VERDIGRIS.
Verdigris is little used as a pigment, but is important to the colour
maker, because copper colours are made from it. In wine-growing
countries it is made from the marc at small expense and without much
labour.
In commerce several kinds of verdigris are distinguished; they differ
in physical and chemical properties. All verdigris consists of copper
acetate, either alone or combined with varying quantities of copper
hydroxide.
=Blue Verdigris= is made in large quantities in France and is commonly
known as French verdigris (_vert de gris naturel_). It has the
following formula:—
Cu(OH)(C₂H₃O₂).2½H₂O.
The process adopted in southern France, especially in the neighbourhood
of Montpellier, is as follows: copper plates are placed in layers in
heaps of the freshly pressed grape residues, which always contain
a certain quantity of must even when the most powerful presses are
used. The process is conducted either in large heaps or in pots; in
the first case care must be taken that the heaps are in a room whose
temperature does not lie below 15° C. It is important to maintain
this temperature, because the formation of acetic acid takes place
with sufficient rapidity only at temperatures not below 12° to 15° C.
Too high a rise of temperature in the heaps must also be avoided or a
considerable quantity of acetic acid will be volatilised. The heaps
should therefore not be made too large, otherwise the temperature
cannot be kept within the proper limits.
The residues contain a considerable quantity of sugar, which can be
transformed by fermentation into alcohol. If air has free access, the
alcohol is at once oxidised to acetic acid, which is recognised by
the acid smell given off. In order to make possible the entry of air
into the interior of the heaps, rectangular wooden bars are introduced
in piling up the heaps. These are carefully withdrawn when the heap
is finished, so that it is traversed by passages through which air
can enter. By the simultaneous action of air and acetic acid copper
acetate is formed, and since copper is always in excess, the acetate
produced is a basic salt. In consequence of the chemical processes, the
temperature of the heap rises to 35° to 40° C.; it is endeavoured to
attain this temperature by artificial heat. It has already been stated
that too great a rise of temperature is accompanied by a considerable
evaporation of acetic acid. The process is then finished in a short
time, but the loss of acetic acid is remarkable. When the temperature
is so regulated that it varies between 25° and 30° C. the process takes
a normal course—four to five days may be regarded as its duration.
The grape residues are effective on account of the sugar, which is
transformed into alcohol and then into acetic acid. Residues which
have been extracted with water after pressing should not be used in
the manufacture of verdigris; they contain but little sugar and hence
undergo a very feeble acetic fermentation. When such residues are used
the verdigris is often accompanied by black spots of copper sulphide
upon the plates; this is due to incipient decomposition of the residues.
As a practical test for the termination of the process a strip of
copper is plunged into the heap and left for some hours. It should
be covered by a uniform coating of verdigris; if it is covered with
small drops that is a sign that the process is not completed and the
heaps must be left some time longer. The course of the process can be
simply and safely followed by means of a thermometer. This is placed in
a perforated copper tube and plunged into the interior of the heap. A
continuous rise of temperature indicates a steady increase of chemical
action; the temperature of the room is regulated in accordance. When
the temperature in the interior of the mass begins to fall the process
is nearing the end. More heat is now supplied to assist the chemical
action. The process is at an end when, in spite of external heat, the
temperature of the heap decreases and approaches the temperature of the
air of the room.
Verdigris can also be made by placing sheet copper and grape residues
in layers in pots, which are deposited in a room of fairly uniform
temperature, such as a cellar. This method has the advantage that the
formation of the verdigris is finished in a shorter time, which is more
than counterbalanced by the labour and expense of filling the pots.
The copper plates, when removed from the heaps or pots, are covered by
a crust of thin needle-shaped crystals of a bright green colour. They
are shaken to free them from the adhering grape skins and seeds, and
then treated with weak vinegar, in which they are dipped; they are then
left standing on edge for several days. This treatment with vinegar has
the object of converting the neutral salt formed on the plates into
basic salt. The dipping into vinegar and exposure to air are repeated
six to eight times, when the originally pure green crust on the plates
is gradually changed into bluish-green verdigris. These processes
are continued until the plates are covered uniformly by a layer of
verdigris about 5 centimetres thick. The crust is then scraped off by
copper knives and stirred with water to a paste, which is pressed in
leather bags in rectangular moulds. The lumps are then slowly dried in
the air.
Plates which have been once used give a larger quantity of verdigris
in a second operation, which is to be ascribed to the fact that they
possess a larger surface than unused plates. New plates are made more
susceptible to the further action of the acid by dipping in strong
vinegar, by which they become covered with a layer of copper acetate.
The verdigris obtained by this method consists of crystalline scales of
a pale bluish-green colour, which produce a pale blue powder. In crude
verdigris grape seeds are often found, and occasionally stalks and
pieces of metallic copper. These admixtures are a consequence of the
method of manufacture and cannot be regarded as adulterants. Gypsum is
to be regarded as an adulterant. Verdigris often contains basic copper
carbonate.
Verdigris behaves in a peculiar manner with water. In contact
with a small quantity it swells to a bluish-green mass, which
becomes quite warm. Neutral copper acetate and the basic salts
2Cu(C₂H₃O₂)₂.Cu(OH)₂.5H₂O and Cu(C₂H₃O₂)₂.2CuO.1½H₂O are formed, the
latter of which is insoluble.
By the addition of a larger quantity of water the basic compounds are
decomposed, neutral copper acetate dissolves, and a mixture remains
of the compound Cu(C₂H₃O₂)₂.2CuO.1½H₂O and a brown basic acetate
containing still less acetic acid. On account of this peculiar
behaviour of verdigris towards water some care is required when it is
used as a water colour. A very dilute solution is not green, but has an
indefinite shade.
