METALLIC COMPOUNDS.
ALKALIS.
The compounds of the alkali metals, potassium and sodium, play a
considerable part in colour making. Formerly the potassium compounds
were in general use, but the sodium compounds are at present obtainable
at a much lower price, and in most cases they can be used equally well.
Thus, in colour making, sodium compounds are chiefly employed. The
cyanogen compounds are an exception; their potassium compounds are used
exclusively.
=Potassium Compounds.=—The potassium compounds which are chiefly
used in colour making are potassium carbonate (potashes, pearl-ash),
potassium hydroxide (caustic potash), potassium nitrate (saltpetre),
potassium tartrate (tartar), and potassium ferrocyanide and
ferricyanide (yellow and red prussiate of potash). The cyanogen
compounds have peculiar properties. We shall describe them separately
after the potassium and sodium compounds.
=Potassium Carbonate= (carbonate of potash), K₂CO₃ = 138, is known
commercially as potashes, a name derived from its former method of
preparation by heating the ashes of plants in pots. At present potashes
are prepared in large quantities from other sources.
Pure potash forms crumbling lumps with a slight yellow or bluish grey
tinge, rapidly absorbing moisture from the air, and in time completely
liquefying. The yellowish tinge is caused by oxide of iron, the bluish
by manganese compounds. The so-called calcined potash has been strongly
heated, and thus all organic substances contained in it have been
destroyed.
Potashes are in no way pure potassium carbonate; they contain a mixture
of all those salts which are found in plants—potassium sulphate and
chloride, small quantities of silicic acid, etc. These impurities are
rarely harmful, still it is generally necessary to know the percentage
of pure potassium carbonate contained in potashes.
Although at present in commerce the strength of potashes is frequently
guaranteed, it is still desirable to estimate the strength. It is
sufficient for practice to allow a small quantity, say 100 grammes, to
stand with an equal quantity of very cold water for some hours, then
to filter and pour a similar quantity of water over the residue on
the filter. The weight of undissolved substance subtracted from 100
gives with sufficient accuracy the weight of pure potassium carbonate
contained in 100 parts of potashes. This method is founded on the fact
that potassium carbonate dissolves readily even in cold water, but the
other salts with difficulty. This procedure can also be used to obtain
pure potassium carbonate from crude potashes; it is only necessary to
filter and evaporate to dryness the solution obtained by pouring very
cold water on crude potashes.
=Potassium Hydroxide= (Potassium Hydrate, Caustic Potash), KOH =
56.—The commercial variety consists of very deliquescent white lumps,
generally containing a large quantity of impurities. On this account
caustic potash, or rather a solution of it, is prepared in the colour
works.
With this object 11 parts of potash, contained in a tub with an opening
at the bottom, are mixed with 100 parts of cold water. Two hours
later, the clear solution is run off into a clean iron pan, in which
it is heated to boiling. To the boiling solution is added milk of
lime prepared from water and 3·5 parts of quicklime. After the liquid
has boiled a few minutes, a small portion is filtered and hydrochloric
acid added to the clear filtrate; if no effervescence occurs, then
all the potassium carbonate is converted into caustic potash. Should
effervescence occur, milk of lime is added until a new portion no
longer effervesces on the addition of hydrochloric acid. Then the pan
is covered with a well-fitting lid, and the cooled liquid, if not
required for immediate use, preserved in well-corked glass bottles.
The strength of a caustic potash solution can be found by means of a
hydrometer. The following table shows the relation between the specific
gravity of a solution and the percentage of caustic potash it contains:—
+----------+----------------+
| Specific | Caustic Potash |
| Gravity. | per cent. |
+----------+----------------+
| 1·06 | 4·7 |
| 1·11 | 9·5 |
| 1·15 | 13·0 |
| 1·19 | 16·2 |
| 1·23 | 19·5 |
| 1·28 | 23·4 |
| 1·39 | 32·4 |
| 1·52 | 42·9 |
| 1·60 | 46·7 |
| 1·68 | 51·2 |
+----------+----------------+
=Potassium Nitrate= (Saltpetre), KNO₃ = 101, consists of large
crystals, which quickly dissolve in water. On heating it readily gives
up oxygen, and thus finds use as an oxidising agent. In former times,
when the colour manufacturer was compelled to make his own materials,
saltpetre was of great importance in colour making; at present, when
such materials are to be bought at low prices and no colour maker
prepares his own, saltpetre is little used.
=Potassium Bitartrate=, C₄H₅KO₆ = 188.—This salt, known as tartar
in large crystals, and as cream of tartar in the form of meal, is
occasionally used in colour making. It is little soluble in cold water,
but more easily in hot. The hot solution is generally used.
=Potassium Bichromate= (Bichromate of Potash), K₂Cr₂O⁷ = 295.—This salt
is made in special works, by melting chrome iron ore with saltpetre and
extracting the mass with water, when a yellow solution of potassium
chromate is obtained; to this sulphuric acid is added, which unites
with half the potassium, thus leaving potassium bichromate, which is
obtained by evaporation of the solution in fine red crystals. These
are purified by recrystallisation. At present, in place of the above
method, calcium chromate is formed by roasting chrome iron ore with
lime; the calcium chromate is then decomposed by a soluble potassium
salt, thus forming potassium chromate.
Potassium bichromate is unaltered in air; it dissolves easily in
water, and is of great importance in the preparation of many colours,
in particular chromium oxide and the lead pigments. The commercial
salt generally contains potassium sulphate, with which at times it is
intentionally adulterated. The adulteration is detected by dissolving
in water, adding half the volume of pure hydrochloric acid, and
cautiously and carefully dropping in spirits of wine. A rapid action
takes place, which is only assisted by warming when necessary. The red
liquid changes to emerald green. If barium chloride be now added, a
white precipitate is obtained in the presence of potassium sulphate.
=Potassium Sodium Chromate=, KNaCrO₄ = 279, is also used in colour
making. Its solution is made by adding soda to a solution of potassium
bichromate so long as an effervescence of carbonic acid occurs, and
until the liquid turns red litmus paper blue; the solution of the
double salt is yellow.
=Chrome Alum=, KCr(SO₄)₂.12H₂O = 499.—This salt occurs in commerce
as beautiful violet crystals. It is obtained as a by-product in the
manufacture of aniline and anthracene dyes, and may often be bought
at lower prices than other chromium salts; 100 parts of water dissolve
approximately 20 parts of chrome alum.
=Potassium Ferrocyanide=, K₄Fe(CN)₆.3H₂O = 422.—The potassium iron
cyanogen compounds are made in special works, particularly in the
neighbourhood of large towns, by melting potashes with nitrogenous
organic substances and iron, washing out the mass and purifying the
salt so obtained by recrystallisation. Potassium ferrocyanide (yellow
prussiate of potash) forms large transparent crystals of a peculiar
soft nature, which dissolve readily in water. It often contains
considerable quantities of potassium sulphate, up to 5 per cent., and
it is to be noted that the impurity is much the cheaper of the two
salts. When barium chloride is added to a solution of the salt, a white
precipitate forms if sulphate be present.
The behaviour of yellow prussiate towards iron salts is noteworthy.
With ferrous salts, for example green vitriol (copperas), it gives a
white precipitate which gradually turns blue in the air; with ferric
salts, for example ferric chloride (“nitrate of iron”), it at once
gives a blue precipitate.
=Potassium Ferricyanide= (Red Prussiate of Potash), K₃Fe(CN)₆ = 329,
is obtained by passing chlorine through a solution of yellow prussiate
until the liquid smells strongly of chlorine and no longer gives
a precipitate with a solution of a ferric salt. The solution then
contains potassium ferricyanide and chloride. The former is obtained by
evaporating and allowing to crystallise.
