Coloration, Mordant Dyes
A mordant is a substance applied to a textile substrate in parallel with the dyeing process that modifies the interaction of dye and fiber (and remains present in the subsequent dyed material) to provide better uptake, better fastness, and/or a wider range of colors than would otherwise be achieved.
Nevertheless, there are substances that fall into this definition which are not called mordants, and some substances described as mordants that do not fit the definition. O’Neill in the 1860s  suggested that a mordant should “exert an affinity for the fibrous material to which it is applied, and ……. an attraction for coloring matters,” and this is largely in agreement with the definition given here, but implies that the attractions are simultaneous which is rarely the case. Shore  gives a definition based on the twentieth-century commercial practice but recognizes the implicit ambiguities.
From the definition it is clear that mordants compensate for some lack of substantivity or fastness of a dye. The watershed between a dyeing world that relied heavily on mordants and one that was becoming relatively mordant-free coincides with the move from natural to synthetic dyes. Mordants were the means of getting good results from natural dyes that were not “designed” for dyeing textiles. Mordants were used for centuries to achieve remarkable results. That strong historical aspect complicates the definition: more recent understanding of the chemistry behind the dyeing process has revealed that substances formerly included in the broad scheme of mordants do not truly behave as such.
For several decades prior to the full development of synthetic dyes, chemical knowledge was increasing, and as new elements were identified, and new compounds prepared, a wide range of substances was tested as mordants to expand the coloration technology of natural dyes. Even after the first synthetic dyes were developed, their usefulness was broadened by the use of mordanting materials. A notable development occurred in the 1880s, when a new group of synthetic dyes was named “substantive dyes” (later “direct dyes”), because they were substantive toward and dyed cotton “directly,” without the use of a mordant. Since then, the skill of the chemist has obviated the need for mordants by creating dyes that not only have color but which also have the molecular configuration to provide dye-fiber substantivity, ultimate insolubility, or functional groups to react covalently with the fiber. The result is a much-simplified (and more efficient) dyeing process that readily provides fast colors. The (synthetic) dyes logically form groups of suitability for certain fibers and of similar dyeing behavior. These groups were codified in the second edition of the Colour Index in 1956 . One such group was “mordant dyes.” These dyes were applied to wool in conjunction with a chrome mordant: no other synthetic dyes routinely employed a mordant, and few mordants other than chromium salts were being used. The use of these mordant dyes has lessened as environmental concerns over the use of heavy metals, such as chromium, increase. Together with the very limited use of natural dyes, it is not surprising that mordants are a rarity in (commercial) dyeing today. Current use of mordants is largely confined to home and craft dyeing, where the purported advantages of natural dyes are emphasized, and the inefficiencies of the processes and environmental limitations of the required mordants are not. In an interesting paradox, the scientific ignorance that accompanied the historic use of mordants is becoming apparent once more. A proportion of the community of interest in applying natural dyes is unaware of the temporary function of dyebath additives (described below), and it is not unusual to read of any chemical adjunct to the dyeing process being inappropriately referred to as a mordant.
Synthetic dyes were well established before the development of synthetic fibers, and thus when such fibers were introduced, dyers had mordant-free processes with which to dye them. With one or two exceptions, mordants have been almost exclusively associated with the dyeing of natural fibers. In some cases when materials have been used to improve the interaction of dyes and synthetic fibers, they have not been referred to as mordants, even though they fulfilled essentially the same function.
In a dyeing process, dyes are soluble (or sparingly soluble) and a (textile) substrate (fiber, yarn, fabric) absorbs the dye through forces of attraction from an external solution  ( Coloration, Textile). The sum of the attracting forces is referred to as substantivity. The concept of “substantivity,” relating to dyes that impart color without the use of a mordant, is attributed to Bancroft  and significantly predates the development of synthetic dyes, even though inherent substantivity is rare in natural dyes. Substantivity also contributes to the subsequent resistance of the dye to removal in use ( Coloration, Fastness).
A dyer seeks to achieve a steady and even uptake of dye by the substrate. For a given dye/substrate/machine, this is achieved with control of temperature, agitation, and liquid volume, augmented with dyebath additives. Such adjuncts to the dyeing process provide the correct ionic strength (as electrolyte), pH (acids and bases), and moderation of the dye-fiber-water interactions (often surfactants). While it may not be obvious, similar interactions take place in textile printing with dyes, albeit in the confines of a small volume of print paste, which will also include substances (“thickeners”) to control its rheology and balance the ready transfer of paste onto the fabric with the need to maintain a sharp printed mark. These various dyeing and printing adjuncts, however, do not fall under the definition of mordants. They remain within the exhausted dyebath or print paste at the end of the process and are lost from the colored fiber in any subsequent rinsing.
