Red wood is obtained from species of Cæsalpinia growing in South America. The following varieties are distinguished: Fernambuco or Brazil wood, Bahia wood, St. Martha wood, Lima wood and Sapan wood, all of which contain the same colouring matter, but in different quantities. Red wood comes into commerce either as thick logs of a fine red colour or as finely-rasped chips. Fernambuco and Sapan woods are accounted the best varieties. Although the colouring matters contained in these woods have been investigated for a long time, an accurate knowledge of them has only recently been obtained. The pure colouring matter of red wood, brasilin, forms orange-coloured needle-shaped crystals, soluble in water, alcohol and ether. On careful heating it partially sublimes undecomposed. Brasilin most probably results from the alteration of a substance contained in the inner less coloured portion of the wood; this compound, brasileïn, has been obtained in colourless needles; its solution acquires a red colour on boiling, and produces the pure colouring matter.
Brasilin is a weak acid of the composition C₁₆H₁₄O₅. It forms handsomely coloured compounds with most metallic oxides, which are much used in dyeing. A decoction of red wood gives with alum and soda solution a handsome lake of a pure red colour. With a solution of stannic chloride it at once produces a red precipitate. When a solution of potassium chromate is added to a decoction of redwood, a dark brown precipitate separates, which is a compound of chromium oxide with the partially altered colouring matter. The decoction produces a brownish red compound with ferric oxide.
The decoctions of red wood contain, in addition to the colouring matter, a number of other substances which would injure the shade of the lake obtained from the decoction. The greater portion of these substances may be separated by allowing the decoction to stand for several days, when a dirty reddish brown mud forms at the bottom of the vessel. This simple method of purification is attended with the disadvantage that, especially in summer, the decoction rapidly becomes mouldy. This may be prevented by the addition of a little carbolic acid; not more than 0·01 per cent. of the quantity of liquid is required. The foreign matters may also be separated by adding milk or a solution of glue to the decoction, but the simpler process of allowing it to stand gives the best result.
From the behaviour of brasilin towards the metallic oxides, as given above, it follows that compounds of different colours can be made from red wood. These are utilised in dyeing, but in colour works red wood is only used for the preparation of red lakes. The lakes are found in commerce under numerous names, the most common of which are Venetian lake, Vienna lake, Florentine lake, Berlin lake, etc. According as the colouring matter is combined with alumina alone or with stannic chloride in addition, pale or deep lakes are obtained. The different shades of red wood lakes found in commerce are obtained by additions of a white pigment. For this purpose levigated chalk, gypsum, lycopodium powder, and other white pigments light in weight are used.
In preparing red wood lakes the decoction should always be purified by the process mentioned above; there is then more certainty of obtaining a bright lake. It is advisable to dilute the decoction considerably with pure water before precipitating the lake; experience has shown that brighter colours are thus obtained. To 100 parts of wood 130 to 150 parts of alum are used. The latter is dissolved alone and mixed with the decoction, the lake is then precipitated by soda solution. As the lake separates the liquid becomes lighter; portions should be taken out of the vessel in order to see when all the colouring matter is thrown down. If too much soda solution is added the lake takes a violet shade.
A particularly fine lake is obtained from Fernambuco wood when the extract is acidified with a weak acid, such as acetic, and stannic chloride used together with alum in precipitating the lake. One hundred parts of rasped wood are boiled with 300 parts of water to which acetic acid has been added; the extract is then boiled with 130 parts of alum until the latter is dissolved. To the filtered liquid 20 to 25 parts of stannic chloride are added whilst stirring well, and the lake is at once precipitated by soda solution.
If the lake from Fernambuco wood does not turn out well, a better product may be obtained by treating the lake with such a quantity of hydrochloric acid that a small portion remains undecomposed. This is filtered off, and the colouring matter reprecipitated in combination with alumina by neutralising with soda solution.
By adding a little Dutch pink more orange shades of the Fernambuco lake are obtained. In this manner any intermediate shade between pure red and pure yellow can be produced. It is important to lay down a standard series of shades, and always to make pigments which exactly correspond with one of them. It is not possible to make a determined shade of red wood lake or any other lake obtained from a decoction by working with accurately weighed quantities. In general the result will be near the desired shade, but only by chance will it be hit exactly. This is because different samples of the dye-wood do not always contain the same quantity of colouring matter. It is indeed possible with practice to estimate approximately from the colour of the decoction the shade of the lake which will be obtained from it; but this can only be done exactly by comparing the shade of a small batch of the lake with the standard shades. Thus in the case of a Fernambuco lake it will be seen by comparison with the standard shades whether the colour of the small batch is too deep, too violet, etc.; then by using more alum or more tin solution the defects can be removed.
CHAPTER LIX.
THE COLOURING MATTERS OF SANDALWOOD
AND OTHER DYE-WOODS.
In addition to the dye-woods in more common use, the tropics furnish a number of woods which contain fine colouring matters, but which are only in restricted use, either on account of their cost or because they are not always to be bought. To these belong the camwood or barwood of Madagascar, the East Indian sandalwood, and other woods which have not yet been used for colouring purposes even in the tropics.