One of the decisive factors involved in creating a paper project is to choose a bright colour that represents the concept perfectly. However, there is another aspect that is sometimes overlooked but is no less essential if we are to obtain good quality beyond the initial visual impact: that it keep its hue over time.
Colour ageing is one of paper’s greatest enemies, since with time its brightness will either be lost or dulled by external agents that gradually modify the paper, such as humidity or heat. This is because paper is alive, as Guarro Casas is wont to repeat: we cannot prevent it from ageing, although we can take certain precautions in the manufacturing process to improve its durability.
Therefore, four different factors are involved in keeping colours vivid or bright and they must be monitored in the manufacturing process in order to achieve a superior outcome.
The first one is the use of water with neutral pH during production. pH establishes the degree of acidity or alkalinity in an aqueous solution, indicating the concentration of hydrogen irons, and is measured on a scale of 0 to 14. Solutions with values below 7 are acid (with a greater number of hydrogen ions) and those above it are alkaline (fewer hydrogen ions). Neutral pH is 7, meaning that it is in equilibrium.
But how does it impact the preservation of colour? This is due to its degree of acidity. Acid is the cause of the material’s natural degradation and this process is what produces “yellowing”. In order to prevent time from taking its toll on a book or a packaging made with this material it should be manufactured at a neutral pH to prevent its subsequent acidification and thus help to preserve the paper.
Not everything depends on the composition of the water used when paper sheets are formed, as the way the colour is added also has an influence. Mass pigmentation, unlike surface painting, prevents the hue from dulling quickly with time, since the wear on the outer part of the material does not affect the pigments or colourants added since they are evenly distributed throughout the fibres inside. For this reason, all Guarro Casas papers are mass-pigmented to guarantee that the paper will maintain its brightness from the first time it is touched and over time.
Another factor that affects hue brightness is bleaching. The pulp used for production is supplied to the factory pre-bleached to ensure that it delivers the best possible colour characteristics, although in some cases optical bleaching agents can be added so that certain whites will be more luminous. For example, when recycled fibres are used, the pulp is supplied with residual colours that need to be eliminated by adding these components to achieve a whiter finish.
However, although the initial result is good, the properties of optical bleaching agents gradually diminish over time and the paper ultimately turns yellow. On the other hand, a natural white maintains its hue much better over the years.
To ascertain whether a material contains these compounds it can be placed under a black or UV light: if it shines, it means that these compounds have been added to yield a brighter colour. This is because they absorb ultraviolet light between 340 and 400 nm of the electromagnetic spectrum and re-emit it in the blue region, which is in the 420-470 nm spectrum.
LONG LIFE certification
The LONG LIFE certification is also linked to the passing of time and is the only one that guarantees the prevention of long-term yellowing, among other things. In general, it certifies compliance with the requirements of the ISO 9706 which specifies paper permanence criteria. It makes provision for two of the factors we have addressed up until this point: that the water used in the process must have a pH of between 7.5 and 10 and that the alkaline reserve applied does not surpass 2%.
Although this certification endorses a paper’s quality in the manufacturing process, which can be regulated through the components used, proper preservation depends on the subsequent use made of the paper once the certification has been awarded. This second part includes circumstances such as the humidity and light conditions in which the paper object will be stored, which can be highly variable depending on the environment and are also difficult to control.
Beyond ageing: metameric colours
However, besides these four factors that may cause the hue to change, there is another concept that may impact the way that we perceive it and which must be taken into account. Metameric colours are those which, when their coordinates are measured with the help of a colorimeter are an exact match although visually they are perceived differently.
Moreover, when observing a paper, certain conditioning factors (such as the temperature of the light, the observer’s eyesight, geometry or angle of observation, etc.) that may change the way we see it must be taken into account. Therefore, for example, we will not see the same colour under natural light as in a room with LED or fluorescent lighting, as we explained in a previous post about the visual perception of colour.
As we have already seen, many elements impact not only the formation of colour and how it is preserved over the years, but so too does perception affect the hue perceived by our eyes. A meticulous production process with suitable components is the best way to guarantee a paper’s quality and that it will remain in perfect condition for a long time, even with repeat use.