YELLOWS.
The yellow pigments do not form a large or important group, and beyond
a few organic colouring matters which have a limited use in artistic
painting, they are chiefly confined to the ochres of natural origin and
to the chromes, and one or two other kinds artificially prepared from
mineral substances.
ARSENIC YELLOW.--Another name for Orpiment (see p. 280).
AUREOLIN YELLOW.--This colour is prepared by precipitating
carbonate of cobalt from a solution of a cobalt salt with carbonate
of potash; this precipitate is dissolved in acetic acid, and to it is
added nitrite of sodium; nitrite of potassium and cobalt are thrown
down as a yellow powder. Experiments made on the freshly-precipitated
colour prepared in this way, have proved it to be invariably destroyed
by caustic potash; but aureolin yellow prepared in another way, put
into a flask with some caustic soda, has remained unchanged for several
days, and proved to be perfectly stable under this treatment. Samples
of the same yellow, exposed to the action of ammonia, soda, and potash
for a considerable length of time, have not changed. This is really a
most important fact, because aureolin yellow is so beautiful a pigment
that one wishes it would stand for ever. It is, in fresco and silicious
painting, a great desideratum to have colours that will stand the
action of lime and caustic alkalies. Our colours for fresco painting
are very limited, and if aureolin will stand the action of caustic
alkalies, it may be safely used both for fresco and silicious painting.
CADMIUM YELLOW.--The pigment known as cadmium yellow is
important on account of its permanence and brilliance. It is prepared
by passing a stream of sulphuretted hydrogen gas through a slightly
acid solution of a salt of cobalt, usually the nitrate or sulphate,
whereby the sulphide of cadmium is precipitated as an impalpable
powder. It is filtered off, well washed with water, and finally dried
at a low temperature.
By this method there is considerable difficulty in producing any
precise, shade of yellow, which is dependent on the proportion of
precipitant used, and can only be controlled after long practical
experience. Therefore some modifications have been introduced with
the object of securing definite tints. The ordinary colour is a pure
chrome yellow. For a lemon-yellow shade, a solution of yellow sulphide
of ammonium is added to one of sulphate of cadmium. For an orange
tint, the cadmium solution (chloride or sulphate) is made very acid by
addition of hydrochloric acid before passing the sulphuretted hydrogen;
or the cadmium solution, as for lemon-yellow, is boiled, and receives
the ammonium sulphide solution while still boiling. But none of these
modifications results in a really durable pigment, on account of the
presence of traces of free sulphur or acid.
The best brands of this pigment being somewhat expensive, there is
great inducement for adulteration, which usually takes the form of
orpiment or of chrome. The former can be detected by the ordinary tests
for arsenic, and the latter by the process described on p. 266.
CHROME YELLOWS.--A very important family of yellow pigments
are the “chromes,” consisting mainly of chromic acid in combination
with lead, iron, or zinc.
Chromates of lead, says Prof. Barff, are produced by precipitating a
lead salt with a salt of chromic acid, and the difference in tint is
owing to the different quantity of the chromic acid which is present
in the salt. The orange chrome is a basic chromate of lead, and basic
chromate of lead contains more of the chromic acid than is present in
the lemon chrome. The lightest chrome contains some sulphate of lead
precipitated with the chromate. All these colours contain lead, and
are therefore liable to the influence of sulphuretted hydrogen. Now,
if a chromate of lead is bought hap-hazard anywhere, it may or may not
be pure, but generally speaking, unless the chromates are obtained
from makers who are careful in the preparation of their colours, they
contain many other substances besides chromate of lead. For instance,
they contain a quantity of the solution, in a dry state, from which
they have been precipitated, and are by no means pure. Obviously, also,
there is an inducement to men who sell cheap colours to adulterate
them, to bring them down with whiting, and so forth. In that case the
chrome loses its body; if it is brought down with lead sulphate it has
more body; but when chrome yellows are prepared and mixed with good
oils, and put on carefully, and those oils have time allowed them to
become oxidised, perfectly dry in fact--not dry in the sense in which
an artist considers a painting dry, but perfectly hard--then the action
of sulphuretted hydrogen in the atmosphere will not be of such great
moment as if the colours are impure, or if they are submitted to the
influence of that deleterious gas before they have become perfectly
hard.
