ULTRAMARINE.
At the present time blue, green, red and violet pigments come into the
market under the name of ultramarine. The green and blue have been
commercial articles for about seventy years; the violet and red were
introduced about the year 1860.
At first the name ultramarine was restricted to a natural blue pigment
obtained from _lapis lazuli_, which was extremely costly. Accounts
of the payments of Italian artists, still extant, show the expensive
nature of the ultramarine blue used in their paintings. At that time,
when artists were compelled themselves to make the majority of their
pigments, ultramarine was made in a most laborious manner from _lapis
lazuli_, for which incredibly high prices were paid. In order to make
the mineral easier to powder the lumps were heated, and, whilst hot,
thrown into water. They were then powdered as finely as possible. The
powder was mixed with melted resin, and the mixture kneaded under
water for a long time. The ultramarine suspended in the water by this
crude method of levigation was obtained by allowing the wash waters
to settle. Few places are known at which _lapis lazuli_ occurs in
quantity. It is chiefly obtained in China and Thibet, and, considering
the little intercourse between Europe and these distant countries at
that time, it is no wonder that the price of ultramarine was fabulously
high. One ounce cost about £8—a price which is explained by the small
yield of ultramarine from the best _lapis lazuli_. By the most careful
work not more than from 2 to 3 per cent. of the mineral was obtained,
the residue consisted of foreign minerals.
The enormously high price of this pigment was the stimulus for the
endeavours to make it artificially. The attempts are to be regarded not
only as completely successful, but it must be allowed that science has
gone a considerable step further than nature, since the researches have
made it known that there is not only a blue ultramarine, but also a
green, and, according to the latest researches, there exist in addition
violet, red and white compounds, also to be described as ultramarines.
With the discovery of the methods by which ultramarine can be made
artificially, the preparation of this pigment from _lapis lazuli_
came to an end, and has now but historic interest. The discovery of
artificial ultramarine is due to the French chemist Guimet and to the
great German chemist Gmelin. Ultramarine was first manufactured in
Germany in the year 1828 by A. Köttig, as a branch of the porcelain
works at Meissen in Saxony, where the manufacture was continued for
about fifty years.
The discovery of the aniline dyes is rightly called a triumph of human
intellect. The artificial manufacture of ultramarine deserves the same
description in no less degree, although it has not effected so great a
revolution as the former.
Attempts to make ultramarine artificially would naturally be based on
the analysis of the natural product. The following comparison of the
compositions of natural and artificial ultramarines shows how nearly
the artificial product approaches the natural:—
NATURAL ULTRAMARINE.
Clément and Desormes. Gmelin.
Silica 35·8 per cent. 47·31 per cent.
Alumina 34·8 ” 22·00 ”
Soda 23·2 ” 12·06 ”
Lime 3·1 ” 1·55 ”
Sulphuric acid —— ” 4·68 ”
Sulphur 3·1 ” 0·19 ”
Water and organic matter —— ” 12·21 ”
ARTIFICIAL ULTRAMARINE.
+------------------------------------------------------------------+
| | Blue. | Green. |
+-------------+------------+------+--------+-----+-----------------+
| Observer. |Warrentrapp.|Elfen.|Brunner.|Pohl.| Gentele. |
| | | | | | |
| Silica | 45·60 | 40·0 | 32·54 |36·67|} 47·31 |
| Alumina | 23·31 | 29·5 | 25·25 |32·12|} |
| Soda | 21·47 | 23·0 | 16·91 |21·45| 39·93 |
| | | | | |(sodium silicate)|
| Potash | 1·75 | —— | —— | —— | 3·92 |
| Lime | 0·02 | —— | 2·38 | —— | 1·13 |
| Sulphuric | | | | | |
| Acid | 3·83 | 3·4 | —— | 2·08| —— |
| Sulphur | 1·69 | 4·1 | 11·63 | 7·22| 6·62 |
| Iron | 1·06 | 1·0 | 2·25 |trace| 1·95 |
| | | | | |(ferric silicate)|
| Water | —— | —— | —— | —— | —— |
| Oxygen | —— | —— | 9·04 | 0·58| —— |
+-------------+------------+------+--------+-----+-----------------+
I. Szilasi found three samples of green ultramarine to have the
following composition:—
Water 2·20 1·20 1·19
Aluminous residue 1·80 1·42 1·41
Silica 16·73 17·18 16·74
Aluminium 15·92 15·87 16·15
Sodium 18·42 18·18 18·10
Sulphur 7·19 6·97 6·85
There are many other analyses in addition to those we have given and
agreeing with them, so that there is no doubt as to the composition of
ultramarine, but as to the manner in which the elements are grouped
nothing is definitely known. Some chemists are of the opinion that
the colouring principle of ultramarine is a sulphur compound of iron,
whilst others oppose this view and consider that the colour is due
to the combination of a double silicate of alumina and soda with an
unknown sulphide of sodium. Although no blue or green compound is
known of corresponding composition, the majority of chemists incline
to the latter view. Experience has shown that the presence of iron in
any of the materials used in the manufacture of artificial ultramarine
is very dangerous to the success of the operation, and at the least
considerably injures the beauty of the product.
