What are dyes

A dye can generally be described as a coloredas an after-treatment. It is important to note that many
substance that has an affinity to the substrate tomordants, particularly those in the hard metal category,
which it is being applied. The dye is generally applied incan be hazardous to health and extreme care must
an aqueous solution, and may require a mordant tobe taken in using them.
improve the fastness of the dye on the fiber.Look up leuco form in Wiktionary, the free dictionary.
Both dyes and pigments appear to be coloredVat dyes are essentially insoluble in water and
because they absorb some wavelengths of lightincapable of dyeing fibres directly. However, reduction
preferentially. In contrast with a dye, a pigmentin alkaline liquor produces the water soluble alkali metal
generally is insoluble, and has no affinity for thesalt of the dye, which, in this leuco form, has an affinity
substrate. Some dyes can be precipitated with an inertfor the textile fibre. Subsequent oxidation reforms the
salt to produce a lake pigment.original insoluble dye.
Archaeological evidence shows that, particularly in IndiaReactive dyes utilize a chromophore containing a
and the Middle East, dyeing has been carried out forsubstituent that is capable of directly reacting with the
over 5000 years. The dyes were obtained fromfibre substrate. The covalent bonds that attach
animal, vegetable or mineral origin, with no or very littlereactive dye to natural fibers make it among the most
processing. By far the greatest source of dyes haspermanent of dyes. "Cold" reactive dyes, such as
been from the plant kingdom, notably roots, berries,Procion MX, Cibacron F, and Drimarene K, are very
bark, leaves and wood, but only a few have evereasy to use because the dye can be applied at room
been used on a commercial scale.temperature. Reactive dyes are by far the best
Organic dyeschoice for dyeing cotton and other cellulose fibers at
The first human-made (synthetic) organic dye,home or in the art studio.
mauveine, was discovered by William Henry Perkin inDisperse dyes were originally developed for the dyeing
1856. Many thousands of synthetic dyes have sinceof cellulose acetate, and are substantially water
been prepared.insoluble. The dyes are finely ground in the presence of
Synthetic dyes quickly replaced the traditional naturala dispersing agent and then sold as a paste, or
dyes. They cost less, they offered a vast range ofspray-dried and sold as a powder. They can also be
new colors, and they imparted better properties uponused to dye nylon, cellulose triacetate, polyester and
the dyed materials.[1] Dyes are now classifiedacrylic fibres. In some cases, a dyeing temperature of
according to how they are used in the dyeing process.130 °C is required, and a pressurised dyebath is
Acid dyes are water-soluble anionic dyes that areused. The very fine particle size gives a large surface
applied to fibers such as silk, wool, nylon and modifiedarea that aids dissolution to allow uptake by the fibre.
acrylic fibers using neutral to acid dyebaths.The dyeing rate can be significantly influenced by the
Attachment to the fiber is attributed, at least partly, tochoice of dispersing agent used during the grinding.
salt formation between anionic groups in the dyes andAzo dyeing is a technique in which an insoluble azoic
cationic groups in the fiber. Acid dyes are notdye is produced directly onto or within the fibre. This is
substantive to cellulosic fibers.achieved by treating a fibre with both diazoic and
Basic dyes are water-soluble cationic dyes that arecoupling components. With suitable adjustment of
mainly applied to acrylic fibers, but find some use fordyebath conditions the two components react to
wool and silk. Usually acetic acid is added to theproduce the required insoluble azo dye. This technique
dyebath to help the uptake of the dye onto the fiber.of dyeing is unique, in that the final color is controlled by
Basic dyes are also used in the coloration of paper.the choice of the diazoic and coupling components.
Direct or substantive dyeing is normally carried out in aSulfur dyes are two part "developed" dyes used to
neutral or slightly alkaline dyebath, at or near boilingdye cotton with dark colors. The initial bath imparts a
point, with the addition of either sodium chloride (NaCl)yellow or pale chartreuse color. This is oxidized in
or sodium sulfate (Na2SO4). Direct dyes are used onplace to produce the dark black we are familiar with in
cotton, paper, leather, wool, silk and nylon. They aresocks and the indigo blue of the common blue jeans.
also used as pH indicators and as biological stains.Food dyes
Mordant dyes require a mordant, which improves theOne other class which describes the role of dyes,
fastness of the dye against water, light andrather than their mode of use, is the food dye.
perspiration. The choice of mordant is very importantBecause food dyes are classed as food additives,
as different mordants can change the final colorthey are manufactured to a higher standard than
significantly. Most natural dyes are mordant dyes andsome industrial dyes. Food dyes can be direct,
there is therefore a large literature base describingmordant and vat dyes, and their use is strictly
dyeing techniques. The most important mordant dyescontrolled by legislation. Many are azoic dyes, although
are the synthetic mordant dyes, or chrome dyes, usedanthraquinone and triphenylmethane compounds are
for wool; these comprise some 30% of dyes used forused for colors such as green and blue. Some
wool, and are especially useful for black and navynaturally-occurring dyes are also used.
shades. The mordant, potassium dichromate, is applied