Pure potassium ferricyanide forms beautiful dark red crystals, which
readily dissolve in water. The solution gives a blue precipitate with
ferrous salts, but only a brown colouration and no precipitate with
ferric salts. Both yellow and red prussiate are used in the preparation
of several much used colours, for Prussian and Chinese blues, and
several others. All cyanogen compounds, with the exception of yellow
prussiate, are extremely poisonous. The following table gives the
solubility of potassium ferricyanide at different temperatures:—
+-----------------------+------------+----------------+
| 100 Parts of Water |Temperature.|Specific Gravity|
|dissolve Parts of Salt.+------------+ of Solution. |
| | °C. | |
+-----------------------+------------+----------------+
| 33 | 4.44 | 1.151 |
| 36 | 10.00 | 1.164 |
| 40.8 | 15.50 | 1.178 |
| 58.8 | 37.80 | 1.225 |
| 77.5 | 100.00 | 1.250 |
| 82.6 | 104.40 | 1.265 |
+-----------------------+------------+----------------+
=Sodium Salts.=—In chemical properties the sodium salts are very
similar to the potassium salts, and, being cheaper, they are generally
used in place of the latter.
=Sodium Carbonate= (Soda Crystals), Na₂CO₃.10H₂O = 286, is made in
enormous quantities in great works and in a very pure state. It forms
large transparent crystals, which effloresce in the air, losing a
large quantity of water, and so falling to a white powder. Although
this property does not interfere with the use of soda, since it is
generally used in solution, yet efflorescence should be as far as
possible avoided by keeping the salt in well-closed packages, because
effloresced soda dissolves more slowly than crystallised, since it has
to combine with water before it can enter into solution.
In the retail trade a form of soda is found which is adulterated with
very large quantities of Glauber’s salt. This is recognised by the
different form of the crystals. Manufacturers sell soda stating its
strength. The colour maker should only buy with this guarantee.
=Sodium Hydroxide= (Sodium Hydrate, Caustic Soda), NaOH = 40, comes
into commerce in the form of hard masses, the use of which would be
very convenient for the colour maker if it were not often very impure.
Thus it is better to prepare a solution oneself, which is accomplished
in the manner given above for caustic potash.
Caustic soda and caustic potash have similar properties; they have a
corrosive action on the skin, readily unite with carbonic acid from the
air, and separate the heavy metals from their solutions in the form of
hydrated oxides.
The strength of caustic soda solutions is given in the following table:—
+----------+----------+----------+----------+
| Specific | Caustic | Specific | Caustic |
| Gravity. | Soda | Gravity. | Soda |
| | per cent.| | per cent.|
+----------+----------+----------+----------+
| 2·00 | 77·8 | 1·40 | 29·0 |
| 1·85 | 63·6 | 1·36 | 26·0 |
| 1·72 | 53·8 | 1·32 | 23·0 |
| 1·63 | 46·6 | 1·29 | 19·0 |
| 1·56 | 41·2 | 1·23 | 16·0 |
| 1·50 | 36·8 | 1·18 | 13·0 |
| 1·47 | 34·0 | 1·12 | 9·0 |
| 1·44 | 31·0 | 1·06 | 4·7 |
+----------+----------+----------+----------+
Besides soda and caustic soda, few soda salts are used in colour
making. However, sodium nitrate (Chili saltpetre), NaNO₃, is frequently
used instead of ordinary saltpetre, from which it differs in being
deliquescent.
=Sodium Thiosulphate= (Hyposulphite), Na₂S₂O₃.5H₂O = 248, is a common
article of commerce, being much used by photographers; it is used in
a few cases in colour making. It forms large crystals with a somewhat
bitter taste, permanent in air and readily soluble in water.
=Sodium Chloride= (Common Salt), NaCl = 58·5, which has a little
application in colour making, is sufficiently pure in the form in which
it is generally used for household purposes.
SALTS OF THE ALKALINE EARTH METALS.
The metals which are known as the alkali earth metals have much
similarity with the alkali metals in their compounds, with the
difference that their alkalinity is much less, and that their salts
are much less soluble in water. For colour making the compounds of
three of these metals—calcium, barium and magnesium—are used.
CALCIUM COMPOUNDS.
The most important calcium compounds are lime and carbonate and
phosphate of lime. Carbonate of lime is used as a pigment, and will be
dealt with in detail among the mineral colours; it will be but briefly
described here.
=Calcium Oxide= (Quicklime), CaO = 56.—When chalk and limestone, which
consist of calcium carbonate, are heated, carbonic acid is evolved, and
calcium oxide, commonly called quicklime, is left. For our purposes
only very pure quicklime is to be used. Its ordinary impurities are
iron oxide and magnesia: the former is found in lime made from red or
brown limestone; the latter in lime made from dolomitic limestone. The
presence of oxide of iron is recognised by the reddish tinge of the
quicklime; if magnesia be present, a small quantity of the quicklime,
when mixed with a very large quantity of water, leaves an insoluble
residue which consists of magnesia.
When quicklime and water are brought together they unite very
energetically and form calcium hydroxide or slaked lime. According to
the quantity of water used for slaking, either dry slaked lime, lime
paste, or milk of lime is produced, all of which find a use in colour
making.
=Calcium Hydroxide= (Slaked Lime), Ca(OH)₂ = 74.—In order to prepare
slaked lime, which contains lime united with just the necessary
quantity of water, the pieces of quicklime are sprinkled with water
from a watering can. The water is rapidly taken up and the sprinkling
is repeated until the lumps begin to fall to a fine powder; in the
process the lime becomes very hot. The slaked lime is then passed
through a sieve in order to separate the larger pieces of quicklime
which have not been slaked; the powder must be kept in well closed
packages, since it energetically absorbs carbonic acid out of the air.
If so much water is added to the lime that a homogeneous wet mass is
formed which can be readily moved with a shovel, one has then lime
paste, which can be conveniently kept in pits as the masons do; it may
be stored in this way for many months without appreciable alteration,
still it is better to keep it covered. To prepare milk of lime, so
much water is used in slaking that a milky liquid is formed, or the
lime paste is mixed up in the proper quantity of water. Slaked lime
dissolves in 700 to 800 parts of water; on standing, the undissolved
slaked lime settles to the bottom of the milk of lime: thus it is
better to prepare milk of lime immediately before use, and to stir it
well to prevent the settling of the solid particles.
Slaked lime in one of its forms is often used instead of the more
costly caustic soda in order to precipitate metallic oxides from their
salts.
At times one finds a too strongly burnt lime, so-called “dead-burnt”
lime, which is very slowly slaked by water. Such quicklime is slaked by
allowing it to lie in water for days, or by means of hot water, which
accomplishes the slaking more quickly.
=Calcium Carbonate=, CaCO₃ = 100, is found naturally in large
quantities as chalk, which consists of the skeletons of extremely small
animals. By powdering and levigating, it is converted into a soft
powder, which is used to lighten the shade of lakes and other colours.
=Calcium Sulphate= (Gypsum), CaSO₄.2H₂O = 172.—This mineral, when
finely powdered, is added to some colours.
=Calcium Phosphate= (Bone Ash), Ca₃(PO₄)₂ = 310, is sometimes used to
lighten the shade of certain colours which might be injured by calcium
carbonate. It comes in large quantities as a fine, white powder from
South America. Naturally, only quite white bone ash can be used; if not
completely burnt it is grey, and will then impair the shade of colours
with which it is mixed.
=Magnesium Carbonate= (Magnesia), MgCO₃ = 84, is also used as an
addition to colours in order to obtain pale shades. It is most cheaply
obtained by dissolving magnesium sulphate (Epsom salts) in water and
adding soda solution so long as a precipitate forms, which is then
washed and dried. The magnesium carbonate prepared in this way is a
very fine, light powder, insoluble in water, which can be mixed with
the most delicate colours without harming them. White magnesia is an
extremely light powder, which may be used when it can be bought at as
low a price as the above preparation.
=Barium Compounds.=—The raw material used for the preparation of barium
pigments is either barium sulphate (barytes, heavy spar) or barium
carbonate (witherite). The latter is much more rare than barytes, which
is almost exclusively employed in the preparation of barium compounds.
The barium compounds of particular importance for our purpose are
barium chloride and nitrate.
