YELLOW LAKES.
Yellow colouring matters are widespread in nature; from them are
obtained many yellow pigments used in dyeing and in the manufacture of
colours. The colours produced by the yellow vegetable colouring matters
are not particularly bright; thus the yellow lakes are used chiefly
to produce cheap pigments, whilst the inorganic pigments are used for
bright yellows.
=Dutch Pink.=—Several species of buckthorn (_Rhamnus_) contain a yellow
colouring matter—xanthorhamnin—which is obtained pure by extracting
the yellow berries with hot alcohol. On cooling, the impure colouring
matter separates; by repeated recrystallisation from alcohol it is
obtained in the form of crystalline needles, which are soluble in water
and alcohol.
The yellow lake known as Dutch pink is prepared from yellow (Persian)
berries, by boiling the crushed berries with water and mixing the
extract with a solution of alum. The lake is then precipitated by the
addition of powdered chalk. As a rule, 500 parts of water are used to
100 parts of berries, 20 parts of alum are added to the decoction, and
the mixture poured upon 75 parts of finely powdered chalk. The liquid
is decanted off, the residue filtered, washed and dried. Commercial
Dutch pink is made from a mixture of the decoctions of yellow berries,
quercitron bark and turmeric, to which the alum solution is added, and
then chalk. The precipitate is made into conical lumps, which are sold
as Dutch pink, and used for ordinary painting and for colouring leather.
=Weld Lake.=—The dyers’ weld (_Reseda luteola_) contains a yellow
colouring matter formerly much used in dyeing; a yellow lake can also
be obtained from it. When weld is boiled with water a deep yellowish
green decoction is obtained, from which yellowish-green flocks separate
on cooling. The lake can be obtained from all parts of the plant except
the root, the flowering shoots giving the largest yield of colour; the
lake is generally precipitated by alumina. Equal parts of weld and alum
are boiled with water until the latter is dissolved and the liquid is
coloured deep yellow; the hot solution is quickly filtered through a
thick linen cloth and soda solution gradually added in small quantities
with continual stirring so long as effervescence follows.
Alum entirely free from iron is required to produce a bright yellow
lake; a small quantity of iron has great influence on the shade. Weld
contains more or less tannin, which gives very dark blue or green
compounds with iron salts, the production of a very small quantity of
which suffices to convert the yellow shade into an ugly dirty colour.
Chalk may be used instead of soda to precipitate the alumina; in this
case the precipitate will contain calcium sulphate in addition to the
alumina compound of luteolin.
Weld lake may be used in oil, or size, or as a water colour.
=Gamboge Lake.=—Crude gamboge is the dried sap of certain East Indian
trees; it is used as an artists’ colour, but only in water; for use in
oil it is converted into a lake. A particular treatment is necessary
to obtain a handsome lake. Gamboge is treated for several days with
water and the soft mass ground on the slab or in a mill until it forms
a uniform paste, which is mixed with water to a thick liquid, which is
then put through a fine sieve. A hot solution of alum is then added
in the proportion of three parts of alum to one part of gamboge, the
mixture is boiled in a wooden vessel and two parts of nitric acid added
whilst well stirring; finally a dilute solution of potash is added
in small quantities until the liquid solidifies to a jelly, which is
spread out in thin layers on filter cloth, well washed with water and
dried at a gentle heat.
=Prepared Gamboge.=—Crude gamboge cannot be used in oil painting
because it never gives a uniform shade, but when the colouring matter
is freed from admixtures and ground with oil, it gives a deep durable
colour of great beauty.
The colouring constituent of gamboge is a resin which is fairly soluble
in strong alcohol, whilst the impurities are insoluble. The pure
colouring matter is obtained without great expense in the following
manner: Coarsely powdered gamboge is placed in a large flask with
strong alcohol; the flask is well closed, placed in a warm position
and repeatedly shaken; the deep yellow solution is carefully poured
off from the sediment, water is then added, when the colouring
matter separates in flocks. Only sufficient water should be added to
precipitate all the colouring matter; test portions of the liquid
should be examined from time to time, the alcohol can then be recovered
by distillation and used again. The colouring matter separated from the
alcoholic solution forms a hard mass on drying; it must be very finely
powdered before it is ground in oil.
