Flax must undergo several prepara tions before it is fit to receive the dye ; the first is the watering, by which the fi brous parts of the plant become dispos ed to separate, so as to be rendered fit for spinning.
In watering, a glutinous juice, which holds the green colouring part of the plant in solution, and which is the me dium of union between its cortical and ligneous parts, undergoes a greater or less degree of putrefaction, according to the mode of conducting the operation. This process is performed to the great est advantage in pits situated on the banki of rivers, where the water may be chang ed often enough to prevent a degree of putrefaction that would injure the flax, and be prejudicial to the workmen, yet not so often as to hinder the degree of putrefaction necessary for rendering the glutinous substance soluble in water. After watering, the flax is dried, and the ligneous parts separated by a mechanical operation.
Some have proposed the mixing a small quantity of caustic alkali with the water, to increase its solvent power, but it ap pears from Dr. Home's experiments, that the alkali retards the operation, and ren ders the flax liable to break. But after the watering and drying, alkaline sub stances dissolve the greatest part of the colouring matter, on account of the change it has undergone from the expo sure to air and light, and the consequent absorption of oxygen.
The processes published by the prince of S. Sever, for obtaining fine dressed hemp, depend on the solution of the co louring matter by alkali. He directs that dressed hemp be lixiviated in a so lution of two parts soda and one of lime, then impregnated with soap, and kept in digestion ; and afterwards well washed and hackled; but in this process only that portion of the colouring matter is dissolved, which would have been carried off by the first leys used in the beginning of the bleaching of the cloth. The great fineness given to it probably can not be produced, but at the expense of the length and firmness of the fila.. ments.
A clergyman of the department of Somme, in France, employs a process not liable to the inconveniences caused by 'eying the dressed hemp. He waters the hemp as soon as it is pulled, and separateS the cortical part by a peculiar operation immediately after the watering, and hay ing soaked it in a weak solution of black soap, he washes it with great care; pre vious to the drying, the colouring matter, (which would afterwards have been so luble only in alkali) may be dissolved and extracted by water, with the addition of a small quantity of soap ; the hemp be comes much whiter, and divides better and more minutely, without, however, having been injured ; and the leys pre paratory to the bleaching become unne cessary. Thread and linen contain then
a colouring substance, most of which may be extracted by simple ley a, but there is a part of it, which is really combined with the vegetable fibres, and which can only be taken away by the destruction of its nature, effected by the combustion it undergoes during its combination with oxygen. Thread loses, by the operations employed in bleaching, from one-fourth to one-third of its weight.
Flax, or linen, intended to be dyed, must be subjected to the sanie operations of scouring: aluming, and galling, which cotton undergoes.
The well known greater difficulty with which' linen, cotton, and silk, take dyes than wool, have been accounted for by supposing the pores of their fibres to be smaller ; this, however, appears not to be true, from the greater quantity of co louring matter which they absorb. Un bleached cotton is always preferred for dyeing Turkey red, because in this state its colour is more permanent. The same thing is observed of raw or unscoured silk, which is found to combine more ea sily with the colouring' matter, and to re ceive a more permanent colour in this state, than after it has been scoured and whitened. This has been accounted for also on mechanical principles, but it more probably is owing to the difference of the affinity, which exists between the co louring matter and the substance separat ed from the silk or. cotton in bleaching Scouring. This substance acts probably the part of a mordant, and having a ptronger affinity for the stuff and the co louring matter, than the stuff has for the latter, the colour communicated is more durable, when the silk or cotton is dyed in the unbleached or unscoured state.