Another objection to the chromes, on the authority of Prof. Barff, is
that they are soluble in alkali, and so are many other colours. If
a painting is painted with chromes, and if that painting be washed
with an alkaline soap, it is quite certain that some of the chromates
will be dissolved. Consequently the minute and delicate touches, upon
which the artist depended for some of the best effects of his picture,
are removed with soap and water. No painting should ever be washed
with soap and water at all; but there are certain colours which will
withstand the action of soap, even if it is intensely alkaline, while
others will not. Taking a precipitate of lemon chrome, if we act upon
this with potash or soda, it dissolves up the yellow precipitate, and
destroys it altogether.
According to Weber, the preparation of chrome yellow presents
difficulties in practice, because products differing in shade and
structure, although of uniform chemical composition, are obtained
according to modifications of the method of manufacture or nature of
materials employed. A special difficulty is the turning of colour,
whereby a “turned” yellow has a dirty orange-yellow colour, which when
mixed with barytes gives a yellowish-brown leather-coloured shade and
not a light pure yellow. Other derived colours are similarly affected.
_Lead Chromates._--The acetate and nitrate of lead are the soluble
lead salts generally used. Basic carbonate of lead (white lead) is
also much used, the yellows prepared from this material being cheaper,
having a large covering capacity, and being particularly adapted for
ordinary greens; but these have not the smoothness and lightness so
much prized in some of the yellows obtained from soluble lead salts.
The oxide, sulphate, and chloride of lead have been proposed for the
manufacture of chrome yellow, but their treatment would be very tedious
and the products obtained only of medium quality. In the manufacture of
chrome yellow from acetate of lead and bichromate of potash, different
proportions of these materials are given by different authorities, but
some of these are obviously wrong where an excess of bichromate is to
be used, for every light chrome yellow is liable to turn by the action
of chromic acid or a chromate and thereby become of little value.
Proportions should be used, whereby the acetate of lead remains in
excess, and the reaction should take place in a solution as cold and
dilute as possible. In this way a brilliant yellow tint is obtained.
For the lighter chrome yellows, lead sulphate is precipitated
simultaneously with the chromate by adding sulphuric acid or a soluble
sulphate to the solution of the bichromate; these yellows have less
tendency to change colour than the pure chromate of lead, if the above
precautions are observed. Chrome yellow, precipitated from an excess of
lead acetate solution, by means of potassium bichromate, corresponds
to the formula PbCrO_{4}. When dried it forms light pieces, which show
a conchoidal fracture. A still more voluminous product corresponds to
the formula PbCrO_{4}, PbSO_{4}. The yellow having the composition
PbCrO_{4}, 2PbSO_{4}, is very heavy and shows a smooth fracture.
Lightness is often imparted to chrome yellow by the addition of
magnesium carbonate.
Nitrate of lead offers no advantages over the acetate, and is generally
more expensive to use. Free nitric acid is more objectionable than
free acetic acid, because it may act as a solvent on chrome yellow
and liberate free chromic acid, which is liable to “turn” the yellow.
When using lead nitrate it is preferable to neutralise potassium
bichromate with soda, to avoid the presence of free nitric acid. The
yellows mostly in demand are the inferior qualities, prepared by mixing
pure chrome yellows with white mineral matters, generally barytes,
gypsum, and kaolin, usually stirred in with the bichromate solution
before adding the lead salt. Barytes injures the colour least, but
kaolin has the advantage that it does not increase the weight of the
colour so much. Gypsum occupies an intermediate position; it is much
more voluminous than barytes, and does not injure the colour so much
as kaolin, but it is generally used together with barytes. It is not
advisable to use this combination, for the reason that the colour on
drying forms very hard pieces, which offer difficulties in grinding.
Gypsum, too, from being more easily acted on by reagents than barytes
or kaolin, tends to take part in the reaction by decomposing the
potassium bichromate before the addition of the lead salt, and this is
objectionable. (Weber.)
A writer in the _Chemical Trade Journal_ gives the following formula
for the production of various yellows:--
(1) For soluble lead salts:--
Kilos.
Lead acetate 100
Potassium bichromate 18
Sulphuric acid (66° B.) 12
This mixture yields a yellow of the formula PbCrO_{4}, PbSO_{4}. To
obtain good shades, the amount of water employed should not be less
than 1000 litres, or double as much with the nitrate. In the latter
case, it is better to neutralise the lead nitrate and to replace the
acid by the sulphate of an alkali or of magnesium, or preferably of
aluminium; for the neutralisation of the bichromate, the best substance
is magnesite. The formula thus becomes--Lead acetate and bichromate as
before; magnesite, 6 kilos.; aluminium sulphate, 27 kilos.
