DYEING. 



Puranilitv 

 of Colours. 



17-i. With a view to know what colours were, and 

 what were not durable and fading, he exposed to the 

 sun and air, for twelve days, some patterns of all co- 

 lours, which had been dyed in his own house with 

 known compositions. It appears, that this time was 

 sufficient for a trial of them ; for the durable colours are 

 not at all, or but little injured, while the fading ones 

 are almost entirely obliterated ; so that, after twelve 

 days exposure to the heat of the sun, and the dampness 

 of the night air in summer, there can be no doubt as 

 to the class in which each colour should be placed. 



175. But there still remains a difficulty, which was 

 this, that as all these colours had not been exposed to 

 the air precisely at the same time and in the same sea- 

 son, some might have had more sun, and, consequent- 

 ly, might have suffered a greater change in the twelve 

 days, than others exposed in more cloudy weather, or 

 when the days were shorter. But he discovered a re- 

 medy for this inconvenience, which removed all diffi- 

 culty and doubt as to the accuracy of the experiments ; 

 he chose one of the worst colours, that is, one on which 

 the sun had produced the greatest effect in twelve days. 

 This colour served as a standard in the experiments ; 

 and whenever he exposed patterns to the air, he always 

 exposed a piece of the stuff along with them ; so that he 

 did not calculate by the number of days, but by the co- 

 lour of his standard, always keeping the pattern expo- 

 sed, till it had lost as much as that would have done by 

 twelve days exposure in summer. As he always noted 

 the day on which he exposed his patterns, he was led 

 to observe, that in winter it was only necessary to leave 

 them four or five days longer exposed to the air than 

 in summer. By pursuing this method, he removed all 

 doubt as to the accuracy of his experiments. In this 

 trial, by exposure to the air and to the rays of the sun, he 

 had a still farther object, which was, to find the proper 

 proof for each colour. What is called proof, is the trial 

 of a stuff, with a view to discover whether its dye be 

 permanent or not : a pattern is boiled with aluni, tar- 

 tar, soap, vinegar, lemon juice, &c. and its quality is 

 judged of by the effect these substances produce. The 

 proofs used in 1 733 were so insufficient, that they did 

 not even assist Mr Dufay in discovering more certain 

 ones ; they even destroyed some good colours, while 

 they produced very little effect upon the bad ; so that 

 he was obliged to f;x upon several, each of which serves 

 for a great numbei .,i' colours. The following is a short 

 description of the method he pursued in order to dis- 

 cover them. 



1 70'. After having observed the effect of air on each 

 colour, whether good or bad, he tried the same stuff 

 with different proofs, and stopped when he found one 

 which produced the same effect with the air; then no- 

 ting the weight of the ingredients, the quantity of the 

 water, and the length of the trial, he was sure to be 

 able to produce on a colour an effect equal to that 

 which the air would have produced, supposing it had 

 been dyed in the same way with his ; that is, accord- 

 ing to the methods employed by the dyers of true and 

 false colour-. Having thus examined all the colours, 

 and all the ingredients employed in dyeing, he found 

 out a method, which may be considered as accurate, of 

 distinguishing the good or bad qualities of each colour, 

 by making, by means of the proof, a sort of analy-i- of 

 the materials which composed it. We cannot, w'ithout 

 injustice, refuse to acknowledge, that the means Mr 

 Dufay employed, in the discovery of these proofs, or 

 testa for colour, are ingenious ; for the trial by air and 

 sun cannot be made on those, where it is necessary to 



judge immediately, whether a stud' exposed to sale, at Durability 

 a fair or elsewhere, as if a true dye, be really so or pf Colours, 

 not. ""Y""" 



177. The proofs .mentioned in the new regulations, 

 made in consequence of Mr Dufay's memoirs, dischar- 

 ges in a few minutes as much of a colour, where it is 

 not dyed of a durable colour, as would be lost by 

 twelve or fifteen days exposure to the air. But, as ge- 

 neral rules for such trials must be liable to many ex- 

 ceptions, which either cannot be foreseen, or, though 

 foreseen, cannot be particularized, without the risk of 

 causing confusion, or of affording matter for enumera- 

 ble disputes, it follows, that these rules, considered per- 

 haps as too general, are also too severe in many cases 

 wherein light colours require salts, or quantities of 

 salts, which shall be less active than those necessary for 

 deep or loaded colours, which may allow a considerable 

 portion of their colouring matter to be carried off by a 

 proof, without shewing a very visible alteration. It 

 would, therefore, have been very necessary to prescribe 

 a proof for almost every shade, which, from their infi- 

 nite variety, would be impossible. Thus the air and 

 the sun will always be the true test; and every co- 

 lour which is not changed by them in a certain length 

 of time, or which, by exposure, acquires what the 

 dyers call a body, ought to be considered as a standing 

 colour, even though it should be considerably changed 

 by the proofs. Of this we have an example in scarlet; 

 as soap almost entirely discharges this colour, it has 

 been submitted to the trial by alum ; and when it has 

 been dyed with cochineal alone, without any mixture 

 of other ingredients, it ought to assume a purple co- 

 lour in a boiling solution of alum. If scarlet be expo- 

 sed to the sun, it loses some of its brightness, and be- 

 comes deeper ; but this dec)) shade is not similar to 

 that imparted by alum. Proofs, then, in certain cases, 

 cannot be substituted for the action of the air and sun, 

 at least so far as respects similarity of effect. 



17**. By means of Brazil wood, which, like almost all 

 the other woods loaded with colour, gives a fading 

 dye, I prepared a red which was much finer than the 

 madder reds, and as bright as those made with kermes ; 

 this red remained exposed to the air for the two last 

 months of the year 17 W, which were very rainy, and 

 for the two first of 1741 ; notwithstanding the rain and 

 bad weather, it stood, upon account of its peculiar 

 preparation, v Inch will be mentioned- in the proper 

 place, and was so far from losing, that it acquired body. 

 Vet the same red which is so durable in the air, does 

 not resist the trial by tartar. Would it then be proper 

 to proscribe this colour, because that salt discharges it, 

 or are the stuffs we employ for clothing intended to be 

 boiled with tartar, alum, or soap ? I do not, however, 

 mean to disapprove of the trial by proofs ; they are 

 useful because they arc ready, but there are cases in 

 which they ought not to serve as a ground for pro- 

 nouncing a sentence of confiscation, especially when 

 they cannot show that a colour which should have been 

 dyed for materials for durable colours, has been dyed 

 with those of a fading nature." 



1 7f). Upon the whole, then, it appears that the only Exposure 

 sure test we can employ, is direct exposure to air and to light, 

 light, for those stuffs which are not to be leyed ; and alr> ^ c - tllc 



washing with alkalies and soap for such as are to be " n y '!"? 



test ot the 



occasionally exposed to these substances. In order to gooUncss of 

 try the colours of silk, it is generally thought sufficient a colour. 

 to expose them to heat in acetous acid, or lemon juice ; 

 and they are considered as good and permanent colours 

 if they island tlu's test. And, indeed, when the woods 