=Barium Chloride=, BaCl₂.2H₂O = 244.—This salt is now a common article
of trade, and can be bought at a low price. When pure it forms
colourless crystals readily soluble in water. If the colour maker is
able to get cheap barytes and fuel, it may be advantageous for him to
prepare barium chloride himself.
To prepare barium chloride from barytes, the latter is very finely
ground and levigated, intimately mixed with coal, and the mixture
subjected to a very high temperature, when barium sulphide is formed,
which is dissolved by washing out the mass with water and converted
into barium chloride by adding hydrochloric acid, sulphuretted hydrogen
being evolved.
The best method is to mix 4 parts of barytes with 1 part of bituminous
coal and so much coal-tar that a plastic mass is formed, which is well
kneaded and made into small cylinders 3 centimetres in diameter and 10
centimetres long. These cylinders are placed in layers in a cylindrical
furnace with a good draught, which contains at the bottom a layer of
coal 15 to 20 centimetres thick, then a layer of the cylinders, then
again coal, and so on until the furnace is full. The lowest layer
of coal is lighted, and the whole burnt at a bright red heat, when
the barium sulphate is changed into sulphide. Hydrochloric acid is
poured over the residue and the insoluble part, consisting chiefly of
unaltered barytes, is used for the next operation.
Witherite (barium carbonate) can be converted into barium chloride in a
very simple manner. Hydrochloric acid is added, in which it dissolves
with the evolution of carbonic acid. The solution is allowed to stand
twenty-four hours with excess of witherite; the whole of the dissolved
iron is thus precipitated. The solution is then filtered, evaporated
down and left, when pure barium chloride crystallises out.
Barium chloride and all soluble barium salts should only be dissolved
in pure water (rain or distilled). Water which contains carbonates
or sulphates always gives a turbid solution by precipitating barium
carbonate or sulphate.
ALUMINIUM COMPOUNDS.
The compounds of the earth metal aluminium play a very important part
in colour making, since they form beautifully coloured compounds with
many organic colouring matters. Formerly alum was the only material
used in colour factories for the preparation of the alumina compounds;
at present aluminium sulphate is used, and when it is sufficiently pure
it is the most valuable material, because it contains the greatest
quantity of alumina.
Under the designation of alum only one compound, the so-called potash
alum, was at one time found in commerce, but now there are other alums,
which contain soda or ammonia in place of potash. These salts are of
equal use in colour making to potash alum. The preference is to be
given to the compound which contains the largest proportion of alumina.
The chief point to be observed in connection with alumina compounds for
use in colour making is that they shall be free from iron, because iron
oxide, which would be precipitated out of the solution along with the
colours, in consequence of its red colour would spoil the shade of the
pigment.
=Aluminium Sulphate= (Sulphate of Alumina), Al₂(SO₄)₃.18H₂O = 664.—Any
manufacturer who can obtain cheap china clay (kaolin) and sulphuric
acid can himself prepare this compound with advantage. The apparatus
used for this purpose is an iron pan containing sand, in which is
placed a large earthenware dish. In this dish are put very finely
ground kaolin and strong sulphuric acid, and the mixture is heated so
strongly that the acid boils, evolving heavy, white vapours. It is
absolutely necessary to heat in this manner in order to avoid dangerous
accidents. Sulphuric acid bumps so violently on boiling that it may
even break a thick earthenware dish. The use of a sand bath makes the
bumping harmless.
China clay, which consists of silicate of alumina, is decomposed by
heating with sulphuric acid into silicic acid and sulphate of alumina.
The original milky liquid becomes more transparent during boiling, and
has at last the appearance of starch paste. Kaolin contains varying
quantities of silica. The quantity of sulphuric acid necessary for its
decomposition can only be found by trial. The quantities are chosen so
that a small amount of kaolin remains undecomposed in order that the
aluminium sulphate shall contain no free sulphuric acid.
When the decomposition is finished the pan is allowed to cool and the
solid mass is brought into a vat filled with water, in which it is
stirred until dissolved; then the liquid is left until the jelly-like
mass of silicic acid has sunk to the bottom, when the clear solution of
aluminium sulphate is drawn off and can at once be used.
If solid aluminium sulphate is required—and this is to be recommended
when large quantities are to be prepared—the solution is evaporated
in earthenware dishes until a portion solidifies when dropped on a
cold plate. The molten aluminium sulphate is then cast in prismatic
blocks, which are preserved in boxes. These blocks are of a pure white
colour and very crystalline; they dissolve with difficulty in cold, but
readily in hot water without residue. The solution has an acid taste,
even when it contains no excess of sulphuric acid. When the blocks have
a yellowish tinge, this denotes the presence of iron, and the solution
must be freed from this impurity. Nowadays sulphate of alumina can be
obtained so cheaply that it is hardly of advantage to make it.
THE ALUMS.
These are double salts of aluminium sulphate and potassium, sodium or
ammonium sulphate. There are also other double sulphates known as alums
which, in place of aluminium, contain chromium, iron, etc., but they
are not of interest here. It may still be observed that all alums,
whatever their composition, possess the property of crystallising
together from mixed solutions, so that crystals can be obtained in
which every existing alum is contained.
The potassium, sodium and ammonium aluminium alums are used in colour
making.
=Potassium Aluminium Alum=, KAl(SO₄)₂.12H₂O = 474.—This is the
substance commonly called alum. Like all alums it crystallises in
fine octahedral crystals, which at first are quite transparent, but
slowly effloresce in the air and become covered by a white powder.
It dissolves with difficulty in cold, but readily in hot water. The
solution has at first a sweet taste, with an astringent after-taste.
Alum comes into the market in different forms, of which the following
are the most important: as crystallised alum, in the form of large
crystals united together; as alum meal, a coarse crystalline powder
obtained by rapidly cooling and stirring a hot alum solution. On
account of the larger surface this form dissolves more quickly than the
large crystals. Roman alum is the name of a variety chiefly imported
from Italy; it owes its reputation to its great purity—it contains a
very small quantity of iron.
In order to prepare alum quite free from iron from the ordinary alum
containing iron, it is recrystallised, that is, as much as possible
is dissolved in boiling water and the solution quickly cooled with
continual stirring; the small crystals so obtained are then washed with
cold water. The residual saturated solution of alum, which contains
the greater part of the iron, can be used for the preparation of those
colours which are not injured by the presence of iron.
The solubility of alum in water varies greatly at different
temperatures. The table gives the weight of alum dissolved by 100 parts
of water at different temperatures.
+------------+------------------+---------------+
|Temperature.|Crystallised Alum.|Anhydrous Alum.|
+------------+------------------+---------------+
| 0° C. | 3·90 | 2·10 |
| 10° ” | 9·52 | 4·99 |
| 20° ” | 15·13 | 7·74 |
| 30° ” | 22·01 | 10·94 |
| 40° ” | 30·92 | 14·88 |
| 50° ” | 44·11 | 20·09 |
| 60° ” | 66·65 | 26·76 |
| 70° ” | 90·67 | 35·11 |
| 80° ” | 134·47 | 45·66 |
| 90° ” | 209·31 | 58·64 |
| 100° ” | 357·48 | 74·53 |
+------------+------------------+---------------+
When potash alum is heated it loses water, 75 per cent. of the total
at 61° C.; at 92° C. it melts completely, and all the water is lost by
continued heating at 100° C. The residue is known as burnt alum.
In alum the whole acidity of the sulphuric acid is not neutralised;
the solution has always an acid reaction; if soda solution is added,
the escaping carbonic acid causes the liquid to effervesce. If soda
solution is added until a further addition would cause a precipitate,
a solution of so-called neutral alum is formed which has no longer
an acid reaction. Neutral alum is occasionally required in colour
making. In preparing it the soda solution must be added with great care
when the liquid is near its point of neutralisation. Any addition of
soda solution after this point is reached will cause a separation of
alumina. This is not desirable, since it is generally only wished to
precipitate the alumina in combination with colouring matters.