(2) The Basic Acetate Method.--Litharge, 76 kilos.; acetic acid (30 per
cent.), 42; bichromate, 21·5; sulphuric acid, 21·5; water, 2000 to 3000
litres. To obtain a denser chrome, 10 kilos. of soda should be added to
the acetate, and 5 of the sulphuric acid replaced by 10 of aluminium
sulphate. For the production of an orange chrome, the following
formula is given: Litharge, 76 kilos.; acetic acid (30 per cent.), 42;
bichromate, 24; Solvay’s soda, 15; and caustic soda (100 per cent.), 5.
Care must be taken that the temperature does not rise sufficiently high
to spoil the shade.
(3) The White Lead Method.--In this case the white lead must be in
the finest state of subdivision possible, and suspended in the water:
White lead, 100 kilos.; nitric acid (36° B.), 12; bichromate, 13;
and aluminium sulphate, 10: or nitric acid (40° B.), 44; bichromate,
24; sulphate, 20, the latter giving the more fiery shade. For the
production of an orange: white lead, 100 kilos.; nitric acid (36° B.)
18; bichromate, 28; and caustic soda, 8, the latter being best added to
the bichromate before precipitation, and the temperature kept between
150° and 165° F.
(4) The Basic Chloride Method.--The same proportions and temperature
are suitable here as in the case of the white lead.
(5) The Sulphate Method.--Lead sulphate, 100 kilos.; bichromate, 24
to 25; Solvay’s soda, 8·75 to 16; ammonia (24 per cent.), 1 to 2; and
acetic acid (30 per cent.), 5 to 10. The sulphate, in the form of a
cream, is gradually added to the other ingredients after solution.
A new and improved process for manufacturing from galena chemically
pure chrome yellow having great colouring power, according to the
_Paper Trade Journal_, consists in first dissolving pulverised
galena with nitric acid to produce liquid nitrate of lead, and then
precipitating the chromate of lead by subjecting the nitrate of lead
to the action of bichromate of potash, neutral chromate of potash or
chromate of potash-soda.
The galena (sulphide of lead) is first pulverised by suitable means,
and in case it contains foreign minerals or other impurities, it is
washed or otherwise treated in a suitable manner to remove these
substances. The pulverised galena is then placed in acid-proof vessels
and is dissolved by adding nitric acid diluted with water, the entire
mass being well stirred. A slow dissolving takes place at the ordinary
temperature; but when the mass is heated artificially, either by
heating the vessel, or by using hot water added to the nitric acid,
or by the use of steam, more rapid solution is effected. The product
obtained is nitrate of lead in a liquid state.
The quantity of nitric acid necessary for dissolving a certain quantity
of galena depends on the percentage of lead contained in the ore, and
to a certain extent on the amount and nature of impurities present in
it, and also on the length of time in which the dissolving takes place.
In treating 100 lb. of galena having 80 per cent. of metallic lead,
about 90 to 100 lb. of nitric acid of 36° to 38° B. are used, and the
nitric acid is diluted with 100 to 200 lb. of water. This mixture is
left for about 24 to 36 hours, and is stirred up occasionally, as above
stated.
After the galena is dissolved by the nitric acid, and the sulphide of
lead is changed into liquid plumbic nitrate, then the sulphur which
floats occasionally on the surface of the solution is removed, and the
substance which remains undissolved is washed out and is also removed.
The liquid nitrate is then passed through filters of felt, linen, hemp,
flannel, &c., or is left standing for about 12 to 18 hours for settling
and clearing.
Now in order to produce the chrome yellow from this nitrate of lead,
bichromate of potash is dissolved in water, and a sufficient quantity
of this solution is poured into the plumbic nitrate solution until all
the plumbic nitrate is changed into chromate of lead, called “chrome
yellow.” Instead of the bichromate of potash, neutral chromate, or
chromate of soda may be used, and for the purpose of obtaining lighter
tints they may be tempered with sulphuric acid or any other compound
of sulphur. The liquid nitrate of lead is placed, preferably, in
large open receptacles of wood, clay, earthenware, or other suitable
material, and the chromate of potash solution is put into similar
vessels, and then placed above the receptacles containing the plumbic
nitrate. The chromate of potash can then easily be run into the lower
receptacles containing the liquid nitrate of lead, and this mixture is
constantly agitated by similar means until all the plumbic nitrate is
changed into chromate of lead, which is precipitated on the bottom of
the larger receptacles.