Although the manufacture of ultramarine is now very well known, it
cannot be denied that some works produce a pigment of a shade which
cannot be obtained by others. These works keep their method very
secret, so that it is not possible to say with certainty whether they
have introduced a process varying from that commonly known, or whether,
by carefully watching the process, they have achieved great technical
dexterity in the manufacture of this product. The latter appears the
most probable, for in order to obtain a good result, many experiments,
and an accurate knowledge of the raw materials, are necessary.
The raw materials used are as follows: pure aluminium silicate, sodium
sulphate, soda, sulphur, coal. The aluminium silicate is used in the
form of fine china clay or kaolin, sodium sulphate and soda must be
used in the anhydrous form, the sulphur is the ordinary commercial
substance purified by distillation. Charcoal or coal containing little
ash can be used.
Whilst the remaining raw materials are always of similar composition,
the china clay from different localities possesses a very varying
composition. This substance must be carefully chosen. There is hardly
any kaolin which is naturally of sufficient purity to be used without
purification; it is well known that the china clay used for porcelain
is subjected to a thorough preparation before it is used. Kaolin, like
all clays, has been produced by the decomposition of feldspar; when
the aluminium silicate so formed was able to deposit without foreign
admixtures, that mineral was formed which is the purest of all clays
and is called kaolin. The more foreign substances are mixed with
the aluminium silicate the further is the clay removed from kaolin.
The impurities which generally accompany the aluminium silicate are
quartz sand, chalk and ferric oxide. We distinguish accordingly
between kaolin, white clay or pipe clay, clay, and lastly marl, a clay
containing much chalk.
Even the purest kaolin contains certain impurities, of which quartz
sand is the principal and the least harmful. Before kaolin can be
used in the ultramarine manufacture it must always be purified by
levigation; it is then ignited at a low temperature and powdered under
stamps or in mills. The other materials required are generally produced
by the chemical works in a condition of such purity that they can be at
once used.
Occasionally Glauber’s salt contains iron, which would spoil the shade
of the ultramarine. The iron can be easily removed by dissolving the
crystallised salt in water, adding a little milk of lime and leaving
the liquid for several days, stirring frequently. The lime neutralises
every trace of free acid, and at the same time produces an ochre-yellow
precipitate of ferric hydrate. The solution of Glauber’s salt thus
freed from iron is evaporated in reverberatory furnaces, in which
the salt is then calcined. This operation is not only simpler than
evaporating the solution in iron pans to crystallisation and subjecting
the dried salt alone to calcination, but it also guards against fresh
contamination by iron, which might be caused by the use of iron
evaporating pans.
The sulphur and coal are employed in a soft powder, which is most
readily obtained by placing the coarsely powdered materials in rotating
drums containing a number of iron balls. By continued rotation the
materials are converted to any desired degree of fineness without the
production of dust. The powder is then put through fine sieves by which
the larger particles are retained.
The proportions in which the raw materials are mixed vary within
certain limits; fixed quantities can only be given for a kaolin of
definite composition. Definite formulæ are known by which ultramarine
is made, but these can only be regarded as approximate. The proportions
employed by French manufacturers differ considerably from those usual
in German works. This variation is chiefly due to the difference in
the composition of the kaolin employed in the two countries.
From the composition of the different mixtures one conclusion may be
drawn with certainty—sufficient sodium is always used to neutralise
half the silicic acid of the kaolin and to form some quantity of
sodium sulphur compounds. In the successful process of the German
makers a portion of the soda unites with the silica during heating. By
the action of the coal on the Glauber’s salt it is reduced to sodium
sulphide, which, since sulphur is present, unites with a further
quantity of that element. The sodium sulphur compounds then unite with
the silicates of aluminium and sodium to form a green compound, which
is converted into blue ultramarine by a further treatment with sulphur
in the presence of air.
Instead of using Glauber’s salt, which must always be decomposed in
the first process, the sodium sulphide may be formed by the action of
sulphur on soda in the presence of coal. This procedure is adopted in
the French process. The proportions of the mixture used in different
works vary. If the kaolin employed is assumed to be bisilicate of
alumina—a somewhat arbitrary assumption—the following mixture can be
successfully used:—
Anhydrous kaolin 100
” Glauber’s salt 42
” soda 42
Sulphur 60
Coal 13
In working by the French method the following formula is suitable:—
Anhydrous kaolin 100
” soda 100
Sulphur 62
Coal 14
These formulæ are not to be regarded as unalterable. In the different
works such varied mixtures are used that it may be said with truth that
each works has its particular formula for the mixture, the composition
of which depends on the nature of the clay used in the works. The
composition of the mixtures used in works is kept secret as far as
possible. The formulæ given above refer to a clay approximating in
composition to the bisilicate of alumina.
It will appear from the description of the manufacture of ultramarine
that certain quantities of sodium sulphide are produced. The process
is, however, conducted so that sodium sulphide shall be formed; thus
the soda and Glauber’s salt used in the mixture may be replaced by the
sodium sulphide produced in previous operations. The liquors in which
the sodium sulphide is contained are evaporated to dryness, and as
much of the residue added to the clay as corresponds to the quantity
obtained from the usual amounts of soda and Glauber’s salt. Assuming
that these latter materials are pure, 80 parts of the sodium sulphide
residue correspond to 100 parts of soda, and 60 parts to 100 parts of
Glauber’s salt.