DYEING. 



243 



Of Mor- 

 dants. 



Astrin 

 gem*. 



likewise constitutes the basis of black colours ; so that 

 it may justly be regarded as one of the most useful sub- 

 stances employed in dyeing, whether in producing co- 

 lours by itself, or in acting as a mordant to madder red, 

 with tannin, or the astringent principle. 



Saltsof iron. 2'26. The mordant of iron is applied under different 

 forms. Some manufacturers employ the sulphate with- 

 out any addition ; others compose it by dissolving iron 

 in vinegar; some add to it a decoction of rye flour, 

 while others mix it with urine, herring brine, &c. The 

 longer the composition is kept, the better it becomes. 

 At present, instead of the sulphuric, acetic, or other 

 acids, is substituted, as we formerly observed, the py- 

 roligneous acid, which differs from the acetic by hold- 

 ing in combination a portion of empyreumatic oil. 



227. Mordants are not procured from the earthy and 

 metallic bodies alone; for, under certain circumstances, 

 vegetable and animal substances serve as mordants for 

 each other. Thus, in the complicated process for dye- 

 ing Adrianople red, the stuff is first impregnated with 

 an animal substance ; the astringent principle is then 

 applied, and after this preparation, the cotton is pre- 

 sented to the aluminous mordant. In this case, there- 

 fore, the mordant is a triple compound of oil, the as- 

 tringent principle, and alumine. The astringent prin- 

 ciple merits particular consideration, not merely on ac- 

 count of its extensive use as a mordant, but also as a 

 colouring principle. The class of substances included 

 under the head of astringent principles, is rather vague- 

 ly defined. Frequently some slight resemblance in 

 taste only has been attended to, and under the name of 

 astringents, alum, and many vegetables possessing very 



lilar properties, have been confounded together, 

 both in medicine and the arts ; and more frequently 

 still, every substance which renders a solution of iron 

 black, has been considered as astringent. It does not 

 appear, however, that either the property of corrugating 

 the animal fibre, or producing a dark precipitate with 

 the salts of iron, is sufficiently distinctive of the sub- 

 stances to which the term astringent is usually applied. 

 Many vegetable products, which are decidedly astrin- 

 gent to the taste, afford no black precipitate with the 

 salts of iron ; and on the other hand, several substan- 

 oes which yield a copious black precipitate with the 

 salts of that metal, exhibit no traces of astringency. 

 The substance frorJi A'hich the astringent principle is 

 usually extruded, is an excrescence formed on the 

 branches of the oak. and known by the name of gall- 

 nut: (See C'IIFMISTBV, p. 6'1.) As this is almost the 

 only substance employed to afford the astringent prin- 

 ciple for the purposes of dyeing, we- shall confine our 

 fi'ivrvatiiins respecting astringents chiefly to its proper- 

 ties. 



228. There are different kinds of the gall-nut ; some 

 inclining to white, yellow, green, brown, or red; others 



'i-( olonred, or blackish. They also differ greatly 

 in magnitude, and are either round or irregular, heavy 

 or light, smooth or covered with protuberances. Those 

 which are small, knotted, and heavy, are reckoned the 

 best ; they are known by the name of Aleppo galls, 

 and come from Aleppo, Tripoli, and Smyrna. Galls 

 arc .-ilmnst merely soluble in water by long ebullition. 

 Sixteen drachms afforded Neumann fourteen of extract ; 

 from tin 1 remaining two drachms alcohol extracted only 

 four grains. 



Of Mor- 



dants. 



Gallfc 



Propertie* 

 f the <le- 



wn> 



229- The decoction of galls precipitates the oxide of 

 ,,f a ,1^, black colour, and forms the 



ron ,, a ,^, blac colour, and forms the well-known 

 substance ink. The solutions of mercury, mixed with 

 an infusion of galls, assumes a brick colour, -which soon 



becomes of a reddish grey. Copper is easily precipita- 

 ted: the precipitate is at first green, but becomes of an 

 ash grey, and acquires a reddish coppery tinge in dry- 

 ing. Zinc yields a dirty green precipitate. Cobalt 

 forms a light blue precipitate, which quickly becomes 

 of an ash grey. 



230. If the astringent property was owing to some Objection* 

 individual principle, which was always the same in the w 

 different vegetables in which it occurs, the precipitates ^ 

 obtained by their means from a solution of iron, would 

 always form the same compound, and exhibit the same 

 general appearances ; but this is not the case. The 

 precipitate produced by galls is of a blackish blue ; that 



by logwood has a different shade of blue; that by oak 

 is of a fawn-colour; that by quinquina of a blackish 

 green, &c. These precipitates are also formed with dif- 

 ferent attendant circumstances, and when fixed on stuffs, 

 are differently affected by alum and tartar. So that the 

 substances denominated astringents form, with iron, 

 different kinds of precipitates, and, consequently, do 

 not derive their properties from an individual principle 

 found in different vegetables. This diversity of pro- 

 perties has induced Dr Bancroft to rank the substances 

 denominated astringents as colouring matters. " I have 

 invariably applied the name of colouring matter," says 

 he, " to those parts of vegetable dyeing drugs which 

 are found to produce colour with an earthy or a metal- 

 lic basis ; and I have certainly never been able to dis- 

 cover any good reason for doing otherwise, in regard to 

 those vegetable matters which afford ink, or a black dye 

 with iron ; matters which, indeed, are extremely vari- 

 ous in their other properties, and even in the sorts of 

 black which they produce ; though chemists have, as I 

 think improperly, confounded most of them under the 

 general denomination of astringents ; a term wliich may 

 be unobjectionable, as signifying acerbity in vegetables, 

 but not as indicating, or being invariably connected 

 with, any such property of matter as they have been 

 supposed universally to possess, that of producing a 

 black colour with iron." 



231. Although chemists have considered the astrin- 

 gent principle as always the same, experience had taught 

 us tliat all astringent substances were not equally pro- 

 per for producing a beautifid and durable black. Of 

 twenty-one species of astringents compared with galls, 

 oak saw dust, and yellow myrobolans, were the only 

 substances which produced a fine black, but inferior in 

 beauty and durability to that obtained by means of the 

 common gall M. Beunie, who made the comparative 

 trial of their properties in this respect, found that the 

 oak saw dust w s preferable to the bark, which is em- 

 ployed by the dyers of thread, and he remarks it is 

 el KM per. 



232. Lavoisier, Vandermonde, Fourcroy, and Ber-. Properties 

 thollet, were desired to try experiments on the different of different 

 astringents, for the purpose of making a report on the astringents. 

 subject to the academy. The substances upon which 



they instituted the experiments of comparison, were 

 galls, oak-bark, raspings of heart of oak, of the exter- 

 nal part of oak, of logwood, and sumach. To deter- 

 mine the proportion of astringent principle contained 

 in these substances, they took two ounces of each se- 

 parately, which they boiled half an hour in three pounds 

 of water ; after the first water they added a second, 

 which underwent a similar ebullition, and continued 

 these operations until the substances appeared exhaust- 

 ed ; they then mixed together the decoctions belonging 

 to each. They employed a transparent solution of 

 sulphate of iron, in wliich the proportions of water and 



