334

DYEING AND CALICO PRINTING.

which aid the oxidation by becoming altered somehow themselves. To acertain extent the wood, after having been rasped, gains in tindtftrial value bycontact with air for some time, but it should not be exposed too long, andcertainly not be submitted to the direct adtion of the sun’s rays, which willsoon cause the destruction of the pigment altogether. The decodtion of the redwoods is also very sensitive to the adtion of reducing agents ; sulphurettedhydrogen discolours this liquid, and when boiled even with T , l cu th of sulphideof sodium, the decodtion only exhibits a reddish hue due to the influence of thealkaline reaction of the salt, but this colouration disappears if the alkali isneutralised by an acid. When a decodtion so treated, and quite colourless, isused for a dye-bath, it would at first appear to be worthless, but as the tempera-ture rises the colour returns, and the dyeing operation goes on as well as withthe original decodtion though attended with a great loss of colouring matter.Acids have the effedt of rendering the colour of the decodtions yellow, with amore or less orange shade, which depends on thefadt of the more or less oxidisedstate of the wood used for the decodtions. After the lapse of some time decodtionsso treated deposit a crystalline substance, which is yellow in case the decoc-tion was quite deoxidised, otherwise it exhibits a red colour. Excess ofhydrochloric acid added to the decodtion turns them a bright rose-red, butthis hue disapears on the addition of water. Caustic and carbonated alkaliesturn the colour of the decodtion to a carmine-red. When lime or even chalkis added to a decodtion of these woods the liquid becomes carmine-redcoloured: this base has a specific effedt in promoting the oxidation of thedecodtion. The liquid thus treated soon becomes covered with a blackpellicle, which'after a while sinks to the bottom of the liquid, and the for-mation of another coating in its stead begins. Alum turns the decodtion to ared, but does not cause a precipitate ; if, however, so much of an alkalinecarbonate is added as to make the alum basic and a gentle heat is applied, ared-coloured lake is precipitated. Acetate of alumina heated with the decodtionbehaves in the same manner. Neutral acetate of lead yields merely a slightreddish precipitate. When the substance thus thrown down is removed fromthe fluid by filtration, the filtrate is found to be as fit for dyeing and even togive better and brighter colours than the primitive liquid. Hence it appearsthat neutral acetate of lead precipitates only the super-oxidised matters, and,moreover, removes from the decodtion such compounds, tannin, for instance,as may affedt the purity and brilliancy of the dye. Subacetate of lead yieldsa very abundant precipitate of a bluish colour; the filtrate from the liquid sotreated does not contain anything worth attention. Nitro-muriate of tinyields with the decodtion a red precipitate, which turns to a carmine-red ofthe greater beauty the less the liquid was previously altered. The charadtershere described are those which M. Persoz obtained while experimenting upondecodtions of Bahia wood both old as well as freshly made.

M. Girardin’s experiments on this topic differ in some particulars, but itought to be observed that in the first place the time elapsed since the raspingof the wood has some influence, and, secondly, the different kinds of thesewoods as met with in commerce may, and undoubtedly do, exhibit slightdifferences when the decodtions are tested with reagents.

Brazil wood decodtion is better fit for dyeing when it has been kept for agood length of time in a cool place than when recently made. This fadt,