LICHEN COLOURING MATTERS.
Many lichens contain compounds from which colouring matters can be
obtained by appropriate treatment; the colouring matters known as
cudbear, archil and litmus were formerly much more extensively used in
dyeing than at present: like so many natural colouring matters they
have been replaced by coal-tar dyes.
The lichens which produce colouring matters live upon trees and rocks,
chiefly on the sea-shore; the colouring matters are thus generally
manufactured in places near the coast. The species principally used for
this purpose are _Lecanora_, _Rocella_ and _Usnea_.
The lichens contain colourless compounds, which by treatment with acid
or alkalis produce orcinol. In contact with moist air and ammonia,
orcinol is decomposed into water and orcein, which is the colouring
matter. The lichens are treated so as to produce the largest quantity
of orcinol, which is then converted into orcein. There is not space
here to enumerate and describe the many compounds obtained from
lichens, only the most important can be mentioned; these are erythric,
lecanoric and usnic acids. Lecanoric acid produces, under certain
conditions, orsellic acid, which is again changed to orcinol.
Pure orcinol, C₇H₈O₂. H₂O, is a crystalline substance of sweet taste;
it is soluble in ether, alcohol, water, etc. In damp air containing
ammonia, orcinol gradually acquires a fine red colour, being converted
into orcein, C₇H₇NO₃, according to the following reaction:—
C₇H₈O₂ + NH₃ + 3O = C₇H₇NO₃ + 2H₂O.
When pure, orcein is a brownish-red amorphous powder, soluble in
alcohol to a scarlet solution and in alkalis to a purple solution. On
the addition of water to the ammoniacal solution or of salt to the
alkaline solution, orcein is again separated; this procedure may be
used to completely purify the colouring matter.
=Archil= is principally obtained from the lichens _Lecanora_ and
_Rocella_. These lichens have a wide geographical distribution; archil
is made in Sweden, Italy and Spain. It is obtained in a simple manner:
the powdered lichen is mixed with decomposing urine to a paste, which
is allowed to lie in the air until the colour has changed through
red to violet; on the average fourteen days are required for the
transformation. More recently the process has been made less disgusting
by replacing urine by ammoniacal gas liquor. When the mixture is violet
all through the process is finished; the wet mass is packed into
barrels, which are well closed to prevent it from drying.
The presence of alkalis accelerates the formation of orcein; in many
districts it is customary to add a little lime to the mixture of lichen
and urine; alum is also added to retard putrefaction, which readily
occurs in warm climates.
Instead of using lichens to produce the colouring matter, they may be
extracted by boiling water, the solution considerably evaporated and
then exposed to the air with the addition of ammonia, until the violet
colouration appears. Slight additions of sulphuric acid give a more
purple tint; soda produces a deeper violet.
=French Purple= is a compound of the archil colouring matter with
lime, _i.e._, a lime lake. This beautiful substance is obtained by
treating the lichen for some minutes with ammonia, pressing out
the liquid and neutralising it exactly with hydrochloric acid. The
precipitate obtained consists of the lichen acids; it is again
dissolved in ammonia and the solution exposed to the air until it has
become cherry red. When this is the case the liquid is quickly boiled
and placed in large shallow dishes, which are kept at a temperature
of 75° C. until the colour is purple-violet. Calcium chloride is then
added; a garnet red precipitate is produced, which, when washed and
dried, constitutes French purple.
Archil and French purple produce shades of great beauty and purity,
but they have little durability and are quickly altered by exposure to
light. They are, therefore, seldom used as pigments. The same is the
case with cudbear and litmus.
=Cudbear= is a dirty purple powder with an ammoniacal odour and salt
taste. It is made on a large scale in Holland. It differs from archil
only in containing the colouring matter in the solid form.
=Litmus= is generally made from lichens of the genus _Variolaria_,
though varieties of _Rocella_ and _Lecanora_ are often used. It is not
made in quite the same manner as archil and cudbear. Together with the
ammonia, potash is added to the powdered lichen. The mixture is left
until it has become violet, when urine, potash and lime are added at
intervals to continue the decomposition until the colour is changed to
blue and the whole mass has become a paste. Gypsum and lime are then
added, the mixture formed into rectangular plates, and brought into the
market, described by a number, according to the amount of gypsum added.
Pure litmus should leave little solid residue when dissolved in water,
the solution should be a fine violet. On account of its want of
stability this colouring matter is not used in dyeing. It can be used
for colouring foods since it is innocuous; but indigo carmine has much
greater colouring power and at the same time gives fine shades.
The only use of litmus at the present time is as an indicator to decide
whether a solution is acid, neutral or alkaline. In a quite neutral
solution litmus is violet. Acids turn the solution to red and alkalis
to pure blue. In preparing litmus tincture regard must be given to
the fact that the aqueous solution of the colouring matter is always
alkaline; acid must be cautiously added until the colour is violet, and
the least addition of acid or alkali suffices to change it to red or
blue.