=Roman Alum.=—Under this name, or that of “cubic alum,” a variety of
alum is sold, generally at a rather higher price than ordinary alum,
from which it is distinguished by its crystalline form. Ordinary alum
forms octahedral crystals often the size of a child’s head, but cubic
alum well formed cubes.
The property of crystallising in cubes may be imparted to any alum
solution by the addition of a little potash. Much so-called Roman alum
is made in German works in this way. When this alum contains very
little iron it is quite equal in quality to the best Roman alum, for
the higher value of the latter is entirely due to its small content of
iron. The alum prepared in the province of Naples is still better than
Roman alum; it contains less iron.
When alum is required for the preparation of lakes of bright and
delicate shades, it is indispensable to use a preparation free from
iron, because the brownish yellow oxide of iron would appreciably
injure the shade. Alum, free from iron, is most simply prepared by
dissolving alum in boiling water, running the boiling solution quickly
through a cloth, and quickly cooling with constant stirring. The alum
meal prepared in this way contains much less iron than the original
alum, the iron salts remaining dissolved in the mother liquor. When
this alum meal is collected and cold water poured over it to free it
from mother liquor, it is generally sufficiently pure to be used for
any purpose, but if not, it is again recrystallised. The alum liquors
containing the iron are used for the preparation of colours which are
not injured by the presence of iron.
To test alum for iron yellow prussiate of potash is used, which gives
a blue precipitate with ferric salts. The test is carried out by
dissolving 10 grammes of alum in 1 litre of water, placing the solution
in a tall, narrow cylinder standing on white paper, adding 10 to 20
drops of a saturated solution of yellow prussiate, and well stirring.
On looking down through the liquid, if a distinct colouration is at
once evident, the alum contains much iron, and must be recrystallised;
indeed, the crystals would generally be coloured yellow. On the
contrary, if the solution does not show a blue tint until after
standing several days, the alum contains but a small quantity of
iron, and can be used for most purposes without further purification.
Alum quite free from iron is a rare commercial article; the test will
generally show a feeble blue colouration. If this is not intense and
no blue precipitate is deposited at the bottom, the alum is tolerably
pure, and can be used in colour works. The longer the time before the
blue colouration appears the poorer is the alum in iron.
=Soda Alum=, NaAl(SO₄)₂.12H₂O = 458, is made in some alum works. It
has the greatest similarity in properties with the potash salt, but
is distinguished by a much greater solubility in water and more rapid
efflorescence in air. Soda alum can be bought at varying prices; that
containing iron is much cheaper than that free from iron. When the
latter is to be bought at a fair price it is to be preferred to potash
alum, since, as we shall show later, it contains a larger proportion of
alumina.
=Ammonia Alum=, (NH₄)Al(SO₄)₂.12H₂O = 453.—This compound of aluminium
sulphate and ammonium sulphate is now often met with; the more
expensive potassium sulphate in ordinary alum is replaced by ammonium
sulphate, which is cheaply obtained from the ammoniacal liquor of the
gas works.
Ammonia alum is better for our purpose than potash alum since it
contains more alumina, is generally cheaper and dissolves more easily
in water. Unfortunately most commercial ammonia alum contains so much
iron that it has to be recrystallised before it can be used in colour
works.
One hundred parts of water dissolve at different temperatures the
quantities of ammonia alum given in the table:—
+------------+--------------------------+-----------------------+
|Temperature.|Crystallised Ammonia Alum.|Anhydrous Ammonia Alum.|
+------------+--------------------------+-----------------------+
| 0° C. | 5·22 | 2·62 |
| 10° ” | 9·16 | 4·50 |
| 20° ” | 13·66 | 6·57 |
| 30° ” | 19·29 | 9·05 |
| 40° ” | 27·27 | 12·35 |
| 50° ” | 36·51 | 15·90 |
| 60° ” | 51·29 | 21·95 |
| 70° ” | 71·97 | 26·09 |
| 80° ” | 103·08 | 35·19 |
| 90° ” | 187·82 | 50·30 |
| 100° ” | 421·90 | 70·83 |
+------------+--------------------------+-----------------------+
Of the different alums, ammonia alum contains the largest and potash
alum the smallest proportion of alumina. The composition of the three
commonly occurring alums is given in the following table:—
+--------------------+------------+----------+-------------+
| |Potash Alum.|Soda Alum.|Ammonia Alum.|
+--------------------+------------+----------+-------------+
|Potash, K₂O | 9·95 | | |
|Soda, Na₂O | | 6·80 | |
|Ammonia, NH₃ | | | 3·89 |
|Alumina, Al₂O₃ | 10·83 | 11·20 | 11·90 |
|Sulphuric acid, SO₃ | 33·71 | 34·90 | 36·10 |
|Water, H₂O₄ | 45·51 | 47·10 | 48·11 |
+--------------------+------------+----------+-------------+
Thus ammonia alum is to be preferred to soda alum and soda to potash
alum, whilst the latter is used on account of its greater purity. In
a colour works, in which large quantities of alum are used, it is
advantageous to work with ammonia alum which is recrystallised on the
works.
The alums and aluminium sulphate are the alumina compounds in ordinary
use in colour works; aluminium acetate could also be used if it were to
be had at a reasonable price.
=Alumina=, Al₂O₃ = 102, =and Hydrate of Alumina=.—Pure alumina, or
rather hydrate of alumina, is required in the preparation of many
colours. When the solution of an aluminium salt is precipitated by
soda, the carbonic acid escapes with effervescence, and a gelatinous
precipitate is formed which it is extremely difficult to wash clean.
The precipitate, which consists of hydrate of alumina, shrinks very
greatly in drying, and turns to a horny mass; when strongly heated it
loses water, and becomes a white, insoluble powder of anhydrous alumina.
A variety of hydrate of alumina, heavy, and therefore easily washed, is
obtained by boiling a solution of alum with a plate of zinc lying on a
copper plate until all the alumina has separated. By collecting this on
a filter and pouring hot water over it a number of times it is obtained
quite pure.
Alumina plays a particularly important part in the manufacture of
cobalt colours. When treated with a cobalt salt and heated it takes a
fine blue shade.
In the foregoing those metals and their compounds have been treated
which are extensively used in preparing colours without themselves
forming coloured compounds. The chromates and prussiates are an
exception to this. The ammonia compounds, the alkalis and alkaline
earths, also the acids, are used in making many colours, although
they do not contain colouring principles. The alumina compounds are
in similar case. Themselves colourless, they form at the same time a
carrier for the coloured compound and bring it into a suitable form for
use as a pigment.
An example will explain what we mean by “carrier” of the colouring
matter. Logwood contains a very handsome colouring matter which can be
extracted by water. In order to be able to employ this colouring matter
as a pigment it is combined with alumina, a compound insoluble in water
being formed, which is called a lake. In this compound the alumina is
to be regarded as the carrier of the colouring matter, which it has
fixed in an insoluble form.
In dyeing, which in many respects is closely allied to colour making,
the property of certain metallic compounds, themselves colourless, of
fixing dyes is commonly utilised, the metallic compound being called
the mordant. The fabric is first prepared with the metallic compound or
mordant, and the colour then formed by bringing the mordanted material
in contact with the colouring matter.
The “heavy metals” form, among their numerous compounds, a great number
of coloured substances, and several of them are distinguished by a
great wealth of coloured derivatives; for example, copper, chromium
and cobalt form coloured compounds only. Although the use of pigments
derived from the heavy metals has been considerably restricted in
recent years by the discovery of a series of colouring matters which
replace them, yet they are now, and always will be, of very great
importance in the manufacture of colours. It is, therefore, necessary
briefly to describe the various metals which are used in colour making,
so that the manufacturer may know what metals produce harmless colours,
permanent and unaltered by the atmosphere, and what do not.
The metals are divided into two great groups, designated, according
to their specific gravities, the group of the light and of the heavy
metals. The light metals comprise the alkali, alkaline earth and earth
metals, whose important compounds we have just described. The specific
gravity of each of these metals is less than five times that of water.
