CHAPTER IV._The manufacture of mineral and lake pigments

ASSISTANT MATERIALS.

=Water=, H₂O = 18.[1]—This substance plays a tremendous part in colour

making; almost all the substances which are used in solution are

dissolved in water; the removal from precipitates of admixed foreign

bodies, the so-called washing, is always accomplished with water. The

chemist does not understand by water quite that liquid which in general

speech is so designated. We must consider the water which is at the

disposal of the colour maker.

[1] We append the chemical formula and the molecular weight to the

description of each compound.

Water, in the chemical meaning of the word, is a liquid composed only

of hydrogen and oxygen, and leaving no residue when evaporated. Such

water is not found in nature; it can only be obtained by distillation

of well or river water. The water which falls in long continued rain,

or is obtained by melting snow, is most nearly like distilled water; it

contains only small quantities of dissolved substances, and generally

such as would be without influence in colour making. Water of this

description is available for but a limited use; the large quantities of

water required in a colour works must be taken from springs or streams.

These waters contain, however, more or less large amounts of dissolved

salts, which act in a marked manner upon the substances dissolved in

them.

In almost all spring and well waters is found carbonate of lime;

such waters are called “hard”. River water contains generally little

carbonate of lime; it is then called “soft water”. The influence of the

carbonate of lime is especially evident when salts of lead, copper,

iron and other heavy metals are dissolved in water; the carbonate of

the particular metal gradually separates from solution, and the liquid

becomes very turbid.

When only hard waters containing much lime are at the service of the

manufacturer, turbid solutions are often obtained, which must be

filtered before use. In many cases this can be avoided by adding milk

of lime to the water in a large vessel; the free carbonic acid unites

with the lime, and thus the carbonate of lime, which is only soluble in

water containing free carbonic acid, separates as a fine precipitate.

Water which has been treated in this way becomes clear after some time,

through the deposition of the carbonate of lime; it is then soft water.

In order to separate the carbonate of lime in this way, no more than

the requisite quantity of milk of lime should be added, so that no lime

remains in excess, since this would cause precipitates when salts of

lead, copper, iron, etc., were dissolved. In many cases—for example,

when lead or barium salts are dissolved—the lime contained in the water

can be made harmless by slightly acidifying with acetic or nitric acid.

Water which contains sulphate of lime (gypsum) is equally useless for

many purposes, as, for example, the solution of lead and barium salts.

These metals form insoluble compounds with the sulphuric acid, which

render the solution turbid, and can be removed only with difficulty by

filtering, on account of their great fineness. They are more easily

removed by allowing to settle.

Water containing gypsum often contains in addition small quantities of

sulphuretted hydrogen. However small the quantities of this gas may

be, they still make the water absolutely useless for certain purposes

in the manufacture of colours; for example, for the preparation of

all pigments containing lead which are obtained by precipitation. The

sulphuretted hydrogen forms black compounds with lead, copper, bismuth,

mercury and other metals, which impair the brilliance of the colour. A

colour made under these conditions is never clean, its hue is injured

by the admixture of the black substance.

Water which contains much common salt (sodium chloride) is unsuitable

for the solution of lead, mercury and silver salts. In consequence of

the great affinity of these metals for chlorine, turbid solutions are

obtained when their salts are dissolved in water containing common salt.

Some waters contain a considerable quantity of iron. Such waters

deposit on evaporation, and often on standing exposed to the air, a

brown powder of ferric hydrate, which would have considerable influence

on the shade of a pigment. White pigments, in the preparation of which

such a water is used, have always a brownish tinge; yellow and red

pigments are also unfavourably affected.

Carbonate of lime and common salt occur in small quantities in every

well water. The colour maker must do the best he can with such a water;

its use will not particularly harm the shade of the colours prepared

with it if the amount of the impurities is not very large. Water

containing much iron is practically useless; the oxide of iron would

injure the colours so much that it would not be possible to obtain

brilliant shades. Water from wells in the neighbourhood of deposits of

turf or cemeteries often contains considerable quantities of organic

substances which act injuriously on the shade of pigments; such water

should not be used in colour making.

