CHROME YELLOWS.
The chrome yellows are the lead, zinc or barium salts of chromic acid.
In describing the different methods used in making these colours, lead
chrome yellow will be taken first, since this is the one most largely
used.
LEAD CHROME YELLOW.
Both neutral and basic lead salts of chromic acid are known. Neutral
lead chromate is found in nature as the somewhat rare mineral
crocoisite, which is found in very small but perfectly shaped crystals
in many lead mines.
=Neutral Lead Chromate=, PbCrO₄, is formed as a very heavy precipitate
of a fine deep yellow colour when a solution of potassium chromate or
bichromate is added to a solution of a lead salt in water. When exactly
equivalent quantities of potassium chromate and lead solution are used,
and the strength of the solutions is the same, the product has the
same shade each time the operation is performed. It is not immaterial
whether the one or the other salt is in excess, or whether strong or
weak solutions are used; all these conditions modify the shade of
the chrome yellow produced. Many colour makers are apparently of the
opinion that some particular skill of the workman is necessary to
produce chrome yellow of a particular shade. This is, however, not the
case; manufacturers who know the simple conditions which are important
in making chrome yellows may produce any desired shade without
difficulty.
Neutral lead chromate readily parts with half the chromic acid it
contains. When treated with alkalis, such as lime or caustic soda, or
even when digested with finely ground litharge, the neutral salt gives
up half of its chromic acid, and is converted into the basic chromate
or chrome red, Pb₂CrO₅.
Basic lead chromate has, as the name chrome red indicates, a fine red
colour. If the quantity of lime or caustic soda used is sufficient to
decompose only a portion of the neutral lead chromate, a mixture of the
yellow neutral and the red basic chromate is formed, the shade of which
will incline to yellow or red according as it contains a preponderance
of one or the other compound. The pigment known as orange chrome is
a mixture of approximately equal parts of the neutral and basic lead
chromates.
In order to brighten the deep yellow shade which distinguishes
neutral lead chromate, it is either mixed with a white pigment, or
a white substance (lead sulphate) is precipitated from the solution
simultaneously with the lead chromate. In this manner all the
imaginable pale yellow shades, lemon yellow, sulphur yellow, etc., can
be obtained.
Just as the quantities of the solutions used and their strength
influence the shade of the chrome yellow they produce, so it also
appears to be not immaterial which lead salt is employed. Colour makers
are generally agreed that the finest product is obtained from neutral
lead acetate. Any lead salt, even insoluble in water, may be used for
the preparation of chrome yellow. The affinity of chromic acid for
lead is so great that an interchange of constituents occurs between
the insoluble salt and the potassium chromate. Chrome yellow may be
made from lead acetate, chloride or sulphate; the resulting substances
are the same, but there is a considerable difference in regard to the
beauty of the product. The finest chrome yellow, which leaves nothing
to be desired in beauty of shade, is obtained by proceeding in the
following manner: Lead acetate is dissolved in water, the solution
diluted with an equal volume of water, and then mixed, under constant
stirring, with a similarly diluted solution of potassium chromate or
bichromate. The precipitate is immediately formed, and quickly sinks to
the bottom in consequence of its high specific gravity. It is washed
with clean water so long as this removes soluble salts. The precipitate
is then drained on cloths and dried in the air.
The finest product is obtained by working with the following
proportions:—
Sugar of lead 100
Potassium bichromate, or 50
Potassium chromate 40
If lead sulphate is used the following quantities are to be employed:—
Lead sulphate 100
Potassium bichromate 25
In the case of lead chloride the following is the proportion:—
Lead chloride 100
Potassium bichromate 27
The chrome yellows prepared from insoluble lead salts have no
particular beauty, but they may be used for mixed colours such as the
spurious chrome green.
=Preparation of the Lead Solution.=—Many makers of chrome yellow do not
use commercial lead acetate, but prepare its solution themselves. The
preparation of this solution requires neither much space nor labour, so
that considering the high price of lead acetate this procedure may be
regarded as advisable, but only when acetic acid is obtainable at a low
rate.
The following is the method by which lead acetate solution is made.