The heavy metals have a specific gravity exceeding 5; they are
generally divided into groups, which are known by the name of the
commonest metal in the group. These are as follows:—
Zinc group Zinc, Zn = 65.
Iron group Iron, Fe = 56.
Tungsten group Tungsten, W = 184.
Antimony group Antimony, Sb = 120.
Tin group Tin, Sn = 119.
Lead group Lead, Pb = 207.
Silver group Silver, Ag = 108.
Gold group Gold, Au = 197.
Platinum group Platinum, Pt = 194.
To the zinc group belong the metals zinc, cadmium and indium, of which
only the two first are of importance here. The iron group comprises
iron, manganese, cobalt, nickel, chromium and uranium, all of which
are used in the manufacture of colours. The antimony group contains
antimony and bismuth, the latter of which is of little importance.
To the tin group belong tin and the rare metals titanium, zirconium,
thorium, niobium and tantalum; of these tin alone is important in
colour making. In the lead group are lead and thallium; lead produces
many important pigments. The silver group contains silver, copper and
mercury, the two latter of which are important. Of the gold group gold
alone is of interest, and its importance has been diminished by the
discovery of far cheaper substances which replace it. In the platinum
group, which contains platinum, iridium, rhodium, ruthenium, palladium
and osmium, only platinum itself is used as a colour; it is employed
in porcelain painting to produce the peculiar metallic shimmer known
technically as “lustre.”
The behaviour of the compounds, of the metals mentioned above towards
sulphuretted hydrogen is of the greatest importance to the colour
maker, since on it depends the alterability of the pigments when
exposed to the atmosphere. Many metallic compounds are unaltered by the
sulphuretted hydrogen present in the air, whilst others are in a high
degree affected by it and become gradually darker, so that their colour
may in the end approximate to black.
The pigments containing lead, copper, mercury and bismuth are extremely
susceptible to the action of sulphuretted hydrogen, by the action of
which they form black compounds. Since colours which contain these
metals are not permanent, but darken considerably, endeavours have been
made for a long time to replace them by others not susceptible to the
action of sulphuretted hydrogen. Thus it is desirable to manufacture
only colours free from metals forming black compounds with sulphuretted
hydrogen. For the same reason pigments which contain sulphur should
not be mixed with those containing metals which form black sulphur
compounds.
The rules laid down in the preceding paragraphs are of the greatest
importance for the artist, for by following them he will succeed in
composing a “permanent palette,” that is, containing only such colours
as will not, by their composition, bring about the speedy ruin of the
painting.
Although the majority of the compounds of the heavy metals are
poisonous, some possess this property in an eminent degree. These are
chiefly colours which contain arsenic, antimony, copper and lead. So
far as it is possible, these colours should be dispensed with and
harmless pigments sold in their place, though this is not always
possible, since several poisonous colours cannot be replaced by
innocuous ones.
COMPOUNDS OF THE HEAVY METALS.
=Zinc Compounds.=—Zinc oxide, ZnO, and zinc sulphate, ZnSO₄.7H₂O, are
the compounds of this metal used in the colour industry. Zinc oxide,
which is used as a white pigment, is a powder which turns yellow when
heated, and is not acted upon by sulphuretted hydrogen. Zinc sulphate
(white vitriol) occurs as colourless crystals, or more frequently as
greyish white crystalline masses. The freedom of zinc sulphate from
iron is of particular importance; the commercial article is seldom
satisfactory in this respect. In order to free commercial zinc sulphate
from iron, the property of zinc hydroxide of precipitating iron oxide
from neutral solutions may be employed. The zinc sulphate is dissolved
in water, and ammonia added in small quantities until the precipitate
of zinc hydroxide remains on stirring. When the liquid is left in
contact with the precipitate and stirred up once or twice a day, if
iron is present the precipitate will turn yellowish brown, owing to the
separation of ferric hydroxide, and in the course of a few days all the
iron will be removed from solution. The liquid should then give no blue
colouration with yellow prussiate of potash.
Zinc oxide is used as a white pigment, zinc chromate as a yellow, and
zinc cobalt compounds as green colours.
=Cadmium Compounds.=—Cadmium is a metal which possesses great
similarity to zinc, with which it occurs in nature. In the preparation
of cadmium compounds the metal is generally used. This is dissolved in
dilute sulphuric acid, hydrogen is evolved, and a solution of cadmium
sulphate obtained.
Cadmium is used in colour making only for the preparation of the
beautiful cadmium yellows.
=Iron Compounds.=—These are of the greatest importance to the colour
maker. Several, in which iron alone is the colour principle, are very
valuable: ochre, rouge, Venetian red, sienna and umber, for example.
Iron compounds are also used in the preparation of many colours. The
most important is:—
=Ferrous Sulphate= (Green Vitriol, Copperas), FeSO₄.7H₂0.—This
substance, which occurs commercially in a form of great purity at a
very low price, is generally the starting point in the preparation
of iron pigments. When pure, it forms fine sea-green crystals, with
an astringent metallic taste, which are not poisonous and are easily
soluble in water. After long exposure to the air, ferrous sulphate
becomes covered with an ochre-coloured crust, consisting of basic
ferric sulphate. The ferrous oxide contained in the green vitriol has
united with oxygen and been converted into ferric oxide. The latter
requires a larger quantity of acid than ferrous oxide for the formation
of soluble salts, so that an insoluble basic salt is separated. The
same thing occurs when a solution of ferrous sulphate is exposed to the
air.
When green vitriol, or any other ferrous salt, is exposed to the
action of oxidising agents, such as chlorine or nitric acid, the iron
is rapidly changed into the ferric state. This transformation is of
particular importance in the manufacture of certain blue pigments.
We give in a table the relation between the percentage of crystallised
ferrous sulphate (FeSO₄.7H₂O) contained in a solution at 15° C. and its
specific gravity:—
+--------+-----------------+--------+-----------------+
|Specific| Percentage of |Specific| Percentage of |
|Gravity.|Ferrous Sulphate.|Gravity.|Ferrous Sulphate.|
+--------+-----------------+--------+-----------------+
| 1·000 | 0 | 1·118 | 21 |
| 1·005 | 1 | 1·125 | 22 |
| 1·011 | 2 | 1·131 | 23 |
| 1·016 | 3 | 1·137 | 24 |
| 1·021 | 4 | 1·143 | 25 |
| 1·027 | 5 | 1·149 | 26 |
| 1·032 | 6 | 1·155 | 27 |
| 1·037 | 7 | 1·161 | 28 |
| 1·043 | 8 | 1·168 | 29 |
| 1·048 | 9 | 1·174 | 30 |
| 1·054 | 10 | 1·180 | 31 |
| 1·059 | 11 | 1·187 | 32 |
| 1·065 | 12 | 1·193 | 33 |
| 1·071 | 13 | 1·200 | 34 |
| 1·077 | 14 | 1·206 | 35 |
| 1·082 | 15 | 1·213 | 36 |
| 1·088 | 16 | 1·219 | 37 |
| 1·094 | 17 | 1·226 | 38 |
| 1·100 | 18 | 1·232 | 39 |
| 1·106 | 19 | 1·239 | 40 |
| 1·112 | 20 | —— | —— |
+--------+-----------------+--------+-----------------+
Yellow and red prussiate, which have been already mentioned, also
belong to the iron compounds. They have been separately mentioned
because the iron is contained in them in a peculiar form as a portion
of an organic radical.
=Ferrous Chloride=, FeCl₂, may sometimes be used instead of green
vitriol. When iron is dissolved in hydrochloric acid hydrogen is given
off and a solution of ferrous chloride is obtained; but when rouge is
dissolved in the same acid, ferric chloride is formed. When iron is
dissolved in nitric acid, in consequence of the oxidising properties of
this acid a ferric salt is obtained. Iron forms two series of salts:
in the ferrous compounds the iron is in the same form as in green
vitriol and the corresponding salts; in the ferric compounds the iron
is contained in a higher state of oxidation. By powerful oxidising
agents, as nitric acid or chlorine, ferrous compounds are converted
into ferric.