The impurities in a water are more or less harmful according to the

purpose for which it is to be used. Sulphate of lime is generally

more injurious than carbonate of lime, since the precipitates which

the latter causes in solutions of the salts of certain metals can be

prevented by the addition of acids. This is not the case with sulphate

of lime; when lead or barium salts are dissolved in water containing

this substance, a precipitate of lead or barium sulphate is obtained,

which is insoluble.

In dealing with the salts of costly metals, such as mercury or silver,

it is better to dissolve them in distilled water, or, at least, very

pure rain water. The rain water which runs from zinc or well tiled

roofs is generally very pure; for practical purposes it may be regarded

as free from carbonate and sulphate of lime, sulphuretted hydrogen and

common salt. The colour maker should take care to obtain as much of

this pure water as possible by erecting large rain-water tanks.

The less impurity a water contains the more useful it is for our

purpose. After rain water soft river water is the best, and after this

the softer well waters. All mineral waters distinguished by a high

content of salts or gases are quite useless for colour making; for this

reason sea water is disqualified.

An accurate analysis of a water is much too complicated for the

manufacturer; it is sufficient for him to convince himself of the

absence of certain substances. Water which, some time after the

addition of a little tannic acid solution, acquires a clear green or a

bluish to black shade contains much iron, and is useless. Water which

coagulates a large quantity of a solution of soap in alcohol is very

rich in carbonate or sulphate of lime. In order to decide approximately

in what relative proportion these salts are present, a solution of

barium chloride is added to the water so long as a precipitate forms.

If this disappears completely on the addition of nitric acid, the

water contains only carbonate of lime; if it only partly dissolves,

sulphate of lime is also present. The presence of chlorine is shown

by a considerable turbidity on acidifying the water with nitric acid,

boiling and adding silver nitrate. If the precipitate obtained on the

addition of a lead salt is not pure white, but discoloured, the water

contains sulphuretted hydrogen, which has formed black lead sulphide.

In order to test the water for organic substances, about a litre is

evaporated to dryness in a porcelain dish, and the residue heated to

redness; if it turns brown and black, and possibly gives off a smell of

burnt feathers, the water contains much organic matter.

Pure water is coloured permanently red by a solution of potassium

permanganate; but if it contains organic matter, the solution is

decolourised after some time and a brown precipitate is deposited at

the bottom. From the amount of this precipitate an idea of the quantity

of organic matter present may be obtained.

It is only necessary to be very scrupulous concerning the quality

of the water when it is to be used for the solution of salts or the

extraction of dye-woods. For washing precipitates, which requires a

large volume of water, there can generally be used, without detriment,

water containing much lime, but it must be free from iron and

sulphuretted hydrogen. The latter is particularly harmful to most of

the lead colours, which would lose in beauty by washing with water

containing this substance.

It is hardly necessary to say that the water used in colour making must

be quite clear. Muddy river water must in every case be completely

freed from the solid particles contained in it, either by settling or

by filtering. Filters filled with well washed sand give good results

for this purpose.

=Chlorine=, Cl = 35·5.—For some operations in colour making it is

necessary to employ chlorine. This is a greenish yellow gas at

ordinary temperatures, which is characterised by a suffocating smell

and the energy with which it unites with most elements. On account of

its injurious effects on man certain precautions have to be observed

in preparing chlorine, and it is advisable to erect the apparatus

necessary for its production in a separate room, so that the workmen

are not injured by the gas.

[Illustration: FIG. 1.]

Formerly chlorine was exclusively made in lead apparatus, because this

metal is one of the least readily attacked. When such an apparatus is

used for the first time a layer of lead chloride is formed, which,

like a varnish, protects the metal beneath from further attack. Fig.

1 represents an apparatus formerly employed for the preparation of

chlorine in chemical works. In the upper part of the pear-shaped

vessel, K, there are four openings, two of which, D and C, are provided

with water lutes. This means that the opening is surrounded by a moat

containing water, into which the rim of the cover dips, thus making

a joint. Through the middle opening goes the axle of the stirring

apparatus, R; in the fourth is a lead safety funnel, J. Solid materials

are introduced through D, liquid through J; the tube C carries away the

chlorine formed; the tube A, furnished with a stop cock, can draw off

the fluid contents of the apparatus.