As mentioned earlier, the use of mordants was and is largely confined to the use of natural fibers. These fall into two distinct groups: protein (wool and silk) and cellulosic (chiefly cotton and linen). Mordants behave somewhat differently for each of these two groups of fibers. Protein fibers comprise many different amino acids with a range of functional groups that provide binding opportunities for colorants of all kinds . Natural dyers overwhelmingly prefer to dye wool for this reason. Mordants are/were nevertheless widely used to provide better fastness and to broaden the range of colors obtainable from a limited number of natural dyes. Cellulose lacks this natural affinity for colorants and, with the notable exceptions of indigo (which does have substantivity, albeit low), turmeric, archil, saffron, and annatto (which have fastness limitations that make them of limited value), natural dyes do not readily dye cellulose. Mordants provide the main means by which cellulose may be satisfactorily dyed with natural dyes.
Over time, expert European dyers learned how to apply these dyes with the appropriate mordants to give bright and fast colors, versus those that were less satisfactory. Thus Venetian dyers were either of greater or lesser arts. French dyers were “au grand teint” and “au petit teint” . In each case the former were the more expert. Global exploration would eventually provide better natural dyes. Woad was the original blue in Europe, replaced with indigo (the same essential colorant) when that became available. Reds and purples were from madder and kermes, the latter replaced when cochineal from Central America was discovered. Yellows were from weld or Persian berries, later from quercitron found in North America. Dye woods (sappan, brazil, cutch, fustic, logwood) were also important. Other than woad or indigo, these were used in conjunction with a mordant. The inventive chemistry that allowed these natural dyes to give good results when they were all that was available today allows craft dyers and researchers to reexamine a wide range of plants as sources for natural dyes in the supposition that these dyes are somehow more “sustainable.”
Mordants for Cotton and Cellulosic Fibers
Since few natural dyes display substantivity toward cellulosic fibers, the use of mordants is essential to achieving satisfactory dyeings and prints with those dyes. While mordants were being widely used, the understanding of the chemistry behind their use was limited: as knowledge increased, the use of mordants was declining. Two sources [10, 11] give a valuable survey of the use of mordants on cellulosic substrates when these were being widely used. The most successful mordants were metallic salts that could be applied as a soluble salt, rendered less soluble, and subsequently formed a complex with the dye. For dyeing a solid color, the fabric would be padded with this metal salt. For printed designs, the metal salt would be incorporated into a print paste. Such a metal salt, if it remained soluble, would redissolve in the subsequent dyebath: the dye-mordant complex formed in the bath would tend to deposit on the textile surface and later be prone to rub off. Thus some means of reducing the solubility of the metal ion and keeping it in place on the substrate as the dye-mordant reaction took place in the dyebath was required, especially in a print where reserve of the unprinted (white) areas was essential .
A soluble aluminum salt such as aluminum sulfate or aluminum potassium sulfate (a.k.a. potassium alum) could be neutralized with carbonate or bicarbonate to produce a “basic aluminum sulfate” or “basic alum,” respectively. Padding with this, followed by treatment with ammonia, would precipitate aluminum hydroxide on the fiber. Alternatively, the mordant metal could be precipitated as the phosphate, arsenate, or silicate .
Co-application of a complex anionic substance would provide insoluble complex salts. Tannins from a range of sources were ubiquitous in this regard, from galls, myrbolam, cutch, or (later) purified tannic acid, resulting in a deposit of aluminum tannate . The effectiveness of “mordants” based on modified vegetable oils (usually castor or olive oil) was probably due to a similar mode of action. (Note: cotton will absorb tannic acid from solution, and cotton so treated does dye more readily, so tannic acid does have a moderate effect as a mordant by itself.)
Salts such as aluminum acetate could be “aged” in moist conditions under which acetic acid would be lost, generating an insoluble hydroxide. A “dunging” (originally with cow dung, later with synthetic alternatives such as sodium phosphate or arsenate) process would both complete the fixation of the mordant and remove unfixed mordant. This process was especially important in printing .
Given the above it is not surprising that nineteenth-century references to mordants usually include a similar application of a metal salt that would subsequently be reacted with a second metal salt to generate a colored inorganic compound. For example, a “lead mordant” (lead acetate) applied by padding and drying would be passed through a bath of sodium or potassium dichromate to produce insoluble yellow lead chromate . This precipitation reaction is sufficiently rapid that the insolubilization of the original “mordant” was not required. This use of the word mordant is not current.
