.'hi 



DYEING. 



Doe not 

 ratdein 



t-ular |irn\i 

 note prin- 

 ciple. 



General 

 properties 

 of colour- 

 ing mt- 

 ten. 



Colouring they appear to have a more powerful affinity, and this 

 ,^1' !! ''' too witliout our being able to refer the ap|x-araii( i s 

 """""' which take place to any known proximate princi- 

 ple. 



41. These facts, however, by no means authorise 

 us to conclude that the colouring matter is a separate 

 and independent principle; nil that can be inli rri d 

 from them is, that the colouring matter does not uni- 

 formly exist in combination with any pnitieular proxi- 

 mate principle, but probably with different proximate 

 principles, or various combinations of them ; though 

 the present state of chemical analy>is does not enable 

 us to ascertain the composition of the colouring parti- 

 cles with a degree of precision sufficient to detect the 

 principles to which they owe their properties. Without 

 taking any notice, therefore, of the theories which have 

 been advanced to explain the cause of colour, we shall 

 be satisfied with presenting a general view of the dif- 

 ferent colouring matters, and leave to future investiga- 

 tion the task of discovering whether these colouring 

 matters form separate substances, or exist uniformly in 

 combination with some particular proximate principle 

 as a basis. 



42. It has been proposed to divide the colouring 

 particles of vegetables into extractive and resinous ; 

 but this division is attended with no advantage, and 

 indeed only serves to convey very imperfect and erro- 

 neous ideas of their properties, as some of the most im- 

 portant dyes are not at all affected by the usual solvents 

 which act upon these two classes of substances. The 

 truth is, that the colouring particles possess chemical 

 properties which are peculiar to themselves, and dis- 

 tinguish them from all other substances ; though these 

 properties are different in different colouring matters. 

 They unite with acids, alkalies, metallic oxides, and some 

 of the earthy bodies, particularly alumine. They are 

 .said to precipitate oxides and alumine from the acids 

 which hold them in combination ; but this is doubtful, 

 or at least the decom]>osition is very partial. They 

 more frequently form triple compounds with the salts, 

 and in this state of combination unite with the stuffs 

 to which they are presented, in a more intimate man- 

 ner than they would do without the intervention of 

 these bodies. 



43. The colouring particles do not, however, unite 

 indiscriminately with these chemical agents, and there- 

 fore require different solvents suited to their respective 

 qualities. The most common solvents are water, acids, 

 and alkalies; alcohol is seldom used, unless when we 

 wish to act on the colouring matter of very small bo- 

 dies. Of these substances, water is most extensively 

 employed, both on account of its abundance, and the 

 great solving jxiwer which it exerts over almost all 

 colouring matters; alkalies are employed to promote 

 the solution of the colouring principle contained in in- 

 digo, the flowers of bastard saffron, Sec. ; and the acids 

 are used in some cases to dissolve colours, and in others 

 to precipitate the colouring principles from their solu- 

 tions in alkalies. 



44. Before colouring principles are exposed to the 

 action of a solvent, they ought to be divided as minute- 

 ly as possible, by mechanical means. This is obtain- 

 ed by triturating them in a mortar, or by means of a 

 muller, and afterwards passing them, in a reduced 

 state, through a fine sieve, that no gross particles may 

 escaj>e notice. The methods by which the division of 

 colouring substances is produced, vary according to 

 the consistence, structure, and volatility of the mat- 

 ters upon which the operation is performed. In some 



Solvents. 



Division by 

 mechanical 



l'i;u:-. 



canes, the sub.-tanre Containing the colouring principle 

 i- reduced to chips or scales, by mcan> of a sharp in- 

 strument; in others, it is perfect I v sufficient to bruise 

 it Milder mullers. in the s:unc manner as tan is ground. 

 In whatever way pulverization i~ performed, the ope- 

 ration is attended with the diffusion of a subtile pou - 

 drr through the workshop, which is lx>th injurious to 

 respiration, and the cau>e of a considerable loss to the 

 manufacturer, particularly if the article be expensive. 

 These disadvantages may l>e guarded against, by per- 

 forming the process of trituration in covered plai . 

 moistening the matters acted upon with water. After 

 the colouring principle is stilliciently comminuted, it 

 may be dissolved, without difficulty, in some of tin 

 vents formerly mentioned. 



45. With the exception of some colouring matters of 

 a resinous or starchy nature, water may be regarded 

 as a universal solvent of that class of bodies; while it 

 possesses the im|x>rtant property of forming n very 

 imperfect union with them, and readily parts with 

 them again to the stuff. Warm water extracts the co- 

 louring principle more copiously than cold water ; but 

 different substances require different degrees of tempe- 

 rature for that purpose. Some colouring principle* 

 can only be dissolved by long protracted maceration ; 

 others require a very gentle heat, and suffer in their 

 brilliancy by being exposed to too elevated a tempera- 

 ture ; and others still yield but little of their colour, 

 unless they be raised to a considerable degree of heat. 

 A knowledge of these circumstances is absolutely ne- 

 cessary to the dyer, in order that he may conduct his 

 operations with certainty and success. 



46'. A great deal has been ascribed to the qualities of 

 waters used in dyeing, nothing being more common 

 than to refer the brilliancy of some colours, and the po- 

 verty of others, entirely to this cause. Without adopt- 

 ing implicitly all that has been published on this sulv- 

 ject, it must be allowed that waters contribute essen- 

 tially to the qualities of dyes; and it may be added, that 

 different colours, and even the same colour in different 

 states, require that waters of very different natures 

 should be employed. Rapid and running waters are 

 generally the purest, and standing waters most impreg- 

 nated with earthy and saline matters. In some cases, 

 however, the latter may be used wiih great advantage 

 in dyeing, because the putrid animal and vegetable 

 matters suspended in them, contribute to form ammonia 

 and sulphureted hydrogen, which precipitate the earthy 

 and metallic principles. 



47. Waters holding calcareous salts in solution, are 

 particularly prejudicial to the dyeing cotton of a red co- 

 lour. The lime which is precipitated during the pro- 

 cess attaches itself to the stuff, and obscures the colour 

 to such a degree that it is almost impossible to revive it. 

 But if the object be to obtain a dark colour, these salts 

 are rather beneficial than hurtful, as they tend to in- 

 crease the body of the red colour and its modifications, 

 hence they may be used with advantage when it is in- 

 tended to convert scarlet into crimson. 



48. Calcareous salts dissolved in water, not only af- 

 fect the brilliancy of some colours, but possess the in- 

 convenience of weakening the solvent which holds them 

 in solution, and thus extract imperfectly the colouring 

 principle. It is of consequence, therefore, to be able 

 to detect the presence of these salts, that we may either 

 avoid the water which contains them, or correct its pre- 

 judicial qualities. Chemistry furnishes us with the 

 means not only of discovering the different ingredients 

 which exist in water, but of ascertaining with precision 



< dlfiuring 





Qualities 

 wluch it 

 ought to 



]H>!-M'S tor 



dyeing. 





Calcareous 

 waters in- 

 jurious. 



