This article was firstly published on Turkotek, a non-commercial site devoted to collectible weavings, where rug enthusiasts can connect.


Fibre
Chemically speaking, wool is keratin, a protein copolymer containing about 17 different amino-acid monomers. The main elements are cystine, leucine, glycine and glutamic acid. Covalent cross-linking of adjacent cystine residues by disulphide bonds is a major factor for the mechanical properties of keratin fibres. The bond fragility to high temperature (over 90-100°C), alkaline pH, reduction or oxidation, must be taken into account in dyeing.

The rather strong disulphide bonds (-S-S-) are supplemented by weaker hydrogen bonds between -NH- and -CO- groups of adjacent keratin chains, ion-ion interactions between amino groups (in their protonized cationic form) and carboxylic acid groups (in their anionic form) which are part of the keratin macromolecules, and hydrophobic bonds between adjacent hydrophobic aliphatic chains. The unusual elasticity of the wool fibre in its relaxed form is explained by keratin's natural folded state.
    
Like most natural fibres, wool has a heterogeneous structure. Its main characteristics include a hydrophobic outer cuticle (scales) and strong, highly oriented fibrous bundles embedded in amorphous protein remnants.

The quality of wool, its textile and dyeing properties are determined by fibre fineness, length, scale structure, natural shade, brightness, cleanliness and freedom from damage. It depends on many factors, mainly sheep breed, age, health and stress of the sheep, season of shearing, body part it comes from (finest comes from shoulders, coarsest from breech), the climate it was exposed to, food, the thoroughness of fibre scouring (full or part removal of lanolin) etc. Central Asian nomads distinguish and have specific names for dozens of wool qualities. In general, they prefer semi-fine and coarse (25-35 microns) spring wool for carpet weaving. The degree of scale abrasion, (due to weathering, to sheep peregrinations through shrubs, to a modern chemical treatment like chlorination, or to an antique one like wool fermentation) has a large impact on wool dye-ability (in particular on levelness, depth and reproducibility of shade), on its tendency for “felting”, on its softness and brightness, etc.

The pile wool of many classical Ushak rugs shows a significant scale opening and exfoliation, probably caused by 8-15 days cold fermentation of the wool under mildly acidic pH in the presence of some alum, before actual mordant dyeing. (Sources: Dr. Manfred Bieber, 7. ICOC, June 1993 and recently confirmed by my friend, Marc Roy, a carpet weaver and expert user of natural dyes.)

Scales have a more hydrophobic and more compact (oriented) structure than the inside of the wool fibre and therefore create a barrier to dye penetration into the fibre, retarding its diffusion. Scales also differentiate between dyes, favouring absorption of the most hydrophobic and small ones, slowing down and decreasing the uptake of the most hydrophilic and bulky ones.


Dyes and dyeing methods

Wool fibre heterogeneity makes level dyeing quite difficult to achieve. This explains why dyeing “in piece” (after a textile is woven) or as a yarn (for example, wool used for the pile of hand woven rugs) seldom result in dyeing evenly. Wool is mostly dyed in form of loose fibre packages (so called “loose stock” ), in which case several batches are carefully blended after dyeing and before yarn spinning, thus minimising the effect of any poor levelness. Colour differences on fibres (some call them micro-abrash) are masked by this blending. Piece- and yarn dyeing are only performed with the best diffusing (levelling) types of dyes. Good levelness requires good dye diffusion, which means "easy in" but, unfortunately, also "easy out". As a consequence, piece- and yarn dyed wool (using rapidly diffusing dyes) has a lower wash- and wet fastness than loose stock-dyed wool (dyed with slowly diffusing dyes). Exceptions to this rule are when wool yarn is dyed with modern reactive dyes.

Since the invention of the first synthetic dyes, ten different types have been used for dyeing wool. Five of them are now fully obsolete, including four of the only five types ever used for dyeing the yarn for hand woven rugs.

The first synthetic dye is Saxon blue (1743, Barth), a natural indigo sulfonated with sulfuric acid (and, incidentally, a lousy dye for wool). It took another century to really get the eventful history of synthetic dyes started. The story of teenager Perkins inventing the phenazinium cationic dye, mauvein, in 1856 by mistake, and of Hoffmann and friends creating a whole new industry based on this invention, is well known by all carpetologists. Only a few of the early synthetic cationic triphenyl methane dyes (misnamed "aniline dyes"), were probably ever used to dye wool (a few bright magenta-, violet-, blue- and green dyes), but all had poor wet-fastness and disastrous light-fastness (rating 1, at best 1-2, of the 1 to 8 ratings on the so-called Blue Scale), because they turned beige or grey in the shortest time. It appears that they were wreaking havoc in some weavers workshops for at least five decades (which rather beats me!).

In 1858, Griess made a much more important discovery, triggering the explosive development of our modern dyestuff and pigment industry: the diazotisation of aromatic amines (such as anilines or naphtylamines) and their coupling with aromatic bases (such as phenols or naphthols), forming a so-called azo bridge (-N=N-). This creates large systems of conjugated double bonds and, therefore, strong chromophores (coloured molecules).

With dozens of suitable colourless building-blocks (aromatic amines and bases) to play with, chemists soon synthesized hundreds of strongly coloured combinations. Most of these new molecules included one or several sulfonic acid groups (-SO3H) making them soluble in water and leading to various new classes of dyes. The first was the so-called acid dyes, which were misused for dyeing rug yarn as early as 1865:

1. These too small and too hydrophilic molecules had insufficient affinity for wool and could not form stable metal complexes by mordanting, unlike the best red and yellow natural dyes. They soon became infamous for severe running (especially the di- and trisulfonated reds).
2. This problem was compounded by the poor light fastness of most red, scarlet and orange elements (2-3 or 3 on the Blue Scale).

Despite these serious handicaps, some carpet dyers continued to use Orange II, Ponceau 2R and 6R, Amaranth, etc., at least until 1930, attracted by their low cost and by their much brighter shades than natural mordant dyes (very bright blue acid dyes appeared later, around 1900, which explains why indigo kept its importance during several decades).  Unfortunately, some carpet weavers used and abused this opportunity to make kitschy shades. In this case (and only in this case) a trained eye can tell that no natural dye was used. These acid dyes were the ancestors of modern levelling acid dyes.

In 1868 Graebe and Perkins developed industrial scale processes for synthetic alizarin (1,2 di-hydroxy anthraquinone), the main and best natural mordant dye in madder. Synthetic alizarin was a brilliant commercial success and quickly replaced madder in most wool markets (including wool for rugs), which it shared for the next 40 years with quite lousy, but cheaper and brighter acid reds.