Since lead melts at low temperature, the apparatus cannot be heated

over the fire without danger, therefore it is surrounded by an iron

jacket, W, which is filled with water, or else the apparatus is heated

by steam introduced into W. Larger quantities of chlorine are more

conveniently prepared in an apparatus, of similar structure, made of

stone or earthenware, which have the advantage over lead that they are

not at all attacked by chlorine.

[Illustration: FIG. 2.]

Fig. 2 exhibits the construction of such an apparatus of medium size.

It is constructed of sandstone or earthenware; the lid and some of the

smaller parts can be made of either earthenware or lead. The pyrolusite

is introduced at _G_ in large pieces; _H_ is the funnel for pouring

in the acid; _E K D_, the steam pipe; _C_, the perforated false

bottom upon which the pyrolusite lies; _F_, the delivery tube for the

chlorine; _J_, the opening for running off the manganese chloride; _a_,

the leaden cover.

To prepare chlorine, 1 part (by weight) of common salt, 1 part of

powdered pyrolusite, 2½ parts of vitriol and 1¼ part of water are

used. The salt and the pyrolusite are introduced through _D_ into the

apparatus (Fig. 1); the acid, diluted with the water, is poured in

through the funnel, the materials are mixed by the stirrer and gently

warmed until chlorine appears, when the application of heat must be

considerably diminished or the chlorine will be violently evolved.

Pyrolusite and hydrochloric acid are now generally used for the

preparation of chlorine, because the solution of manganese chloride,

left at the end of the operation, is valuable.

If all the chlorine made in one operation is not at once required for

the manufacture of a colour, it can be utilised by sending it into a

box filled with slaked lime, which is converted into chloride of lime

or bleaching powder. The liquid run away from the apparatus at the

conclusion of the operation contains manganese and sodium sulphates,

or manganese chloride as the case may be, and can be used for the

preparation of manganese pigments.

=Ammonia=, NH₃ = 17.—Ammonia is obtained from chemical works in the

form of a strong solution of ammonia gas in water, which is generally

very pure. The density of an aqueous solution of ammonia is the smaller

the more ammonia it contains, and thus the strength of a solution of

ammonia can easily be formed by means of the hydrometer. The following

table shows the percentage of ammonia, NH₃, in a liquid of known

specific gravity at the temperature of 14° C.:—

+-------------+------------+------------+------------+

+-------------+------------+------------+------------+

| 0·885 | 36·00 | 0·953 | 11·50 |

| 0·886 | 35·00 | 0·955 | 11·00 |

| 0·889 | 34·00 | 0·957 | 10·50 |

| 0·891 | 33·00 | 0·959 | 10·00 |

| 0·893 | 32·50 | 0·961 | 9·50 |

| 0·895 | 32·00 | 0·963 | 9·00 |

| 0·897 | 31·30 | 0·965 | 8·50 |

| 0·900 | 26·00 | 0·968 | 8·00 |

| 0·905 | 25·39 | 0·970 | 7·50 |

| 0·925 | 19·54 | 0·972 | 7·00 |

| 0·932 | 17·52 | 0·974 | 6·50 |

| 0·947 | 13·46 | 0·976 | 6·00 |

| 0·951 | 12·00 | 0·978 | 5·50 |

+-------------+------------+------------+------------+

_The Hydrometer._—In the above table the percentage content of the

ammonia solution is given according to its specific gravity, that is,

according to the ratio between the weight of any volume of the liquid

and the weight of an equal volume of water. According to scientific

principles, only those hydrometers should be used which are graduated

in specific gravities. In spite of all exertions in this direction,

manufacturers have not yet been induced to use such instruments in

every case. Hydrometers, with quite arbitrary scales, such as those of

Baumé and Twaddell, are frequently found in works. These hydrometers

generally only show that a liquid is of so many degrees on the

particular scale, and the manufacturer in using them is restricted

to the following out of a certain recipe which requires the use of

a liquid of a certain strength which is expressed in degrees Baumé,

etc. He does not learn by this how many per cent. of the particular

substance are dissolved in the water when the liquid has a certain

hydrometric strength.

