LAKES.
The pigments commercially designated lakes generally consist of an
organic colouring matter united with a metallic oxide. In isolated
cases other compounds of colouring matters are included under the term,
such as indigo sulphonic acid, and yet more rarely pure colouring
matters are included, for example carthamine red. But the overwhelming
majority of lake pigments are compounds of a colouring matter with
the oxide of a metal. For this purpose the oxides of tin, lead and
aluminium are commonly used.
Lakes are generally made by mixing the solution of the colouring matter
with the solution of a salt of the metallic oxide, and precipitating
the oxide by an alkali. The colouring matter is separated along with
the oxide and forms with it a substance known as a lake. It has not
yet been decided whether the lakes are true chemical compounds of the
colouring matter with the metallic oxide, or whether the colouring
matter is simply held fast by the surface attraction of the finely
divided oxide. In favour of the latter view is the fact that a larger
or smaller quantity of the colouring matter can be united with a given
quantity of the oxide.
The lakes vary greatly in durability; some, such as the madder lakes,
can be counted among the most durable pigments, whilst others have very
little permanence, as for example, the logwood lakes.
The majority of the colouring matters used in the preparation of lakes
are of vegetable origin, but several are derived from animal sources.
The properties of the colouring matters used for this purpose vary
greatly; each material demands a special treatment for the production
of lakes. Therefore, in describing the methods by which lakes are made,
we shall give first the properties of the colouring matter in question,
and then proceed to the preparation of the lake. In the preceding
portion of the book the pigments have been arranged according to their
colour; the lakes will be described in the same order. The yellow, red
or green lakes of different origin will be treated in the same section
in order to facilitate reference to any particular pigment.
White lakes are not known, neither are there lakes which can be
described as black. With these exceptions lakes of all colours can be
made and also of all shades. As an example of this madder lake may be
given, which is found in commerce in a great variety of shades; it
can be made from the palest rose-red to the deepest purple red, or,
more properly, madder red, which is a characteristic shade. The shades
of any lake are obtained by mixing in a white pigment just as in the
manufacture of mineral pigments. Whilst, however, in the latter case
the white pigment must often be mechanically mixed with the colour, the
shading of the lakes is accomplished in their preparation. Generally
speaking, a pale shade of a lake is obtained by increasing the amount
of the salt, the oxide of which is used to precipitate the colouring
matter. The oxides in question are white (with the exception of lead
oxide); thus in the pale shades a small amount of colouring matter is
precipitated upon a large amount of oxide, whilst in the deep lakes
the reverse is the case, a large quantity of colouring matter is
precipitated upon a small quantity of oxide, and the shade thus appears
very deep.
Apart from the nature of the colouring matter contained in a lake,
and considering only the metallic oxide with which the colouring
matter is united, it appears that lakes containing lead oxide have
little durability. The combination between the colouring matter and
the metallic oxide in a lake is so loose that it is easily destroyed
by sulphuretted hydrogen; if a lake containing lead is exposed to the
action of air containing that gas the colour will, in the course of
time, inevitably blacken. The lakes containing tin are also susceptible
to sulphuretted hydrogen; in air containing but a trace of this gas
they quickly lose their brilliance and in time are quite discoloured.
Alumina is not affected by sulphuretted hydrogen, and must thus be
regarded as the most suitable oxide for the preparation of lakes. It
is generally applied in the form of alum, in the selection of which
great care is necessary if fine colours are to be produced. Commercial
alum frequently contains ferric oxide; when the alumina is precipitated
simultaneously with the colouring matter the ferric oxide is also
thrown down, and is mixed with the lake, its dark colour influencing
the shade of the lake greatly to its disadvantage. The effect of the
ferric oxide upon the colour of the lake is so important that it is
impossible, for example, to obtain a pale red lake with alum containing
iron. To avoid the bad results given by alum containing iron it should
be examined before it is used, and if it is found to contain any
considerable quantity of iron it should be rejected for this purpose.
After alum a solution of stannic chloride is most commonly used to
precipitate lakes, but it must be free from stannous chloride; it
produces as a rule darker lakes than alum. Care must be taken that the
stannic chloride solution is free from iron.
The ordinary process for making lakes is very simple: a clear
aqueous solution of the colouring matter is obtained; to this alum
or stannic chloride solution is added in proportion to the amount of
colouring matter dissolved by the water; the metallic oxide is then
precipitated by an alkali. Sodium or potassium carbonate or caustic
alkalis may be used; ammonia is very suitable for this purpose, since
it is free from iron. The precipitant must be cautiously added, it
is introduced in drops when the greater part of the colouring matter
has been precipitated; an excess of the alkaline solution would have
considerable effect on the shade of the precipitate. The precipitated
lake rapidly settles to the bottom of the almost colourless liquid; the
settling is particularly rapid when stannic chloride has been used. The
liquid is then drawn off, the pasty residue brought upon a strainer,
washed several times with water, and dried in the air or in stoves.
When dry, a properly prepared lake forms a mass of little coherence,
and can be readily ground to a soft powder, which may then be ground
with oil or gum solution to produce oil or water paints. At the same
time the materials are added to the pure lake which are used to shade
its colour; white pigments are generally employed for this purpose—the
paleness of the shade is proportional to the amount of white pigment
introduced. Certain lakes are shaded by small quantities of other
colouring matters; for example, the admixture of a small quantity of
a blue pigment with a pure red lake produces a much deeper colour
inclining to purple. If a small quantity of a red or blue colouring
matter is added to a yellow pigment there is produced a deeper yellow
inclining to orange, or a yellow inclining to green, in proportion to
the quantity of the added colouring matter.
It should be observed that it is of great importance for the colour
maker to have a practised eye, sensitive to fine differences of
colour. He may then produce colours faultless in shade, for if by
chance a colour does not turn out well it may be improved by judicious
admixtures. It is quite impossible to give definite rules for the
mixing of colours; the estimation of a shade cannot be taught in words,
it demands long practice. By the use of a system, provided the user
has normal eyes, very slight differences of colour can be readily
estimated. A scale of colours should be made from pigments of pure
and definite shades, in which scale successful mixtures are gradually
inserted, so that a series is produced in which the separate pure
colours blend regularly into one another. Then, when it is required to
produce a pigment corresponding to a certain shade on the scale, with
a little practice the shade can be readily produced, for the scale
indicates which colour predominates.
The so-called sap colours are generally characterised by very pure
shades. They are produced by decomposing a lake by an acid or a strong
base, evaporating the solution of the colouring matter so obtained at
a gentle heat and mixing with gum, starch, or some other thickening
material to produce a mass of such consistency that it can be formed
into balls or sticks.
The decoctions of the dye-woods always contain other substances in
addition to the colouring matters, which are precipitated with the
colouring matter, and somewhat injure its shade. When such a lake
is decomposed the colouring matter is obtained in a purified form,
thus showing its full beauty. This solution of the purified colouring
matter might be again precipitated with a salt solution and lakes be
produced which would be much brighter than the original, but the losses
incidental to this double precipitation would be so great that the lake
would be made much too expensive.