Excluding, therefore, the use of the word mordant as a component of a precipitated mineral colorant, aluminum and iron were used most extensively as mordants for natural dyes on cotton in both dyeing and printing. Stannic salts were occasionally used. Both iron and aluminum could be produced as the acetate. Iron could be directly dissolved in pyroligneous (acetic) acid, while a reaction between aluminum or ferrous sulfate and lead acetate or calcium acetate would precipitate the lead or calcium sulfate and leave aluminum or ferrous acetate in the supernatant liquid. These acetate solutions were referred to as purple liquor (for iron) and red liquor (for aluminum), based on the colors they would produce in a madder dyebath . Tin salts were common adjuncts to aluminum salts (where they prevented dulling by the presence of small amounts of iron salts). Chromium salts were less easy to precipitate in the fiber and were not extensively used, other than to provide a black color with logwood in which processed dye and mordant were often applied together (compare the dyeing of wool by the metachrome method discussed below). Texts of the time also include a range of other metal salts among the discussions of mordants for cotton, but indicate that their function is often that of an oxidizing (Cu, Cr) or reducing (Sn) agent.
The immersion of a mordanted fabric in a dyebath would allow the formation of a dye-metal complex. (This is discussed in greater detail under mordant dyeing of wool, below.) In a print, the unmordanted dye of the dyebath would inevitably stain the unmordanted portions of the cloth, and the fastness of the dye-mordant complex in the colored areas is indicated by its ability to survive the subsequent “clearing” of dye from white parts of the print by extensive washing processes.
Given the difficulty in dyeing cotton and other cellulosic fibers with natural dyes, it is not surprising that as scientific knowledge increased in the early nineteenth century many attempts were made to modify those fibers to improve their dyeability. Dyers of the nineteenth century attempted such modifications to cotton. The substances have been described as “mordants” , but if these modifications are carried out in bulk, separately from the dyeing process, they stretch the definition somewhat. The modifications were presumably prompted by the greater substantivity of dyes for protein fibers and mostly consisted of depositing protein material onto cotton. Albumen from eggs or blood, gelatin, and casein and lactarine from milk were used in these attempts. The efforts were classified as “animalizing” cotton. Such modifications continue to this day: cotton can be pretreated to have cationic groups and readily takes up anionic dyes . Cotton so treated is not referred to as being mordanted, however.
A new category of mordants for cotton was required when the first synthetic dyes were introduced following Perkin’s synthesis of mauveine in 1856. For almost 30 years, these new dyes comprised what are now classed as acid and basic dyes, which have no substantivity toward cotton. Dyers were obviously eager to make use of the bright colors they offered, and the precipitation of tannic acid with salts of antimony (in the form of antimony potassium tartrate, or “tartar emetic”) on the fiber provided an anionic substrate with which cationic basic dyes would interact sufficiently well to provide substantivity and fastness .
A subscript in the saga of cotton and mordants occurs in efforts in the twentieth century to improve the fastness of the direct dyes. These efforts comprised aftertreatments to a dyed material. Mordants typically being applied before the application of dye, these aftertreatments are not usually regarded as mordants, but the interactions have factors in common with those undergone by mordants. Anionic direct dyes could thus be aftertreated with cationic compounds to form a complex with reduced solubility and thus better fastness to washing. Direct dyes with functional groups capable of chelating metal ions might be aftertreated with copper or chromium salts: a dulling of shade would be compensated by improved fastness to light and (less so) to washing. This technique is technically indistinguishable from the application of mordant dyes to wool by afterchroming but is not regarded as an example of the use of mordants .
Mordant Dyeing of Wool
As discussed earlier, the use of a mordant for applying natural dyes to wool has less to do with achieving substantivity than it was to provide better fastness than the unmordanted dye and to extend the range of colors available. Mordants for wool were (and are) overwhelmingly metallic compounds: no parallel with the animalizing reactions carried out on cotton was developed. The non-mordant cationization of cotton is perhaps echoed in comparatively recent research to render wool dyeable with disperse dyes .
The same (natural) dye will provide distinctly different colors depending on which metallic salt it interacts with. The functional groups on the polypeptide chains that form the keratin of which wool is composed include groups, notably carboxylic acid, with which metal ions can form bonds, holding them in place during subsequent interaction with the dye. Thus the insolubilization required for cotton mordants is not needed. For those reasons, the number of metal salts useful as mordants is greater with wool than with cotton and includes tin, copper, and chrome. Wool additionally has sulfur-containing amino acids that will reduce (or maintain in a reduced form) polyvalent metals: thus chrome is applied as dichromate (CrVI) and interacts with dye as CrIII. Tin is applied and interacts as SnII. Dye recipes often include mild reducing agents (tartrate, oxalate) to assist this.