Lead is granulated by pouring the molten metal from a height of
several yards into cold water, which is kept in rapid motion. The
smaller the particles of lead, the larger surface they will possess,
and the more quickly they will dissolve. For the solution of the
lead small tubs are used, 50 centimetres in diameter and 90 to 100
centimetres in height, provided with a tap immediately above the bottom
to run off the liquid. Four of these vessels are so placed, one above
the other, that the contents of each may be run into the one next below
it. The lead in the top vessel is covered with acetic acid; in a few
minutes this is allowed to flow into the second vessel, and similarly
after a few minutes from the second to the third, from the third to
the fourth, from which it runs away into a receiver below. After this
treatment it contains but a small quantity of lead acetate; the object
of the operation is to start the oxidation of the lead, which quickly
follows when air has sufficient access, the lead particles lose their
metallic appearance and become covered with a white layer. When this is
the case, the acetic acid is pumped from the receiver back into the top
vessel, where it is left one to two hours in contact with the lead. It
is then run off into the second, and thence into the third and fourth,
remaining in each vessel for about the same time; the resulting liquid
is an almost completely neutral solution of lead acetate. The treatment
with acetic acid is continued so long as lead remains undissolved.
The solution of potassium chromate is prepared in a tub. The salt is
easily soluble, and warming is unnecessary if the chromate is placed in
a basket, lined with close linen cloth, hung in the liquid so that it
is immersed to half its depth. The salt rapidly dissolves, its solution
has a greater density than water, in consequence of which it sinks and
fresh quantities of water continually come in contact with the salt.
=Precipitation of the Chrome Yellow.=—Before the chrome yellow can be
precipitated it is necessary to estimate the quantity of lead acetate
contained in the solution, since upon this depends the quantity of
potassium chromate solution to be used. If the lead solution contained
only acetate and water, its strength could be simply found by means
of the hydrometer. It contains, however, varying quantities of acetic
acid, on which account the hydrometer would give very inaccurate
results. The test by which the relation between lead solution and
potassium chromate solution is found is performed in the following
manner: The lead solution is measured off in a cylinder divided into
100 divisions; the same volume of potassium chromate solution is
measured and placed in a high narrow vessel; the lead solution is
gradually added to the chromate solution so long as a precipitate is
formed. The precipitate settles rapidly, and there is no particular
difficulty with a little practice in finding with sufficient accuracy
the quantities required for the precipitation. In order to precipitate
100 litres of potassium chromate solution, there are required as many
litres of lead solution as were used divisions of the cylinder.
The preparation of the chrome yellow is now a very simple matter.
Whilst steadily stirring, the measured quantity of lead solution is run
into the solution of potassium chromate; the precipitate is allowed to
settle, is well washed and dried. It does not make any difference to
the colour whether potassium chromate or bichromate is used; the same
product is obtained in each case.
It is stated by Dullo that chrome yellow prepared by the preceding
process alters in colour on long keeping. This is ascribed to the
formation of a basic compound. According to the same author, a chrome
yellow free from this objectionable property is obtained by using
lead nitrate in place of acetate and an excess of potassium chromate
solution. The writer has kept chrome yellow, made from lead acetate,
for years without observing the slightest alteration in the colour.
On chemical grounds, it is incomprehensible that a chrome yellow
prepared from lead nitrate should have different properties to the
same substance prepared from another soluble lead salt, and freed from
foreign substances by sufficient washing.
The product of this process is that which _par excellence_ is known
as chrome yellow, the chemist’s neutral lead chromate; it exhibits a
characteristic deep yellow colour, a shade which is known as chrome
yellow. Under the microscope chrome yellow is seen to be a crystalline
mass; it will possess greater covering power the smaller the crystals.
Now, the motion of a liquid in which crystals are forming prevents the
production of large crystals, thus the reason is clear for the rapid
stirring of the solutions in the preparation of this pigment.
According to C. O. Weber, who has published an exhaustive account
of chrome pigments (_Dingler’s Journal_, 282), the cost of the
lead chrome pigments varies greatly according to the raw materials
employed. Assuming that 100 kilogrammes of litharge cost 35 marks, 100
kilogrammes of 30 per cent. acetic acid 25 marks, and 100 kilogrammes
of 60 per cent. nitric acid 26 marks, Weber calculates that 100
kilogrammes of litharge, in a form suitable for making chrome yellow,
will cost as follows:—
From lead acetate at 56 marks per 100 kilogrammes 96 marks
” ” solution made in the works 80 ”
” lead nitrate at 50 marks per 100 kilogrammes 75 ”
” ” solution made in the works 64 ”
By the basic lead acetate method 51 ”
” ” chloride ” 40 ”
White lead method 55 ”
=The Pale Chrome Yellows.=—When the solution of the chromate used
for the precipitation of the lead solution is mixed with sulphuric
acid, then a mixture of lead sulphate and lead chromate is formed
on precipitation. Lead sulphate is white, so that the colour of the
precipitate would be paler according to the quantity of sulphuric acid
added to the potassium chromate solution. There are, however, compounds
of lead chromate and lead sulphate, of which we know two. Their
composition is expressed by the formulæ:—
PbCrO₄.PbSO₄ and PbCrO₄.2PbSO₄.
