148. To extract the colour of weld for dyeing, the plant is boiled in a fresh bath, incdosing it in a bag of thin III1C11, and keeping it from rising to the top by means of a heavy wooden cross. Sonic. dyers contieue the boiling till it sinks to the bottom of the copper, and then let a cross down upon it ; others when it is boiled, take it out with a take, and throw it away. The decoc tion should be employed as soon as possible after it has been prepared, as it soon suffers a decomposition, which renders it useless.
149. Common salt added to the weld bath. renders its colour richer and deepen.; sulphate of lime or gypsum also deepens it. Alum renders it paler, but more live ly ; and tartar still paler. Sulphate of iron makes it in cline to brown. The shades obtained from weld may be modified, with these additions, by the proportion of the weld, by the length of the operation, and by the mor dants employed in preparing- the stuff. The colour may be modified also by passing the cloth, when it is taken out of the bath, through other dyes.
150. Dr Bancroft has stated several objections to the use of weld, compared with quercitron, as a dye. When it is employed in topical dyeing, the colouring matter of weld is fixed by a heat very little less than that of boil ing water, and the parts wanted to he preserved white are then so much stained, that it is difficult to brighten them afterwards, particularly during the damp and clou dy weather of winter. Weld also produces another bad effect in topical dyeing upon linens or cottons, which have previously received madder colours : the weld yel low attaching itself so closely to these colours, as great ly to tarnish and impair their lustre. Quercitron is near ly, if not wholly, free from these defects. It is obvious, however, that, in many cases, these qualities must re commend the use of weld.
151. Fustic, the wood of the mortis tinctoria, a tree of consieterable size, which grows in various parts of the West Indies, affords a yellow colouring matter, which is very extensively used in dyeing. The colour which it yields is neither high nor bright, but it pos sesses considerable durability, and is less affected by acids than the quercin on and weld yellows. Fustic abounds much in colouring particles, though in this re spect it is greatly inferior to quercitron, as it only affords one-fourth the quantity of colouring matter.
152. A strong decoction of fustic is of deep reddish yellow colour, which becomes an orange yellow when diluted with water. The greater number of the acids render this liquor turbid; a small quantity of a greenish 'yellow precipitate is formed, and the supernatant liquor is of a pale yellow. Alkalies redissolve the precipitate, and give the liquor a deep reddish colour. Alum forms a small precipitate of a yellow colour; alum and tartar a like precipitate. Muriate of soda makes the liquor a little more deeply coloured, without producing any pre cipitate. Sulphate of iron forms a precipitate, which is at first yellow, but afterwards becomes more and more brown. Sulphate yields a copious brownish yellow pre cipitate. Fustic and weld are sometimes used together, in quantities proportioned to the desired effect. Fustic is also very commonly employed with the sulphate of in digo, in dyeing Saxon greens upon cloth.
153. Fustic, as well as many other woods which yield a yellow colour, contains a quantity of resinous and ex tractive matter in combination with a portion of tannin. This last principle diminishes the brilliancy of the yellow afforded by fustic ; but Chaptal has proposed an easy method of detaching it from the colouring matter, though it may affect its durability. The method to which we allude, is founded upon the attraction of tannin for glue. He accordingly recommends that animal substances, containing gelatinous matter, such as bits of leather, glue, &c. should be added to the decoctions of fustic. This process precipitates the tannin, and enables the co lour to exhibit greater brilliancy.
154. Venice sumach, the wood of the rhus cotinus, a shrub growing principally in Italy, and the South of France, affords a lull high yellow of little durability. Dr Bancroft, indeed, mentions that this defect may, in a great degree, be remedied, by employing tartar, along with the muriate of tin, the mordant by means of which this colouring matter is usually fixed. Four pounds of the thus cotinus chipped, afford no more colour than one pound of quereitron. It is frequently mixed with other colouring substances, particularly cochineal, to give a fine colour to scarlet ; also for pomegranates, oranges, jonquilles, gold colours, buffs, and in general for all those colours which it is wished should have an orange cast.