The chemical action which takes place by this changing of nitrate of
lead into chromate of lead is that the chromic acid of the potash
assumes the place of the nitric acid, which parts from the lead and
combines with the potassium, so that the lead as chromate of lead is
precipitated on the bottom of the receptacle, while the nitric acid of
the plumbic nitrate remains with the potassium, which latter has parted
with its chromic acid, and a quantity of water as solution above the
chromate of lead.
To change the nitrate of lead recovered out of the 100 lb. of galena
above mentioned into chromate of lead, about 56 lb. of bichromate of
potash are used. This change usually takes place in from about 10 to
30 minutes, after which the chrome yellow (chromate of lead) is left
for a few hours to settle, and then the solution standing on top of the
chrome yellow is drawn off by suitable means, or run out of the vessel
by opening a cock placed above the level of the chromate of lead.
The latter is then washed by adding pure water, which is poured
upon the chrome yellow, and the mixture is stirred up, so that all
the remaining liquid nitrate of potash is removed. After this is
accomplished, the mass is left to settle, and the water is again drawn
off from the precipitate, which then settles on the bottom of the
receptacle. This washing is repeated as often as is deemed necessary.
The chrome yellow is next placed in suitable receptacles, and dried in
the open air or in specially constructed drying rooms, after which it
is packed in boxes, kegs, &c., and is then ready for use. The liquid
nitrate of potassium, or saltpetre lye, removed from the receptacles
in which the chrome yellow is precipitated, and the first water used
for washing the chrome yellow, as above described, are placed in
large open flat receptacles or excavations, so as to be exposed to
the action of the air and sun; or the liquids may be operated on by a
small graduation work, so that a great portion of water evaporates. The
residue is then heated in suitable vessels or troughs by a slow heat
until a salt crust is formed, which, when cooled off and left to dry,
is nitrate of potassium or saltpetre in a pure state.
From 100 lb. of galena having 80 per cent. metallic lead, some 28 to
30 lb. of pure and dry saltpetre are produced by the above described
process. The sulphur produced by the dissolving of the galena by nitric
acid is melted in a small stove or furnace in the usual manner, and
then refined, so as to produce bars of sulphur called “brimstone.”
About 10 lb. of such sulphur are produced from 100 lb. of such galena
treated in the manner described.
The chrome yellow thus produced is said to be chemically pure, and of
great covering power, equal to the best chrome yellow in the market.
The process is very simple, and the crude lead ore is transformed into
chrome yellow in from three to four days.
_Characters._--Pure chromate of lead has an orange-yellow colour, and
in whatever manner it be made it always has this tint. Commercially,
chromes are made of a great variety of tints, from a pale-lemon chrome
to a deep scarlet, through all the intermediate shades of yellows and
oranges; in fact, most colour makers produce not less than eight, and
some more shades of chromes, whence it is obvious that they cannot be
chemically pure, but must be mixed with some other bodies.
In commerce, chromes are distinguished as “pure” and “common”: the
distinction between them is that the “pure” chromes are made from a
lead base, and consist of chromate of lead mixed to a larger or smaller
amount with sulphate of lead, the paler shades containing most of the
latter; while the “common” chromes are mixed with china clay, barytes,
gypsum, or similar bodies.
To distinguish the two kinds of chromes, treat with boiling
hydrochloric acid. If pure, the chrome will completely dissolve, the
solution usually having a green colour. On cooling, crystals of lead
chloride separate out. The liquor will give a white precipitate with
barium chloride.
By taking a weighed quantity of chrome, dissolving in hydrochloric
acid, adding excess of barium chloride, filtering, thoroughly washing
the precipitate with _boiling_ water, drying and weighing it, and
making the necessary calculations, every one part of the precipitate
being equal to 1·3 parts of sulphate of lead, the quantity of the
latter in the chrome is obtained.
The “common” chromes, which mostly contain barytes, are not completely
dissolved on boiling in hydrochloric acid, the barytes they hold being
left as an insoluble residue. By taking a weighed quantity of the
chrome, and filtering off, drying and weighing the residue, the amount
of barytes present can be ascertained; the solution can be tested for
sulphate of lead, which they sometimes contain, as described above.
Chrome yellows and oranges should be assayed for colour and tint,
care being taken to compare them with a thoroughly reliable standard
sample. Their colouring power and body should also be assayed. Another
property which should be tested is the colour that they yield on mixing
with Prussian blue. A great deal of chrome is used for making greens
by mixing with Prussian blue, and as there is a very considerable
difference between them in the shade of green they give, it is rather
important to test this property, which can readily be done by mixing
100 grains of the chrome with 10 grains of Prussian blue, grinding them
together in a mortar, and observing the shade of the green which is
produced thereby; if the green is not bright and pure, the chrome is
not fit to be made into greens, and should be rejected for that purpose.
