Lightfastness varies from pigment to pigment. Thanks to modern
techniques we are constantly able to improve the quality of
pigments. At present we have thousands of pigments to choose from.
This enables us to replace traditional pigments with only moderate
light-fastness by superior synthetically produced pigments.
The lightfastness of Talens products is indicated on tubes,
labels and colour charts by means of the following symbols:
+++ = at least 100 years lightfast under museum conditions
++ = at least 25 - 100 years lightfast under museum
+ = at least 10 - 25 years lightfast under museum conditions
º = at least 0 - 10 years lightfast under museum conditions
These degrees of lightfastness are tested under museum
Until 10 to 15 years ago the lightfastness of colours was measured using the blue lightfastness scale. This lightfastness scale consists of eight blue-coloured cotton samples, each separate piece of fabric coloured with another type of blue dye that has been fixed into the cotton fibre: the first sample indicates a very poor lightfastness and progresses incrementally to the eighth sample which indicates a very good lightfastness.
The lightfastness scale, together with the colours to be tested, are partially covered and exposed to a certain amount of highly concentrated artificial light, until step no. 8 begins to fade.
The fading for each tested paint sample is then compared with the various fades of the lightfastness scale and are given the lightfastness scale number that corresponds with the discolouration. If a colour has a lightfastness scale of 7-8 this means that under museum conditions after approximately 100 years or more the colour will either not fade at all or will not fade visibly.
However, the comparison of the tested colours with the steps of the lightfastness scale and their subsequent classification is determined by a panel of three to five people and is consequently subjective. What's more, these results have long been translated by the manufacturers of artists' paints, particularly in Europe, into different symbols, which often leads to confusion.
In order to acquire an objective assessment, as well as an unequivocal indication, research was carried out into a measurement method based on scientific standards. This research eventually resulted in the ASTM (American Society for Testing and Materials) standard D 4303.
The ASTM (American Society for Testing and Materials) has developed a method for objectively measuring the lightfastness of colour-giving substances, indicated by the ASTM standard D4303. This standard describes the method and conditions required to come to the eventual measurement result. A colour has to be exposed to a standardized amount of artificial light, both pure and mixed with a likewise standardized percentage of white (thick paint) or water (water colour). The colour (pure or diluted) with the lowest degree of lightfastness is then used as the benchmark. The classification and the associated indication are also described. The exposure to the standardized amount of artificial light is usually described as "museum conditions".
Colours are measured both before and after the light exposure according to the CIE-L*a*b* method (CIE = "Commission Internationale de l'Éclairage"). L*a*b* stands for a worldwide standardized method of colour measurement, represented by a three-dimensional colour system, where L* indicates the position on the grey axis, a* the position on the green/red axis, and b* the position on the yellow/blue axis. The colour difference is represented by the ∆ E *, calculated from: ∆ E * = √ (∆ L*)² + (∆ a*)² +(∆ b*)². The classification is determined on the basis of this ∆ E*.
If the ∆ E * is less than 4, the colours will show no visible colour change under museum conditions for approximately 125 years. According to the norm, this group is denoted by LF I. If the ∆ E * is between 4 and 8, the colours will show a slight amount of colour change after 50 years. This group is denoted by LF II. Between 8 and 16, this is LF III and so on.
The illustration shows the comparison between the classification of the ASTM and Royal Talens.
We can subdivide colour-giving substances into two types: dyes
and pigments. For the painter, an important difference between the
two is their lightfastness. Blended with paint or ink all dyes have
poor to moderate lightfastness. The lightfastness of pigments
varies from poor to excellent. The degree of lightfastness
indicates the degree to which a colour-giving substance is affected
by ultraviolet light. Ultraviolet is a constituent of both natural
daylight and artificial light. It has the property of breaking down
colour-giving substances: the colour 'fades'. The speed at which
this happens depends on the lightfastness in combination with the
quantity of ultraviolet light. Some colours fade after just a few
weeks, others only after years or not all.
A second difference concerns their solubility. Dyes dissolve in a
liquid, pigments are insoluble.
The lightfastness of dyes in paint or ink is poor to moderate.
For this reason they are not used in artists' products. For
educational uses or illustrative work lightfastness is of less
importance. An original illustration has a temporary function and
after publication can be stored in the dark. In the absence of
light the colour does not fade.
Pigments can be distinguished not only by their degree of
lightfastness but also by other properties such as opacity,
transparency and tinting strength.
When mixing, the
lightfastness of each separate colour plays a role. If, for
example, a violet is made from the poor lightfast traditional
madder (+ = 10 to 25 years lightfast under museum conditions) and
the excellent lightfast ultramarine (+++ = at least 100 years
lightfast under museum conditions), then the red colour of the
madder will fade within the set lightfastness period.
The very lightfast ultramarine, in contrast, will not change.
The red will therefore slowly disappear from the violet, which
means that the violet will change in the same period that the red
disappears. The violet will become increasing blue. The
lightfastness of the mixed colour is therefore the same as the
lightfastness of the madder.