Some other saline substances, as well as the metallic salts, are also employed as mordants. The neutral salts, sal ammo niac, nitre, and particularly sea salt, act as mordants, and modify colours, but it is difficult to ascertain the manner in which they act : salts with calcareous bases also modify colours ; but as these modifica tions are nearly similar to those which would be produced by the addition of a small quantity of lime, it is probable they are decomposed, and that a little of the lime enters into combination with the co louring particles and the stuff.
Astringent substances are often em ployed as mordants. Tan, or the astrin gent principle, having a strong affinity for cloth, and for colouring substances, is found very useful for this purpose. It is commonly prepared by infusing nut-galls in water ; the cloth is immersed in this solution, and allowed to remain till it is sufficiently impregnated with the tan. Sumach, which consists of the shoots of the rhus coriaria (a shrub that grows in the southern parts of Europe,) is often used and prepared in the same way as nut-galls.
Animal substances are sometimes used as mordants; in the process for dyeing the Turkey red, the cotton stuff is im pregnated with animal oil ; and it is pro bable linen and cotton would take other colours better, after some similar prepa ration.
Exsiccation favours the union of the substances which have an attraction for the stuff, and the decompositions which may result from that union ; but the ex siccation should be slow, that the sub stances may not be separated before their mutual attractions have produced their effect.
To judge, of the most advantageous man ner of employing mordants, we must first pay attention to the combinations which may be produced, either by the action of the substances which compose them, or by that of the colouring matter and the stuff; secondly, to the circumstances which may concur in bringing about these combinations more or less quickly, or in rendering them more or less com plete; thirdly, to the action that the liquor in which the stuff is immersed may have, either on its colour or tex ture ; and in order to foresee what that may be, it is necessary to know the pro portions of the principles which enter into the composition of the mordant, and what will he left in an uncombined state in. the liquor.
Of the action of air and light on colours.
The action of atmospheric air on co lours is chiefly owing to the oxygen it contains ; this Berthollet has shewn in some cases to have similar effects to a slight combustion; as when the air ren ders a substance yellow, fawn-coloured, or brown, which he supposes it does by the oxygen combining with the hy drogen of the stuff, and leaving the charcoal predominant, which then com municates its own colour to it. The
action of the air in bleaching he sup poses to be caused by the combination of its oxygen with the colouring matter of the stuff, which renders it soluble in alkaline lixivia ; which for this reason should always be used alternately with exposure to the air.
The changes which occur in the colours produced by the union of colouring mat ter with metallic oxides are effects com pounded of the change that takes place in the colouring matter, and of that which the metallic oxide undergoes.
The light of the sun considerably ac celerates the alteration of colours ; this, according to Berthollet, it effects, by fa vouring the combination of oxygen, and by the combustion thereby produced. Mr. Sennebier (who has published many interesting observations on the effect o light on colours,) on the contrary, attrb, butes these effects to a direct combina tion of light with the substances ; but Berthollet established his opinion by a number of accurate experiments, witich give it a decided superiority.
Colouring substances resist the action of the air more or less, accordi ng as they are more or less disposed to unite with.
oxygen, and thereby to suffer more or less quickly a smaller or greater degree of combustion : light favours this effect ; but the colouring matter, in its separate state, is much more prone to this com bustion that when united to a substance, such as alumen, which may either de fend it by its own power of resisting combustion, or, by attracting it strong ly, weaken its action on other substan ces, which is the chief effect of mor dants ; and this compound acquires greater durability, when it is capable of combining intimately with the stuff. Thus the colouring matter of cochineal dissolves easily in water, and its colour is quickley changed by the air; but when united to the oxide of tin, it becomes much brighter, and almost insoluble in water, though it is still easily affected by the air, and by oxygenated marine acid : it resists the action of these better, how ever, when it has formed a triple com pound with a woollen stuff.