Light promotes this decomposition of the colouring particles, which frequently takes place only with its concurrence, and thus it contributes to the destruction of the colour. Heat also favours the same result, but less efficaciously so, unless it have a certain intensity.
It is concluded, therefore, that colours are more or less fixed in the air, according to the greater or less tendency which the colouring particles have to undergo this change[6]. Hence the utility of mordants in rendering fugitive colours fast.
To prove the colours of Dyed Stuffs, &c.
The natural proofs of a dye's being effectual, are exposure to the air, to the sun, or to rain. If the colour be not changed by such exposure after twelve or fourteen days, it may be considered as fixed. These proofs are not, however, adapted to every colour: for some resist the action of air, light, and rain, yet are nevertheless injured by certain acids. There are also colours which do not resist the natural proofs and yet remain unchanged by acids.
Colours may be arranged in this respect in three classes: the first class is tried with alum, the second with soap, the third with tartar. For the proof with alum, half an ounce of this salt must be dissolved in a pint of water in an earthen pipkin, and into this liquor is to be put half a quarter of an ounce of the dyed thread or stuff, the whole being boiled about five minutes; it is then to be washed clean with water. Thus are tried crimson, scarlet, flesh-colour, violet, ponceau, peach-blossom, different shades of blue, and other colours bordering on these.
The next proof consists in boiling a quarter of an ounce of soap in a pint of water, with half a quarter of an ounce of the dyed stuff or thread for five minutes. With this proof all sorts of yellow, green, madder-red, cinnamon, and similar colours are to be tried.
The proof with tartar consists in boiling one ounce of that salt, previously powdered very fine, with a quarter of an ounce of dyed thread or stuff, in a pint of water for five minutes. This proof is used for all colours bordering upon fallow, or hair-brown.—Journal of Science, vol. xxii. 219.
But notwithstanding these general rules may be given for dye-tests, yet so many are the niceties in this art, that, after all, nothing but long practice combined with scientific knowledge, will enable the dyer to become in this respect, a complete and successful artist.