For the sake of uniformity, it is urgently to be desired that all

manufacturers who use the hydrometer to estimate the content of a

liquid in ammonia, potash, soda, hydrochloric, sulphuric, nitric acids,

etc., should employ simple specific gravities. This is desirable,

because the percentage strength of a solution, corresponding to the

specific gravity, can be at once accurately found from tables. On

these grounds, in the present work, we have restricted ourselves to

tables showing simply the specific gravities of solutions and the

corresponding composition.

=Sal Ammoniac or Ammonium Chloride=, NH₄Cl = 53·5.—This substance comes

into commerce in the form of a white crystalline meal, more rarely in

the form of sugar loaves (crystallised sal ammoniac) or of flat cakes

(sublimed sal ammoniac). It is usually very pure, since impure forms,

generally containing much iron, are difficult of sale. At a particular

temperature sal ammoniac is volatile; it is used in certain mixtures

in order to prevent the temperature, on heating, from rising beyond a

certain point. Like ammonia, it is more used in dyeing.

=Ammonium Sulphide=, NH₄HS.—This compound is obtained by leading

sulphuretted hydrogen into ammonia solution so long as it is dissolved,

and a test portion of the liquid still gives a white precipitate with

a solution of magnesium sulphate. Ammonium sulphide decomposes by long

standing in the air, sulphur being separated. It gives precipitates

with the salts of certain metals, for example, iron, cobalt, manganese,

zinc, nickel. These precipitates, which consist of the sulphides of the

metals, are not formed by sulphuretted hydrogen in acid solutions.

ACIDS.

In colour making many acids are used for the solution of metals, the

production of precipitates, for oxidations and so forth. Commercial

acids, especially inorganic acids, generally contain not inconsiderable

quantities of impurities which are injurious in the manufacture of many

colours.

=Hydrochloric Acid=, HCl = 36·5.—The commercial acid (muriatic acid,

spirits of salt) generally contains large quantities of iron,

which colour it yellow—fortunately, in many cases, this is not a

disadvantage, and also at times the iron can be removed from solutions

made in the acid. Another impurity is sulphuric acid. This can be

detected by diluting and adding barium chloride; if sulphuric acid be

present, a white precipitate, or, at least, a cloudiness, appears.

Ordinary hydrochloric acid is a solution of hydrochloric acid gas in

water. The strongest acid contains 42·85 per cent. of the gas, and has

the specific gravity 1·21. The following table gives the strengths of

acids of various specific gravities:—

+------------+--------------+-------------+--------------+

+------------+--------------+-------------+--------------+

| 1·21 | 42·85 | 1·10 | 20·20 |

| 1·20 | 40·80 | 1·09 | 18·75 |

| 1·19 | 38·88 | 1·08 | 16·71 |

| 1·18 | 36·36 | 1·07 | 15·49 |

| 1·17 | 34·34 | 1·06 | 13·86 |

| 1·16 | 32·32 | 1·05 | 11·49 |

| 1·15 | 30·30 | 1·04 | 8·97 |

| 1·14 | 28·28 | 1·03 | 6·93 |

| 1·13 | 26·26 | 1·02 | 4·89 |

| 1·12 | 24·24 | 1·01 | 2·03 |

| 1·11 | 22·22 | | |

+------------+--------------+-------------+--------------+

=Sulphuretted Hydrogen=, H₂S = 34.—This is a gas of acid properties

smelling like rotten eggs; it precipitates the sulphur compounds of

many metals when led into the acid solution of the corresponding

salt. This substance is seldom required in colour works, so that it

is convenient to have an apparatus which permits of the preparation

of any required quantity. Fig. 3 represents an apparatus devised by

the author, which is well adapted for the preparation of sulphuretted

hydrogen. It consists of a small, wooden tub, on whose upper edge lies

a thick paper ring, so that the lid may be pressed down air-tight by

the screws B. Through the lid pass a tap-funnel, T, a movable screw,

S, and a tube, R, to carry away the gas. On the screw S hangs a basket,

K, by a handle; this is filled with pieces of iron sulphide as large as

nuts. The tub is filled to about one-third of its height with a mixture

of 9 parts of water and 1 part of sulphuric acid.

[Illustration: FIG. 3.]

