RED WOOD LAKES.
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.