DYEING is the art of staining textile sub stances with permanent colours. It was an art known and practised to a considerable ex tent by the ancient Egyptians, Phoenicians, Greeks, and Romans. The moderns have obtained from the New World several dye drugs unknown to the ancients ; such as cochineal, quercitron, Brazil wood, logwood, arnotto ; and they have discovered the art of using indigo as a dye, which the Romans knew only as a pigment. But the vast supe riority of our dyes over those of former times must be ascribed principally to the employ ment of pure alum and solution of tin as mordants, either alone or mixed with other bases ; substances which give to our common dye-stuffs remarkable depth, durability, and lustre. Another improvement in dyeing of more recent date is the application to textile substances of metallic compounds, such as Prussian blue, chrome yellow, manganese brown, &c.
Bergman appears to have been the first who referred to chemical affinities the phenomena of dyeing. Having plunged wool and silk into two separate vessels containing solution of indigo iu sulphuric acid diluted with a great deal of water, he observed that the wool ab stracted much of the colouring matter, and took a deep blue tint, but that the silk was hardly changed. He ascribed this difference to the greater affinity subsisting between the particles of sulphate of indigo and wool, than between these and silk ; and he showed that the affinity of the wool is sufficiently energetic to render the solution colourless by attracting the whole of the indigo, while that of the silk can separate only a little of it. He thence concluded that dyes owed both their perma nence and their depth to the intensity of that attractive force. We have therefore to con. sider in dyeing the play of affinities between the liquid medium in which the dye is dis solved and the fibrous substance to be dyed. By studying these differences of affinity, and by varying the preparations and processes, with the same or different dye-stuffs, we may obtain an indefinite variety of colours of va riable solidity and depth of shade.
Dye-stuffs, whether of vegetable or animal origin, though susceptible of solution in water, and, in this state, of penetrating the pores of fibrous bodies, seldom possess alone the power of fixing their particles so durably as to be capable of resisting the action of water, light, and air. For this purpose they require to be aided by another class of bodies, mor dents, which bodies may not possess any colour in themselves, hut serve in this case merely as a bond of union between the dye and the substance to be dyed. Mordants may be regarded in general as not only fixing but also occasionally modifying the dye, by form ing with the colouring particles an insoluble compound, which is deposited within the tex tile fibres. Such dyes as are capable of passing from the soluble into the insoluble state, and of thus becoming permanent, without the ad dition of a mordant, have been called substan tive, and all the others have been called adjective colours. The first principle of dye ing fast colours consists in causing the colour ing matter to undergo such a change, when deposited upon the wool or other stuffs, as to become insoluble in the liquor of the dye-bath. The more powerfully it resists the action of other external agents, the more solid or durable is the dye.
in the following details concerning the art of dyeing we shall consider principally its ap plication to wool and silk, having already treated, in the article COTTON OR CAI1CO PRINTING, of what is peculiar to cotton and linen.
The operations to which wool and silk are subjected preparatory to being dyed are in tended, 1, to separate certain foreign matters from the animal fibro ; 2, to render it more apt to unite with such colouring particles as the dyer wishes to fix upon it, and also to take therefrom a more lively and agreeable tint, as well as to be less liable to soil in use.