When sulphuretted hydrogen is required the basket is lowered by the

screw, S, until it dips in the liquid; according as the basket dips

more or less into the liquid a fast or slow current of the gas is

obtained. When the gas is no longer required the basket is raised out

of the liquid, and the evolution of gas at once ceases. The funnel, T,

serves for the introduction of the liquid, the tap, H, for drawing off

the iron sulphate solution, which can be used with advantage for the

preparation of fine iron colours. The apparatus should not be opened so

long as sulphide of iron remains in the basket.

=Sulphuric Acid= comes into the market in two different forms: oil

acid; both are used in colour making. _Oil of Vitriol_, H₂SO₄ = 98, is

a colourless, oily liquid of high specific gravity; it is generally

tolerably pure, and contains, as a rule, only a small quantity of

lead, the presence of which is indicated by a turbidity on largely

diluting the acid. The amount of pure sulphuric acid in the liquid

is practically determined by taking the specific gravity. The table

indicates the relation between the specific gravity and the content of

sulphuric acid.

+-----------------+--------------+-----------------+--------------+

+-----------------+--------------+-----------------+--------------+

| 1·8485 | 100 | 1·8043 | 89 |

| 1·8475 | 99 | 1·7962 | 88 |

| 1·8460 | 98 | 1·7870 | 87 |

| 1·8439 | 97 | 1·7774 | 86 |

| 1·8410 | 96 | 1·7673 | 85 |

| 1·8376 | 95 | 1·7570 | 84 |

| 1·8336 | 94 | 1·7465 | 83 |

| 1·8290 | 93 | 1·7360 | 82 |

| 1·8233 | 92 | 1·7245 | 81 |

| 1·8179 | 91 | 1·7120 | 80 |

| 1·8115 | 90 | | |

+-----------------+--------------+-----------------+--------------+

_Nordhausen Sulphuric Acid_, H₂S₂O₇ = H₂SO₄ + SO₃, is generally a

yellowish brown liquid, which gives off white fumes in the air. It

contains varying quantities of sulphur trioxide dissolved in sulphuric

acid. It often contains selenium, which separates as a red powder when

the acid is diluted. The presence of this impurity does not interfere

with the use of the acid for dissolving indigo, the only purpose for

which it is required in the colour factory.

=Nitric Acid=, HNO₃ = 63.—This acid, which is used in the preparation

of many colours, is distinguished by the readiness with which it gives

up part of its oxygen, and thus converts metals like antimony and

bismuth into oxides, and transforms other compounds into a higher state

of oxidation. There are two kinds of nitric acid: ordinary nitric

acid, a colourless liquid which is more or less pure; and fuming nitric

acid, a yellow or orange-coloured liquid, fuming strongly in the air,

which consists of a solution of nitrogen peroxide, NO₂, and nitric

oxide, NO, in nitric acid.

+----------------+--------------+----------------+--------------+

| at 15° C. | per cent. | at 15° C. | per cent. |

+----------------+--------------+----------------+--------------+

| 1·530 | 100·00 | 1·323 | 55·00 |

| 1·520 | 97·00 | 1·284 | 50·49 |

| 1·509 | 94·00 | 1·251 | 45·00 |

| 1·503 | 92·00 | 1·211 | 40·00 |

| 1·499 | 91·00 | 1·185 | 33·86 |

| 1·495 | 90·00 | 1·157 | 30·00 |

| 1·478 | 85·00 | 1·138 | 25·71 |

| 1·460 | 80·00 | 1·120 | 23·00 |

| 1·442 | 75·00 | 1·089 | 20·00 |

| 1·423 | 69·96 | 1·067 | 15·00 |

| 1·400 | 65·07 | 1·022 | 11·41 |

| 1·346 | 60·00 | 1·010 | 4·00 |

+----------------+--------------+----------------+--------------+

Since the action of nitric acid chiefly depends on its oxidising

properties, which are possessed by both kinds, it generally does not

matter which is used. The usual impurities are chlorine and sulphuric

acid; the presence of the first is shown by silver nitrate solution,

of the latter by barium chloride, in each case added after diluting.

When the acid is used for oxidations these impurities do not interfere,

but nitric acid containing chlorine cannot be used to dissolve silver,

because the chlorine would form insoluble silver chloride.