The chelation reactions on the fiber between chrome and synthetic mordant dyes have been most widely studied, but the principles involved are the same as those between other metals and natural dyes. Dyemakers simplified the mordant dyeing process by carrying out the dye-metal binding in dye manufacture and selling “premetallized” dyes (now known as metal-complex dyes), and the chemistry of this complexation is more easily studied in the absence of the fiber [2, 18, 19]. (While these are overwhelmingly complexes involving chrome, cobalt is also used, iron and aluminum have been examined as less polluting alternatives, and copper complex dyes are used on cotton.) 1:1 dye-metal complexes were introduced in the early twentieth century, 2:1 complexes in mid-century. Chromium has a coordination number of six: the dye molecule in 1:1 chrome complexes provides three ligands: the remaining three ligand sites are occupied typically by water. It has been suggested that these sites can interact with groups on the wool fiber, although evidence is lacking. If they do so, however, this would represent one of the few instances where the common mental model of mordant as a bridge linking dye and fiber is relevant. The metal atom in a 2:1 complex has no such spare binding capacity and achieves fastness via large molecular size and low solubility. In theory, either 1:1 or 2:1 complexes might be formed by a mordant dyeing process, but evidence suggests that the 2:1 complexes predominate. Whether produced on the fiber or as a metal-complex dye, the resulting complex is more stable to light and has limited solubility and thus good fastness to wet treatments.
The traditional method of mordant dyeing of (natural) dyes on wool involved pretreating the wool with a solution of the metal salt and subsequently introducing the mordanted wool into a dyebath. This sequence is still used for dyeing natural dyes on wool today. When synthetic (mordant) dyes were developed, their varied chemistry could generate the range of shades required, and the need for multiple metal mordants declined: chrome (as dichromate) provided the best fastness and this became the main choice for mordant dyeing using the “chrome mordant method” and mordant dyes were not referred to as such, but as “chrome dyes” [20, 21]. Interest in making the processes more efficient took advantage of the independent substantivity of both dye and mordant. They could be applied together (the “metachrome method”) in a manner reminiscent of the chrome-logwood black dyeing of cotton. Eventually the process in which the dye was applied first and subsequently treated in a bath of chrome became the most widely used as the “afterchrome” or “topchrome” method. As discussed above, this is technically indistinguishable from the aftertreatment of a direct dyeing on cotton with a copper salt.
Given the similar dyeability of nylon to wool, and the search for fastness in nylon dyeings, it is not surprising that chrome dyes have been applied to nylon. The lack of reducing groups in nylon means that a reducing agent is included in the process to convert CrVI to CrIII . Chrome dyes are rarely applied to nylon today. In an interesting twist, the tannic acid/antimony mordant for basic dyes on cotton has been applied as an aftertreatment (“back-tanning”) to improve the fastness of acid dyes on nylon, although no interaction with the dye is suggested.
Acrylic fibers, introduced in the 1950s, were initially difficult to dye in dark shades with acceptable fastness. Before the development of modified basic dyes, an early solution was the so-called cuprous ion method in which the fiber was treated with a solution of copper ions: they are absorbed, and the fiber is then dyeable to dark shades with acid and direct dyes. A number of variations were published, including the application of dye and copper ions simultaneously and the use of reducing agents, but the process was superseded before becoming a standard dyeing method. This is clearly a mordant dyeing technique, although none of the descriptions of the time refer to it as such .
Mordants have a long history in enhancing the coloration of textile fibers. They are essential in the dyeing of cellulosic fibers with natural and basic dyes and improve the fastness and color range obtainable on wool with natural dyes. With the advent of synthetic dyes, their use became restricted to the chrome dyeing of wool, and they have found little value in any dyeing of synthetic fibers. They are rarely used in large-scale commercial dyeing today, but are widely used among home and craft dyers for whom the use of natural dyes remains popular.
While the details may sometimes be hard to unravel from historic or unscientific descriptions/interpretations, the majority of mordants are metal ions (most notably of Cr, Al, Fe, Sn, Cu) that react with dyes to form coordination complexes of low solubility and better fastness. Other useful mordants are those that interact ionically with dyes, again forming a lower solubility complex.
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