=Manganese Compounds.=—Manganese (Mn) is a metal whose compounds show
great similarity with those of iron, like which it forms two oxides
(also others), manganous oxide (MnO) and manganic oxide (Mn₂O₃). The
salts of manganous oxide are not oxidised in the air like those of
ferrous oxide.
The raw material used in the preparation of manganese compounds is the
mineral pyrolusite, which is manganese dioxide (MnO₂).
Manganese sulphate (MnSO₄) forms rose-red crystals containing varying
quantities of water. The residues from the preparation of chlorine can
be used as the material for the preparation of colours. According as
pyrolusite and hydrochloric acid or pyrolusite, salt and sulphuric acid
are used for this purpose a solution of manganous chloride or sulphate
is obtained.
Manganese compounds have but a restricted use in colour making.
=Nickel Compounds= are generally coloured green, but they are not used
as pigments.
=Cobalt Compounds.=—Among these are found many important pigments. All
cobalt compounds are coloured; in beauty and variety of shade they can
only be compared with those of chromium. In properties cobalt is very
similar to iron and nickel.
The form in which cobalt is used in preparing colours is either cobalt
nitrate, Co(NO₃)₂.6H₂O, or cobalt chloride, CoCl₂.6H₂O. Both salts are
articles of commerce, but generally they are so dear that it is more
profitable for the colour maker to prepare them direct from the cobalt
minerals. A simple method for preparing cobalt compounds from the ores
is therefore given. The most important cobalt ores are speiss cobalt,
a compound of cobalt and arsenic, and cobalt glance, a compound of
cobalt, arsenic and sulphur. The former mineral often contains only
small quantities of cobalt, and it is advisable for our purposes to use
cobalt glance, which contains from thirty to forty per cent. of cobalt.
This mineral is first roasted, that is, is heated with a plentiful air
supply, by which means the arsenic is driven off. On account of the
poisonous nature of the arsenic vapours the roasting must be conducted
in a furnace with a very good draught.
Under the name of zaffre, roasted cobalt ores come into commerce. These
may be used in the preparation of cobalt compounds, by which means
the operation of roasting is avoided. According to the quality of the
ore which has been used to obtain zaffre, it contains a very varying
proportion of cobalt. The varieties richer in cobalt must be used; they
are technically known by the mark FS, or FFS (the best).
The roasted cobalt ores or zaffre are treated with fused acid potassium
sulphate, when the salts of iron and manganese are decomposed, whilst
cobalt and nickel sulphates remain unchanged. In a Hessian crucible
are melted 300 parts of acid potassium sulphate, and 100 parts of the
powdered zaffre are gradually added, mixed with one part of green
vitriol and one part of saltpetre; the mixture is heated so long as
sulphuric acid escapes. The mass is then boiled with water and the red
solution treated with sulphuretted hydrogen so long as a precipitate
is formed; this may contain copper, manganese, and bismuth. After
filtering, soda is added to the boiling liquid; cobalt carbonate is
precipitated, which can be converted into nitrate or chloride by
solution in the corresponding acid. If cobalt sulphate is required,
the solution, after treatment with sulphuretted hydrogen, need only be
evaporated to crystallisation, when the sulphate separates in fine red
crystals. Cobalt nitrate and chloride are very soluble in water; to
obtain them their solutions must be strongly evaporated and quickly
cooled whilst stirring. The crystals of cobalt nitrate and chloride
absorb moisture from the air and deliquesce; they must be kept in glass
vessels with well-ground stoppers.
The cobalt compounds which are to be used in colour making must be free
from iron, nickel and arsenic, which would detract from the cleanness
of the colours. If the precipitate produced by soda contains iron it
is mixed with excess of solution of oxalic acid, and after a few hours
the cobalt oxalate is filtered from the liquid, in which all the iron
is dissolved. The cobalt oxalate can then be converted into nitrate or
chloride by treatment with nitric or hydrochloric acids.
These salts form the material for the preparation of the cobalt
compounds, a large number of which are used as extremely durable red,
blue and green pigments; several of them, such as cobalt blue, cannot
be exactly replaced by other pigments. On account of the industrial
importance of the cobalt colours, these directions for the preparation
of the soluble cobalt salts from the ores have been given with some
detail. The preparation of the cobalt colours will be given _in
extenso_ later on.
=Chromium Compounds.=—As the name indicates, this metal yields numerous
coloured compounds (χρῶμα, colour); in fact, only coloured chromium
compounds are known, and the colours are most varied—yellow, green, red
and violet. On this account the chromium compounds are among the most
important used in colour making; a great number of colours are prepared
by their aid. Chrome ironstone, as we have already stated, is the raw
material for the preparation of chromium compounds. From it potassium
bichromate is made on a large scale in special works, so that no colour
maker is compelled to prepare chromium salts himself.
When the chromium pigments contain no metal blackened by sulphuretted
hydrogen, they have the desirable property of being unaltered by the
atmosphere. Like the cobalt compounds, they are distinguished by their
great stability when heated; on this account, they have a large use in
porcelain painting.
=Molybdenum, Tungsten and Vanadium Compounds= on account of their cost
have a very limited use as pigments. Molybdenum compounds are obtained
from molybdic acid; compounds of tungsten from the metal; and those of
vanadium from ammonium vanadate.
=Antimony Compounds= can be used in the preparation of several
pigments, but, on account of their behaviour towards sulphuretted
hydrogen, the pigments cannot be regarded as really permanent, and
their use is generally diminishing. The so-called antimony vermilion is
the only antimony compound at all extensively employed.
=Bismuth Compounds= possess properties very similar to those of
antimony. Only one bismuth preparation is used as a pigment, and this
is very sensitive to the action of sulphuretted hydrogen, being changed
into black bismuth sulphide.
=Tin Compounds= are employed in two ways: some are themselves colours,
such as stannic sulphide (mosaic gold); others, themselves colourless,
are used in making pigments, as stannous and stannic chlorides.
Stannous Chloride, SnCl₂.2 H₂O, is obtained when tin is dissolved in
hydrochloric acid, hydrogen being evolved. Stannic Chloride, SnCl₄, is
formed when tin is dissolved in a mixture of hydrochloric and nitric
acids (_aqua regia_).
Tin compounds have, similarly to aluminium salts, the property of
forming coloured insoluble compounds (lakes) with many organic
colouring matters. Their use for this purpose is extensive.
=Arsenic Compounds= formerly played an important part in colour making.
They were used in the manufacture of a large number of pigments,
very beautiful but extremely poisonous. At the present time, we can,
fortunately, entirely dispense with arsenic in colour making; the
arsenic colours can be replaced by others equally handsome and less,
or not at all poisonous. The most important of the arsenic compounds
is the trioxide As₂O₃, or Arsenious Acid, commercially known as white
arsenic. This substance is obtained in large quantities as a by-product
in the extraction of several metals. It forms masses which are either
glassy or have the appearance of porcelain. Freshly sublimed arsenic
trioxide is glassy; this form gradually changes into the porcellaneous
variety, it dissolves with difficulty in water. A strong solution can
only be obtained by boiling for many hours.
The compounds of arsenic with sulphur, formerly extensively used as
pigments, have now almost fallen into disuse.
=Lead Compounds= belong to the substances most largely used in making
colours. Unfortunately all lead colours have two very important
drawbacks. They are all very poisonous and at the same time extremely
sensitive to sulphuretted hydrogen, so that they are very considerably
altered by the action of the small quantities of that gas contained in
the atmosphere of an ordinary dwelling. A striking example of this is
seen in the lead paints used on the doors of water-closets. The paint,
at first pure white, becomes gradually darker, and at last almost
black, the lead compound having been changed into black lead sulphide.
On account of this great sensitiveness of lead compounds, it would
be better if they could be excluded from the list of colours. Great
care must be taken not to mix lead compounds with others which contain
sulphur; a discolouration of the mixture would be the inevitable result
in a very short time.