=Distilled or Crystallised Verdigris= consists of neutral copper
acetate, Cu(C₂H₃O₂)₂.H₂O. It can be made from blue verdigris by
treating with the amount of acetic acid required to completely
neutralise the copper oxide, or by decomposing copper sulphate with
the acetate of a metal which forms an insoluble or difficultly soluble
sulphate.
Crystallised verdigris is very simply made from ordinary verdigris;
the latter, whilst still moist, but containing only a small quantity
of water, or the acetic acid would be too largely diluted, is brought
into a pan and strong acetic acid poured over. Strong pyroligneous acid
may be used, its empyreumatic odour is without influence on the quality
of the product. The pan is heated until the contents almost, but not
quite, boil. They are frequently stirred so that the particles at the
bottom are brought in contact with the acid. A dark green solution is
formed; when its colour no longer increases in depth it is allowed to
stand until suspended solids sink to the bottom. The clear solution is
then drawn off and rapidly evaporated in shallow copper pans. It is
important that the liquid should not contain an appreciable quantity
of free acid. When a crystalline skin begins to form on the surface of
the solution it is drawn off into the crystallising vessels. These are
made of glazed earthenware; wooden rods are placed in them, on which
the crystallisation takes place. In order that large crystals shall
be obtained, it is necessary to maintain a regular temperature in the
room in which the crystallising vessels are placed. It is heated at the
commencement, and the temperature is allowed to sink a little towards
the end of the operation. From twelve to fourteen days are required to
obtain large crystals. The mass of verdigris crystals adhering to each
rod weighs 2·5 to 3 kilogrammes. The residual liquid is a saturated
solution of verdigris, and is used in the next operation. It is only
necessary to heat it with the residue from the first operation and some
acetic acid in order to obtain a crystallisable solution of verdigris.
The residue in the pan contains metallic copper, grape stems and
seeds, and basic copper acetate. The copper is extracted by moistening
with acetic acid and exposing to the air, when verdigris is formed,
which is added to a later batch.
It is often advisable to make verdigris from a soluble copper salt; the
process varies somewhat according to the salt employed. From copper
sulphate verdigris is obtained by the action of calcium acetate. When
solutions of these compounds are mixed, calcium sulphate separates
as a white precipitate, whilst a solution of the easily soluble
copper acetate is left. It is only necessary to mix the solutions in
equivalent proportions, separate the liquid from the precipitated
gypsum and evaporate. Gypsum is somewhat soluble in water, so that when
the solution is evaporated a double salt separates out first. This
consists of copper calcium acetate; it may be used as a pigment, but is
of less value than verdigris. It is advisable to prevent the formation
of this double salt, which is accomplished by adding copper sulphate
to a slightly acid solution of calcium acetate until a precipitate
is no longer produced. The solution of copper acetate is boiled for
several hours to effect the separation of the dissolved gypsum; at the
same time the iron contained as impurity in the copper sulphate is
also precipitated. The purified solution of verdigris is concentrated
and allowed to cool, when a further quantity of gypsum separates; the
solution is then evaporated to crystallisation.
If barium salts and lead acetate are cheap, barium acetate can be
obtained by double decomposition. When copper sulphate is added to
the solution of barium acetate so long as a precipitate is formed, a
solution of verdigris is produced, which only requires evaporation
to crystallise. The precipitate is barium sulphate, and can be used
as enamel white. It obstinately retains a quantity of copper, which,
though small, is sufficient to give it a greenish tint. By several
washings with acetic acid this copper is removed; the acid can be used
to obtain fresh quantities of verdigris, so that none is lost, and the
copper retained in the precipitate is regained. The precipitate, after
treatment with acetic acid, only requires thorough washing with water
to produce permanent white, satisfactory in every respect.
Ammonia may also be used in the manufacture of verdigris from copper
sulphate. Strong ammonia is left in contact with copper sulphate in
a covered vessel for several hours; gentle warming accelerates the
formation of copper ammonium sulphate. The liquid has then a fine blue
colour; it should not contain sufficient ammonia to be perceptible by
the smell. It is then gently warmed and acetic acid gradually added.
On continued heating small green crystals separate from the solution,
which is boiled with continual additions of acetic acid in small
quantities until small crystals appear on the surface, when it is
allowed to cool and the crystals are strained off.
”Distilled” verdigris forms deep bluish-green crystals, which
effloresce slightly in the air; they dissolve in 5 parts of boiling
water and in 13·4 parts at 20° C. When the solution is boiled for some
time the tribasic acetate separates; the liquid becomes brown and
acquires an acid reaction.
=German Verdigris= does not differ essentially from ordinary verdigris.
It is a compound of the following basic acetates: 2Cu(C₂H₃O₂)₂.CuO
and Cu(C₂H₃O₂)₂.2CuO. The method is similar to that in which grape
residues are used; where acetic acid is cheap the process is
especially appropriate. In Sweden, where pyroligneous acid is made
in large quantities and copper is also cheap, verdigris is made by
arranging copper plates and flannel in alternate layers. The plates
are frequently turned over and the flannel kept saturated with acetic
acid. The same chemical reactions occur as in the process in which
grape residues are used. When a layer of verdigris has formed on
the copper plates they are taken apart, exposed to the air, and
frequently wetted. The formation of the verdigris is thus completed.
The difference between this process and that adopted in wine-producing
countries lies simply in the use of pure acetic acid; the product is
free from the mechanical impurities introduced by the grape residues.
In properties German or Swedish verdigris is completely identical with
blue verdigris.