single bond. This discovery opens the door for restorers to remove the yellowing from ancient manuscripts.
But paper not only has to fight against lignin in its aging process, it also has to deal with the action of some microorganisms capable of degrading it by hydrolysis or oxidation of cellulose through enzymes -cellulase-, which cause stains. typical of deteriorated paper. Among them are Aspergillus niger and some vibrios.
Finally, note that the Anglo-Saxons use the term “pulp” to refer to the waste of wood pulp with which a yellowish, poor-quality paper was made. From that term a type of literature was generated –known as pulp fiction- that was printed on very low-cost paper, without guillotining and that, of course, yellowed very early.
#paper #yellow #time