Notebook

Notebook, 1993- MATERIALS & METHODS - Pigments

Permanence

The highest possible light-fastness must be expected of high-grade artists' pigments, for many of the works created with them are meant to last for many generations. Unfortunately, quite a few ignominious examples prove that this expectation is not always fulfilled. During the nineteenth century a well-known German paint technologist, Adolf Wilhelm Keim, attempted to compile a range of standard pigments. This was followed by a list of pigments, recommended as reliable, which was given in Max Doerner's book The Materials of the Artist. Contrary to the frequently heard opinion, it was not commercial interest, but rather the ignorance and thoughtlessness of the artists themselves, that caused the list of pigments to be ignored in practice. Some color manufacturers did their best to fulfill the bizarre, technologically impracticable and even dangerous requests of a few painters, but such practices were not to the advantage of the consumers. Other manufacturers had to follow to remain competitive, and as a result we now have a great number of colors of which many do not reach the standard that painters should expect. Artists are spontaneously creative and, seized by an idea, feel compelled to achieve with their material the effect desired at the moment. Only rarely, and usually with reluctance, will they think about the physical consequences of their choice of materials and of their manner of application. Farsighted painters run the risk of being jeered at and looked down on by their colleagues. The old masters were more conscientious in this respect, without impairing their temperament or artistic intuition. [p. 59]

Shortly before World War II, Germany was ready to pass a law covering the production of materials. However, it became evident that in order to standardize p pigments, enormous technical preparations would be needed. Efforts in this direction dragged on until the end of the war. Owing to the political and economic situation, no final result was achieved. In recent years two working groups within the German Standards Committee [DIN] have again taken up the matter, with the object of providing the consumer with safe colors under some quality seal. It has not been decided whether the present term, "artists' color," will be retained for high-quality pigments of the greatest possible light-fastness.

Artists' -color manufacturers usually express their light-fastness ratings by asterisks:

Apart from the fact that the last three designations can be interpreted differently by individual users, these descriptions are a relatively rough classification.

For commercial paint the color industry employs the wool scale used in international trade. The German Standards Committee has taken over this classification and has applied it to pigments. The division into eight levels is considerably more accurate than the four classifications shown above and can also be expressed in definitive adjectives. These are according to DIn 53952:

There have always been some pigments known to the paint trade that would justify an extension of this scale to grade 10. This has not been agreed upon so far, because experts are very reluctant to use the highest possible adjective, "absolutely light-fast." However, pigments that have successfully withstood 2,000 years' exposure to bright southern sunlight on exterior or interior walls without roofs undoubtedly deserve this praise. For grade 9 the term "perfectly light-fast" might be used, as it is in our list of pigments. [The word "perfect" can be intensified by comparison, but not the word "absolute."] As a rule artists are justified in demanding a light-fastness of 8. Grade 7 already means a limitation, while grades 5 and 6 are accepted only as a necessary concession to the behavior of certain pigments in extreme dilution or mixed with a great deal of white. [Unmixed alizarin madder has a light-fastness of 7.] A mural on an exterior wall will naturally be subjected to higher light intensities than a panting on the wall of a room, especially since the glass of the double windows customary in our latitudes absorbs a considerable portion of damaging radiation of which, incidentally, ultraviolet is not the only deleterious factor. Commercial artists do not need the same strict standards as do artists and conservators--nor do students of academies or pupils of elementary or trade schools. [p. 60]

While on this subject, a few words on the effect of light from fluorescent tubes would be in order. At the suggestion of the author's institute, measurements were taken with the active cooperation of the Osram Research Laboratories. They revealed that commercially available discharge tubes emit proportionally far less ultraviolet than natural daylight. The energy levels were so low that the actual values could only be estimated. Whenever actual bleaching of colors has been observed, distances between lamp and object were abnormally short, and heat was involved.

The List of Pigments also contains particulars concerning acid or alkali resistance, and occasionally resistance to weathering and hydrogen sulfide present in city air. Ultramarine is attacked even by weak acids. This can lead to noticeable discoloration of exterior walls. Lead white blackens on exposure to sulfide fumes.

On exterior frescoes cadmium yellow is decomposed to white cadmium carbonate by carbonic acid in the air. This is not due to the effects of light but is a chemical reaction. [pp. 59-60]

[Wehlte, Kurt. The Materials and Techniques of Painting. Translated by Ursus Dix. New York: Van Nostrand Reinhold Company. 1975.]

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