The oxides of lead and a number of its salts are themselves pigments,
for example, litharge, PbO, red lead, Pb₃O₄, and white lead (basic lead
carbonate). These pigments are prepared on a large scale in particular
works. At this point only those lead compounds will be mentioned which
are generally used for the preparation of other lead pigments; they
are: lead sulphate, nitrate, acetate and chloride.
=Lead Sulphate=, PbSO₄, is formed when sulphuric acid or the solution
of a sulphate is added to the solution of a lead salt; so obtained it
is a white crystalline powder insoluble in water. This substance is
generally not made in colour works, but is purchased from chemical
works or dye houses, of which it is a by-product. In this form
(lead bottoms) it is generally not sufficiently pure, but contains
admixtures of sulphuric acid or aluminium salts, from which it is
freed by washing. The lead sulphate is stirred up in water, the heavy
precipitate allowed to settle, the wash water drawn off, and after
repeating this process until the wash water no longer shows an acid
reaction, the purified precipitate is dried. In this condition it is a
heavy, white powder, and can alone be ground into paint. But on account
of its crystalline nature, which reduces the covering power, such use
is inadvisable.
=Lead Nitrate=, Pb(NO₃)₂.—This very important compound may be bought,
but it is advisable to prepare it in the works. Water is placed in a
wooden tub, then half the volume of nitric acid is added, and finely
powdered litharge gradually stirred in, the liquid being kept in
constant movement. When it is seen that the litharge is only slowly
dissolved, the liquid is well stirred after each addition of litharge
and then tested by litmus paper. When this is no longer reddened, the
nitric acid is completely saturated, and the liquid contains only
lead nitrate in solution. It is allowed to stand until the insoluble
portions have settled, and then drawn off into another vessel where
crystals of lead nitrate separate in a few days. If the salt be
required in solid form, the solution may be evaporated in earthenware
dishes; generally the solution is used as it is obtained.
Pure lead nitrate forms white crystals which are not particularly
soluble in water, 1 part requiring 2 parts of water at the ordinary
temperature. Lead nitrate is decomposed on heating, like all nitrates,
and litharge remains. The solution of this salt is used in the
preparation of those lead pigments which are obtained by precipitation,
for example, chrome yellow.
=Lead Acetate=, Pb(C₂H₃O₂)₂.3H₂O.—The compounds of lead with acetic
acid are of great importance. Two of these are to be considered:
neutral lead acetate, commonly known as sugar of lead, and basic lead
acetate. It may be advisable to manufacture both these compounds, the
latter always. Neutral lead acetate comes into commerce in the form of
colourless heavy crystals, which are often covered by a white powder
of the basic acetate; they dissolve readily in water, and the solution
has a sweetish taste, hence the name “sugar of lead”. Frequently the
solution is very turbid; this is caused by the carbonates contained in
the water. The turbidity may be removed by the addition of a little
acetic acid. It is only economical for the colour maker to prepare
sugar of lead when he can obtain cheap raw materials, lead or litharge
and vinegar. Pyroligneous acid may also be used if it is colourless,
its odour being without importance for this purpose.
The best method for preparing lead acetate from litharge is to place
the vinegar in a tub and hang in it a strong linen bag filled with
finely ground litharge. The tub is kept covered for a few days and its
contents then tested with red litmus paper. When this is turned blue,
the liquid is drawn off and vinegar gradually added whilst stirring,
until blue litmus paper is just turned red. In this process, after
neutral lead acetate has been formed, more lead oxide is dissolved, and
the liquid thus acquires an alkaline reaction. The further addition of
acetic acid reconverts the basic salt into neutral lead acetate.
Lead acetate solution may, with advantage, be prepared directly from
metallic lead. For this purpose, lead is granulated by melting and
pouring in a thin stream into cold water, where it solidifies in
irregular pieces. This is done in order to give the lead as large a
surface as possible. Three high narrow tubs placed one above the other
so that liquid may flow from the highest to the middle, and from this
into the lowest, are filled with granulated lead. Vinegar is placed
in the uppermost vessel to cover the lead; after twenty-four hours it
is allowed to flow into the middle, and after a further twenty-four
hours into the lowest tub. In this way, a solution of basic lead
acetate is formed, to which the necessary quantity of acetic acid is
added to bring it into the neutral condition. If crystalline lead
acetate is required, the liquid is evaporated down and quickly cooled
with stirring, so that small crystals are formed. Generally, however,
evaporation is unnecessary, since lead acetate is always used in
solution in preparing colours.
If the lead acetate solution be not colourless, which is generally
the case when coloured acetic acid is used, the defect may be removed
by stirring a little bone black into the liquid and filtering after
twenty-four hours, when a completely colourless solution is obtained.
It is always necessary to know exactly how much lead acetate is
contained in the solutions prepared by these processes. The lead or
the litharge is therefore weighed and the volume of the lead acetate
solution measured. One hundred parts by weight of crystallised lead
acetate are obtained from 62·54 parts of lead.
Lead acetate solutions must be kept in well-covered vessels. The
carbonic acid of the air will turn the liquid turbid. The turbidity may
be removed by the addition of acetic acid.
In the following table is given the percentage of crystallised lead
acetate contained in solutions of different specific gravities:—
+--------+------------+--------+------------+
|Specific|Crystallised|Specific|Crystallised|
|Gravity.|Lead Acetate|Gravity.|Lead Acetate|
| | per cent. | | per cent. |
+--------+------------+--------+------------+
| 1·0000 | 0 | 1·1869 | 26 |
| 1·0064 | 1 | 1·1955 | 27 |
| 1·0127 | 2 | 1·2040 | 28 |
| 1·0191 | 3 | 1·2126 | 29 |
| 1·0255 | 4 | 1·2211 | 30 |
| 1·0319 | 5 | 1·2303 | 31 |
| 1·0386 | 6 | 1·2395 | 32 |
| 1·0453 | 7 | 1·2486 | 33 |
| 1·0520 | 8 | 1·2578 | 34 |
| 1·0587 | 9 | 1·2669 | 35 |
| 1·0654 | 10 | 1·2768 | 36 |
| 1·0725 | 11 | 1·2867 | 37 |
| 1·0796 | 12 | 1·2966 | 38 |
| 1·0867 | 13 | 1·3064 | 39 |
| 1·0939 | 14 | 1·3163 | 40 |
| 1·1010 | 15 | 1·3269 | 41 |
| 1·1084 | 16 | 1·3376 | 42 |
| 1·1159 | 17 | 1·3482 | 43 |
| 1·1234 | 18 | 1·3588 | 44 |
| 1·1309 | 19 | 1·3695 | 45 |
| 1·1384 | 20 | 1·3810 | 46 |
| 1·1464 | 21 | 1·3925 | 47 |
| 1·1544 | 22 | 1·4041 | 48 |
| 1·1624 | 23 | 1·4156 | 49 |
| 1·1704 | 24 | 1·4271 | 50 |
| 1·1784 | 25 | —— | —— |
+--------+------------+--------+------------+
=Basic Lead Acetate=, Pb(C₂H₃O₂)₂.2PbO.—This salt may be regarded as
a compound of neutral lead acetate with lead oxide. It is obtained
by digesting vinegar with excess of litharge or with metallic lead,
and also by treating lead acetate solution with litharge so long as
the latter is dissolved. In the method last given, 100 parts of sugar
of lead require about 118 parts of litharge to produce a saturated
solution of basic acetate. The solution of this compound is alkaline;
it turns red litmus paper blue. When exposed to the air, a turbidity
is quickly produced owing to the separation of lead carbonate. In one
white lead process basic lead acetate is the starting point of the
manufacture.
=Lead Chloride=, PbCl₂, is seldom used in making colours. It may
be prepared by stirring powdered litharge in common salt solution
until the powder appears white. This, when washed, constitutes basic
lead chloride. On adding hydrochloric acid to the washed mass until
the liquid remains acid, lead chloride is obtained in the form of
crystalline needles, which are very little soluble in cold, but more
easily in hot, water.
