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DYEING AND CALICO PRINTING.
The primitive green colour is, however, restored by the addition of analkaline acetate, or of a strong base and simultaneous exposure to air.
Oxidising agents, such as nitric, chloric, iodic, and chromic acids, destroythe Chinese green, the ultimate result being the formation of a red substancedifferent from that which is formed from the lo-kao by reducing agents, sincethe latter is capable of being re-converted into the original green colour.Some salts, among which are alkaline phosphates, pyrophosphates, borates,margarates, oleates, and stearates, assist the solution of lo-kao in water, whilethe salts of magnesia and zinc turn its colour to a pure blue ; nitrate of silverand ferricyanide of potassium oxidise the colouring matter if an alkali issimultaneously present. Lo-kao possesses the peculiar property of so maskingthe presence of certain bases, c.g ., lime, oxide of iron, and others, that theycan only be dete&ed after the complete destruction of the organic matter byincineration. The opinion of M. Persoz on lo-kao is, that it is a mixture oflime, magnesia, and iron-lakes, along with some phosphate of alumina, andpartly soluble in water. He gave the name of cyanin to the pure colouringmatter, freed as much as possible from the accompanying inorganic sub-stances. It has a blue colour, and contains no nitrogen.
Persoz gives the following directions for the purification of the commercialarticle:—(i). Lo-kao is dissolved in a saturated solution of carbonate of potassa;after the liquid has lost its turbidity, water is added to the previously decantedclear liquid; after awhile there is formed a green precipitate, which, afterbeing washed and drained, may be successfully used for dyeing and printing.(2). Lo-kao is first soaked in-water, and after it is swollen and softened it ismixed with one-and-a-half times its weight of acetic acid; next 5$ parts ofwater are added, the liquid filtered, and to the filtrate ammonia is added,whereby a lake far richer than the original substance is obtained.
An aluminous lake may be obtained when a solution of lo-kao, to which analum solution is added, is precipitated by an alkaline carbonate, or also by theaddition of a salt of alumina to a solution of lo-kao in carbonate of soda.The double chloride of ammonium and tin yields, with an aqueous or aceticacid solution of lo-kao, a beautiful blue precipitate if care be taken to addacetate of soda at the same time. The purple-violet precipitate obtained bythe a&ion of the reducing acids, such as arsenious and formic, is soluble inacetate of lime, and that solution deposits after some time a deep blue powder,verging upon violet.
As regards the origin and mode of preparation of lo-kao, the results of the re-searches made in China by Father Helot, by MM. Montigny, Edan, Arnaudtizon,Fortune, Remi, Edkins, and others, coupled with the botanical investigationsof samples of plants sent over to M. Decaisne, prove that lo-kao requires for itspreparation two different plants, viz., the Rhamnus utilis , called in the Chineselanguage, hong-pl-lo-chou, and the Rhamnus chlorophorus, pe-pi 4 o-chou. Oneof these shrubs grows wild, while the other is purposely cultivated, butwhich of the two is not known with certainty. The hong-pi-lo-chou, alsoknown as hom-bi-lo-za, is said to yield by itself a deep and fast colour, butvoid of brilliancy, while the pe-pi-lo-chou yields a weak but brilliant colour.In order to render the mode of preparation of lo-kao in China properly under-stood, we have first to describe the mode of dyeing the cloth by means ofthe vegetable matters in question, the lo-kao being obtained by detachingfrom cloth the excess of colour fixed thereupon.