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Old December 15th, 2014, 03:09 PM   #2
Pierre Galafassi
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Hi Paul,

As your research has already shown, despite being one of the earliest synthetic dyes, Alizarin does not belong to any of those chemical families known for lousy light fastness on wool, like the so-called anilin dyes for example.


Alizarin is only one of about 20-30 natural red dyes belonging to a chemical family called the hydroxy-anthraquinones, which are found in various mixtures in just about all natural dyes of vegetal- and insect origin which our ancestors used to dye wool, silk and cotton, in red shades of decent fatnesses. (Besides, no, the hydroxy-anthraquinone family has no parenthood whatsoever with the chemistry of indigo. They are also applied by a completely different dyeing mechanism).

Together with pseudo-purpurin and purpurin, the alizarin molecule is the dominant hydroxy-anthraquinone in madder ( Rubia tinctoria)
It is only a secondary dye in the mix of hydroxy-anthraquinone red dyes which gives its coloring power to wild madder (Rubia peregrina) and exists only as a trace in Indian madder (Rubia cordifolia, Manjeet).
It is not found in the main red insect dyes (like the various cochineal origins or lac) in which other hydroxy-anthraquinones do the job.

When applied on alum-mordanted wool, a good light- and wet fastness is obtained with all these various mixes of hydroxy-anthraquinones.

Alizarin was the first natural hydroxy-anthraquinone molecule which was copied by 19th century chemists and the only one which was produced in very large quantities. It caused the quick disappearance of the cultures of madder and of Indian madder, all but eliminating the natural red dyes from industrial dyeing- and printing of cotton, wool and silk in Europe.
Only a few decades later, superior synthetic dyes appeared which cancelled synthetic alizarin as well from this industrial market.

Only a handful of scientists ever bothered to analyze the dye composition of a limited number of nineteenth century rugs , I can’t therefore tell whether synthetic alizarin took, or not, a large market share of the red shades in Asian carpet-making as well, during the second half of the nineteenth century. A few cases of red dyes containing pure alizarin (no significant traces of other hydroxy-anthraquinones) have been published so far, which probably indicate that some synthetic alizarine too found its way into Turkestan for example, but this does not tell us how much.

My wild guess is that this yurt-industry probably rather quickly adopted the first generation of bright, cheap and easy-to-apply tri-aryl-methan and azo synthetic dyes, which, as we well know, wreaked havoc with their very poor light- and/or wet fastness and triggered late but irate reactions from the authorities in Iran and Turkey. Before the end of the nineteen century, successive generations of better synthetic dyes appeared, specific for cotton or wool, which allowed to replace the lousy first generation and also achieved to push the natural red dyes into near oblivion.

Now, about the use of madder and alizarin in violin varnish:
I do believe that madder-root extracts were used to varnish violins already in the eighteenth century. I am ready to bet that the artisans used in fact an alum complex of the natural dye, a pigment which has a good inherent light fastness.
The same «alum complex» which a wool dyer creates inside the fibre when he applies a hot decoction of madder-root to an alum-mordanted wool yarn. There is no reason to believe that alizarin would not do the job well. Actually it is highly probable IMHO that violin makers have been trying it a long time ago. I would be surprised, though, to learn that modern pigments would not be technically superior. I suppose that Filiberto has a much better knowledge of these ancient pigments for painting and varnish, than I do.

P.S. It is important to remember that good light fastness on one substrate (wool for example) says nothing about the light fastness of the same dye on another substrate (cotton for example, or wood surface). The mechanism of light-degradation is complicated and influenced in positive or in negative, among many other things, by the chemical structure of the substrate.
Best regards
Pierre

Last edited by Pierre Galafassi; December 15th, 2014 at 03:26 PM.
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