Like any other soluble lead salt, lead chloride may be used in the
precipitation of colours, but is seldom employed on account of its
small solubility. Basic lead chloride was, at one time, used as a white
pigment, and after melting, by which it is turned yellow, as a yellow
pigment; it is no longer in use for these purposes.
=Copper Compounds.=—These are generally green or blue, and have an
extended use in the production of colours. The metallic copper which
is used in the preparation of colours is of the ordinary commercial
quality. The impurities which it contains are generally so small in
quantity that they are without importance for our purpose.
=Copper Sulphate= (Bluestone, Blue Vitriol), CuSO₄.5H₂O.—This is the
commonest of the commercial copper salts, and on that account deserves
our especial attention. It forms large sky-blue crystals, which
effloresce slightly in the air, possess an unpleasant metallic taste,
and are poisonous, like all soluble copper compounds.
Copper sulphate comes into commerce in a very pure form, but some
qualities contain zinc sulphate or ferrous sulphate. The presence of
zinc may be detected most easily by boiling the solution with excess
of caustic soda, when copper oxide separates as a black powder, whilst
zinc oxide remains dissolved. When sulphuretted hydrogen is passed
through the liquid, a white precipitate of zinc sulphide is formed.
Iron is detected by passing sulphuretted hydrogen through the solution
so long as a precipitate is formed, allowing the liquid to stand in
a covered vessel, pouring it off from the precipitate, adding nitric
acid, boiling and adding a solution of potassium ferrocyanide; a blue
precipitate denotes the presence of iron.
Copper sulphate is rarely found which is quite free from iron and zinc.
If these impurities are present in but small quantity, the zinc not
exceeding 1 per cent. and the iron at most 0·5 per cent., the copper
sulphate may be regarded as sufficiently pure for our purposes.
It may be here remarked that copper sulphate obtained from mints is
generally of great purity, and hence particularly adapted for colour
making.
When copper sulphate and other copper salts are dissolved in water,
pale blue flocks of copper carbonate generally separate. This is due
to the carbonate of lime contained in the water. An addition of a few
drops of sulphuric, nitric or hydrochloric acid suffices to prevent
this separation.
=Copper Nitrate=, Cu(NO₃)₂.6H₂O.—This salt may occasionally be
obtained in colour works as a by-product. When nitric acid is poured
over copper, there follows a copious evolution of nitric oxide, which
produces brown fumes of nitrogen peroxide in air. Nitric oxide may be
used to convert ferrous into ferric salts, a transformation required
in making Prussian blue. In working in this way, nitric acid is poured
over copper contained in a vessel provided with a delivery tube for the
gas. The blue solution is at once used. Pure copper nitrate forms fine
blue crystals, which very readily deliquesce in the air. The solution
is therefore generally used as it is prepared.
=Copper Acetate=, Cu(C₂H₃O₂)₂.H₂O.—Copper is readily attacked by acetic
acid. A number of salts are formed, of which some are used as pigments.
For our purpose it will be sufficient to describe the manufacture of
verdigris; few colour makers prepare any other copper acetate. A
solution of this salt is most simply prepared in the following manner:
Slaked lime is stirred with strong vinegar, and the solution left in
contact with the excess of lime so long as it has a weak acid reaction.
The solution, which contains acetate of lime, is poured into a solution
of copper sulphate so long as a precipitate of sulphate of lime is
formed. When this has been separated from the liquid, the latter is
ready for further treatment. It contains only a very small quantity of
dissolved sulphate of lime, which is not harmful in the preparation of
colours.
In addition to the copper compounds mentioned here, several others were
formerly used as pigments, or in the preparation of pigments which are
no longer employed, because copper compounds of good colour can be
obtained in a cheaper manner.
The same precautions should be taken in the use of copper colours
which were mentioned for lead pigments; copper compounds are equally
sensitive towards sulphuretted hydrogen, by which they are gradually
discoloured.
=Mercury Compounds.=—Mercury forms compounds which, vermilion in
particular, are used as pigments, and others which are used in the
preparation of pigments. In many cases metallic mercury is the starting
point in the preparation of the mercury compounds. The compounds
commonly known as calomel and corrosive sublimate are also used.
=Mercurous Nitrate=, HgNO₃.—Nitric acid acts upon mercury in a manner
differing according to its strength, and according to whether the
mercury or the nitric acid is used in excess. In order to prepare
mercurous nitrate, acid free from chlorine must be diluted at least
with four times its volume of water, and the mercury must be in excess.
On warming, the mercury is gradually dissolved, and, on cooling,
the solution deposits colourless crystalline needles of the salt. A
further crop of crystals is obtained after evaporating the solution.
When the action of nitric acid is over, the solution must be at once
separated from the excess of mercury to prevent the formation of basic
salts. If the salt has been properly made, it is completely soluble in
water, but if a lemon yellow precipitate is formed on dissolving, the
nitrate contains a basic salt, which can only be dissolved by warming
and adding more nitric acid.
=Mercuric Nitrate=, Hg(NO₃)₂, is most simply obtained by warming
mercury with very strong nitric acid. The heating must be continued
until a test portion of the solution no longer gives a precipitate with
hydrochloric acid. When this solution is evaporated, nitric acid is
given off, and a salt crystallising in white needles is obtained, which
dissolves in water with the separation of a yellow basic salt. It is
therefore better to use the hot solution, which contains a little free
acid, without evaporating.
Instead of mercurous and mercuric nitrates, the corresponding sulphates
may be used, but the chlorides are more frequently employed since they
can be readily obtained from the makers.
=Mercurous Chloride= (Calomel), HgCl, is obtained pure by adding common
salt solution to a solution of mercurous nitrate and washing the
precipitate, which is insoluble in water.
=Mercuric Chloride= (Corrosive Sublimate), HgCl₂ a common article of
commerce, is prepared by heating a carefully made mixture of mercuric
sulphate and common salt, when mercuric chloride sublimes. It is a
white crystalline mass, soluble in 13·5 parts of water at 20° C., and
soluble in 3 parts of alcohol. Although all mercury compounds are very
poisonous, corrosive sublimate requires particular care in handling,
since its easy solubility makes it surpass all other mercury compounds
in poisonousness.
The mercuric sulphate required in the above preparation is obtained by
heating mercury with sulphuric acid. Corrosive sublimate can also be
prepared by adding hydrochloric acid to mercurous nitrate, and heating,
with gradual addition of hydrochloric acid until a clear solution is
formed, from which mercuric chloride crystallises on cooling.
=Silver Compounds.=—Silver nitrate, AgNO₃, is the only one of
importance here. It is obtained by dissolving silver in nitric acid,
when a blue solution is obtained because commercial silver contains
copper, evaporating the solution to dryness, melting the residue, and
keeping it molten until all the copper nitrate is decomposed. This
point is recognised when a small portion of the melt dissolved in water
does not give a blue colouration with excess of ammonia. Fused silver
nitrate forms a white crystalline mass readily soluble in water and
turning black when exposed to light, like many other silver compounds.
=Gold Compounds.=—Gold is now very little used in preparing colours.
The compound used for this purpose is gold chloride, AuCl₃, which is
obtained by heating gold with hydrochloric acid, and adding nitric
acid in small quantities until all the metal is dissolved. By careful
evaporation of the yellow solution gold chloride is obtained in
brownish yellow crystals, which easily dissolve in water.
The compounds of molybdenum, vanadium and uranium are less used than
those of gold, yet these metals find a special use in the preparation
of colours for porcelain painting and for colouring glass, for which
they are of great importance. The preparation of their compounds from
the raw materials is complicated and not remunerative to the colour
maker; they should be obtained from chemical works.
* * * * *
In the foregoing, the most important compounds of inorganic origin
used in making colours have been briefly described, less in order to
teach the methods for their preparation than to give the manufacturer
the means of learning their properties. Since the great development of
chemical industries during the last decades it is more advantageous for
the colour maker in most cases to draw his supply of these substances
from works of good reputation than to make them himself; only in the
case of a few substances, which are sold at unreasonably high prices,
will it be profitable for him to prepare them himself.