The strength of nitric acid is gauged by its specific gravity as given

in the table.

=Aqua Regia.=—A mixture of 2 parts of hydrochloric acid and 1 part of

nitric acid gradually turns orange or yellow and evolves chlorine. This

liquid, which can dissolve gold in consequence of the free chlorine it

contains (hence its alchemistic name, from gold, the “king of metals”),

is used as a very powerful oxidising agent in colour making.

=Carbon=, C = 12, is the only one of the non-metallic elements to be

mentioned here; by itself it forms a group of very important pigments,

which we shall describe in detail at a later stage.

=Carbonic Acid Gas=, CO₂ = 44, is used in the manufacture of white

lead, which it precipitates from lead acetate. This is, however, a

particular branch of colour making carried on in special works. In

describing this manufacture we shall return to the preparation of

carbonic acid on a large scale.

ORGANIC ACIDS.

The organic acids which are important in colour making are acetic,

oxalic and tartaric acids.

=Acetic Acid=, C₂H₄O₂ = 58.—The very dilute form of this substance

is known commonly as vinegar, the stronger as pyroligneous acid, and

the purest as glacial acetic acid; the latter is, however, scarcely

used. Formerly in colour making ordinary vinegar was used, but now

pyroligneous acid is almost exclusively employed. This is distinguished

by its strong empyreumatic smell, which, however, is without importance

in colour making.

The strength of a solution of acetic acid cannot be found by a simple

estimation of specific gravity, since the density does not increase

with the percentage of acetic acid. If an accurate estimation of

the strength of acetic acid is required, it must be obtained by

neutralising the acid with an alkali by a process of volumetric

analysis.

For practical purposes, where it is generally known whether a very

strong or a more dilute acetic acid is under consideration, the

following table, showing the connection between specific gravity and

percentage strength, is sufficient.

+--------+---------+--------+---------+

+--------+---------+--------+---------+

| 1·0635 | 100 | 1·058 | 48 |

| 1·0670 | 98 | 1·055 | 46 |

| 1·0690 | 96 | 1·054 | 44 |

| 1·0706 | 94 | 1·052 | 42 |

| 1·0716 | 92 | 1·051 | 40 |

| 1·0730 | 90 | 1·049 | 38 |

| 1·0730 | 88 | 1·047 | 36 |

| 1·0730 | 86 | 1·045 | 34 |

| 1·0730 | 84 | 1·042 | 32 |

| 1·0730 | 82 | 1·040 | 30 |

| 1·0735 | 80 | 1·038 | 28 |

| 1·0732 | 78 | 1·035 | 26 |

| 1·0730 | 76 | 1·033 | 24 |

| 1·0720 | 74 | 1·031 | 22 |

| 1·0710 | 72 | 1·027 | 20 |

| 1·0700 | 70 | 1·025 | 18 |

| 1·0700 | 68 | 1·023 | 16 |

| 1·0690 | 66 | 1·020 | 14 |

| 1·0680 | 64 | 1·017 | 12 |

| 1·0670 | 62 | 1·015 | 10 |

| 1·0670 | 60 | 1·012 | 8 |

| 1·0660 | 58 | 1·008 | 6 |

| 1·0640 | 56 | 1·005 | 4 |

| 1·0630 | 54 | 1·002 | 2 |

| 1·0620 | 52 | 1·001 | 1 |

| 1·0600 | 50 | —— | —— |

+--------+---------+--------+---------+

=Oxalic Acid=, C₂H₂O₄.2H₂O = 126, has but a limited use in colour

making. It comes into commerce in the form of more or less pure white

crystals which readily dissolve in water, and are almost pure oxalic

acid, containing only small quantities of oxalate of lime, the presence

of which is without importance for the purposes to which the acid is

put in colour making. Frequently, instead of oxalic acid, the acid

potassium oxalate (salt of sorrel) is used.

=Tartaric Acid=, C₄H₆O₆ = 150, occurs as white or yellowish crystals,

with a slightly burnt smell, which dissolve readily in water, and have

a strong acid taste. The pure acid, which is white and without smell,

is considerably dearer than the yellow variety. The impurities of the

latter, which are small in quantity, are without influence on the

colours prepared by its help, so that this form is generally used.