The former is a beautiful lemon yellow shade, the latter nearly
approaches sulphur yellow. By corresponding alterations in the quantity
of sulphuric acid added, all intermediate shades can be obtained. On
the works these shades are made in the following manner: buckets are
used for taking the potassium chromate solution out of the vessel
in which it was made; these buckets hold 12·5 litres. Now, if the
solution of potassium chromate has been made from 25 kilogrammes of
the salt and 750 kilogrammes of water, one of these buckets holds
exactly 0·43 kilogramme of chromic acid. In order to obtain the lemon
yellow compound, 0·39 kilogramme of sulphuric acid must be added to
a bucketful of solution; 0·78 kilogramme must be added to obtain the
sulphur yellow chrome. These liquids are prepared by pouring the
sulphuric acid in a thin stream into the potassium chromate solution
made as above, the liquid must be stirred whilst the sulphuric acid is
being added. The lemon chrome has the peculiar property of increasing
considerably in volume soon after formation, a property to which
regard must be had in the manufacture. A description of the rational
preparation of the lemon and sulphur yellow shades of chrome yellow on
the manufacturing scale follows.
In making lemon chrome, the tub in which the precipitation is to take
place is two-thirds filled with water, the lead solution is stirred
in, and then the chromate solution, mixed with the proper quantity
of sulphuric acid, is run in; the liquid is well stirred so that
the precipitate may form as quickly as possible throughout the whole
liquid. The precipitate is allowed to settle, the liquid drawn off, and
the colour washed twice with water as quickly as possible. The paste
is then removed from the tub and poured on a strong linen strainer.
At first, the fine precipitate goes through the strainer, the liquid
is poured back on the strainer until the size of its pores is so far
diminished by the precipitate itself that only clear liquid runs
through. The precipitate is left on the strainer until it forms a stiff
mass, which can be easily spread out upon boards by spatulas.
To obtain a good, that is, a loose product, it is necessary to carry
out the processes so quickly that the swelling mentioned above does not
take place while the colour is being strained, but when it has been
spread out on the boards. This swelling only takes place completely
when the layer of precipitate is fairly thin. Large boards should be
used, upon which the precipitate is spread out in a very thin layer.
To prevent the mass—which is still fairly fluid—from running off the
boards, they are provided with raised edges, and the paste is spread
out smoothly in these flat trays. If the operations have been properly
performed, the precipitate at once begins to swell and becomes of
a loose nature. When it has acquired a buttery consistency it is
cut up, by means of a thin sheet of brass, into prisms, which are
placed near one another standing on the narrow side, and dried first
in the shade and then in the sun. It is necessary that the drying
should take place slowly at first, or the cakes will crack or even
fall to pieces. The precipitate cannot be washed completely in the
tub, because it often begins to swell on the strainers, consequently
a crystalline crust covers the surface of the cakes during drying.
This layer must be removed by scraping the cakes of colour, in which
operation small quantities of chrome yellow dust become suspended
in the air. To protect the workman against poisoning by this lead
compound, precautions must be taken against breathing in the dust. The
simplest and most efficacious is to tie a wet sponge over mouth and
nose; this retains the particles of dust in the inspired air. The dust,
scraped off the cakes, is put into water, in which the salts dissolve,
whilst the chrome yellow sinks to the bottom. In this process for
preparing chrome yellow the solution of potassium acetate left after
the precipitation contains a considerable quantity of free acetic acid,
which may be utilised to dissolve lead.
When sulphur yellow chrome is to be made, the process is substantially
the same as for the preparation of the lemon shade, but with the
difference that everything is done to prevent the swelling of the
precipitate. The precipitation and washing of the precipitate are done
as quickly as possible: the washed precipitate is filled into press
bags and strongly pressed. Care must be taken in the pressing that the
pressure is only gradually increased; if powerful pressure is applied
at once, even the strongest cloths will be burst. The more thoroughly
the precipitate is pressed, the closer will be the fracture of the
chrome, a property which is regarded as a sign of good quality in this
species of chrome yellow.
The manufacture of chrome yellow is intimately connected with that of
a number of colours, varying from orange to dark red, which are known
under the names of chrome orange or chrome red. In accordance with the
division adopted of pigments according to their colour, chrome orange
and chrome red will be considered among the red mineral pigments.