There are many other yellow lakes which can be obtained by adding
alum to an aqueous decoction of the colour-bearing material and
neutralising the liquid. Such decoctions are obtained from fustic,
quercitron (_Quercus tinctoria_), young fustic (_Rhus cotinus_), the
root of barberry (_Berberis vulgaris_), annatto (the fruit of _Bixa
orrellana_), turmeric (the root of _Curcuma longa_), etc. The yellow
lakes obtained from these sources are seldom used in painting, the
mineral pigments are preferred on account of their finer appearance.
These colouring matters are largely used in dyeing to produce shades
varying from yellow to brown, the majority are also used in calico
printing.
=Fustic Lake.=—Of the colouring materials mentioned above fustic
produces a lake which is so handsome that it deserves larger use in
painting than it has yet found. ”Old fustic” is chipped immediately
before use and extracted with boiling water; whilst still hot the
extract is mixed with a hot solution of alum; on cooling a fine yellow
precipitate is formed, which when dry is frequently sold under the name
of Dutch pink. (Genuine Dutch pink is made from a decoction of yellow
berries; the majority of the pigments sold under this name are made
from decoctions of mixtures of the different yellow dye wares.)
To obtain the best lake from fustic the alum used must be completely
free from iron, otherwise only a dirty green lake is produced. Since
the least trace of iron damages the shade of the colour, fustic lake
is more frequently made by means of lead oxide. To obtain the lead
fustic lake the decoction is allowed to stand for several days, so that
the morin, which together with maclurin forms the colouring principle
of fustic, may separate; the liquid is filtered from the precipitate,
and lead acetate solution added which has been boiled with litharge to
saturation, so that the most basic acetate is produced. The lead fustic
lake has a deep yellow colour; it is well adapted for use in painting,
but, like all lead pigments, is not specially permanent in air. By
admixtures of levigated chalk or starch paler shades of the dull yellow
lake are obtained.
=Quercitron Lake.=—Lakes are seldom made from quercitron alone,
although they possess a deep yellow colour; they are more frequently
employed in dyeing when they are produced directly upon the fibre.
Quercitron lake can be made by treating the aqueous decoction of the
ground bark with tin crystals and a little alum solution. With tin
crystals alone a darker yellow lake is obtained than when alum is used
at the same time. Different shades may thus be produced.
Extract of quercitron bark comes into commerce as a greyish yellow
powder under the name of flavine, which has great colouring power, and
may be used with advantage in the place of aqueous extracts of the bark.
Since the discovery of the aniline dyes the applications of all lakes
have been largely extended, since the dyes enable brighter shades to be
obtained without the many processes necessary when natural colouring
matters are used, and do not require so much skill on the part of the
workman. This applies not only to yellow colouring matters, but to the
many other colouring matters formerly used by the dyer. The principal
reasons which have so quickly brought the new dyes to the front lie in
their greater beauty and in the diminished labour required in their use.
=Purree or Indian Yellow= must not be confounded with the yellow cobalt
pigment known also under the latter name. It is a compound of magnesia
with an organic acid—euxanthic acid. It is obtained from the urine of
cows fed upon mango leaves. It comes into commerce in lumps weighing
about 50 to 60 grammes, which are dark brown on the exterior, but on
fracture show a fine orange yellow colour. Indian yellow is little
used, and is not likely to attain importance. Many cheaper colouring
matters equal in shade are known.
=The Colouring Matter of Saffron.=—The dried stigma of the flowers of
the saffron, a species of _iris_, has been long used for colouring
foods. It contains a very handsome yellow colouring matter of a
characteristic shade; formerly this was largely used in silk-dyeing,
and especially for dyeing glove leather. It is practically now no
longer used; considerably cheaper aniline dyes produce an equally good
shade. On account of its high price (1 kilogramme of saffron contains
about 60,000 stigmata) this material cannot be used for lakes,
although they leave nothing to be desired in fineness of shade, but
they have no great permanence.
=The Colouring Matter of Gardenia Grandiflora.=—The fruits of this
plant, which are imported from Southern Asia, contain a beautiful dark
yellow colouring matter, which, according to Rochleder, is identical
with the colouring matter of saffron. It is readily soluble in water;
lakes of various shades can readily be obtained by the addition of
salts to the aqueous solution. Alum gives a pure yellow, lead acetate a
yellowish red, and stannous chloride a dark orange red lake.