_Iron Chromate._--If a solution of chloride of iron acidulated with
hydrochloric acid be added to neutral chromate of lead, a light orange
powder is precipitated, which is chromate of iron. Dried at 104° F.,
it is found to consist of 65 to 65·11 chromic acid, and 34·58 to 34·78
oxide of iron. Chromate of iron is insoluble in water, dissolves easily
in hydrochloric, nitric, and sulphuric acids, and decomposes when
mixed with soda lye. Under strong heat, it melts into a brownish mass.
It may be used in painting in oils, as a substitute for chromate of
lead. Although inferior to the latter in brilliancy, it has certain
advantages over it--it does not blacken with exposure to sulphuretted
hydrogen, is not injurious to the user, and withal is cheaper.
_Zinc Chromates._--The before-quoted writer in the _Chemical Trade
Journal_, speaking of zinc chromates, says that although it is possible
to prepare lead chromates having shades varying imperceptibly from
the palest lemon to a deep granite red, in the case of the zinc
compounds scarcely any variation from the normal is possible, this
being, however, different from anything obtainable with lead. Zinc
yellows fall considerably below the ordinary chromes in their colouring
power, but they are faster in light and are less poisonous. More
than 80 per cent. of the amount annually made in Germany is used for
the production of zinc green by mixing with Prussian blues, of which
substance Holland, Switzerland, and Hungary are the greatest consumers.
The zinc yellows met with in commerce vary in their constitution
considerably, being usually acid chromates of zinc and potassium, basic
zinc chromates being rare. Ordinary salts of zinc invariably contain
small amounts of iron, which must be removed before they are used in
the manufacture of colours. The simplest method is to heat them with
the quantity of permanganate theoretically necessary to convert all the
iron present into the ferric state, adding zinc hydrate, which need not
be free from iron; after thorough stirring, the whole is allowed to
settle, and filtered, when the liquid will be found to contain not a
trace of iron.
On the addition of chromate to such solutions, a yellow precipitate
(ZnCrO_{4}) falls, but owing to its great solubility in the liquid
this process is valueless. By using an excess of bichromate, the zinc
chromate combines with some of the alkaline salt, forming the compound
(ZnCrO_{4})_{3}.K_{2}Cr_{2}O_{7}, which may be washed without loss;
but on drying yields an extremely hard, sandy powder, possessing, in
spite of its fine colour, no value as a pigment. By neutralising the
two solutions before precipitating, a much higher yield of chromate is
obtained, but still so much chromic acid is lost as to make the process
too expensive to pay. Formerly an addition of calcium chloride was
made to the neutral solutions, so as to precipitate as calcium chromate
some of the acid which remained mixed with the zinc salt. The best
results, both in regard to yield and colour, are obtained by adding to
the zinc salt sufficient alkali to decompose one-quarter of it, so that
the chrome may have the formula (ZnCrO_{4})_{3}.ZnO. To the bichromate,
enough alkali should also be added to convert it into the normal
salt. It is to be remarked that the nature of the metal combined with
the chromic acid has the greatest influence on the shade of the zinc
yellow, so much so that in manufacturing “acid” zinc yellow, the use of
sodium bichromate is inadmissible. A basic zinc yellow prepared from
the sodium salt has a redder and more cloudy shade than one made from
the potassium compound, but the difference is hardly noticeable when
sodium-potassium chromate is employed.
Modern zinc yellows are invariably prepared from acid solutions, and
consist of a double salt of zinc chromate and potassium bichromate,
mixed with a varying amount of unchanged zinc oxide, which must not
be regarded as an adulteration of the pigment, for its presence gives
the substance “body.” As previously stated, sodium bichromate is
inadmissible, as it does not form similar double salts.
The raw material is usually zinc oxide, which is met with in a state
of great purity; by the addition of sulphuric acid, this is converted
into basic zinc sulphate; potassium bichromate solution is added, and
the whole is stirred vigorously for an hour. At the end of this time,
the zinc chromate, which was previously in a state of partial solution,
begins to separate out in the form of a brilliant yellow scum on the
surface of the liquid, consisting of (ZnCrO_{4})_{3}.K_{2}Cr_{2}O_{7},
while the solution rapidly becomes almost colourless. Suitable
proportions are:--Zinc oxide, 100 parts; sulphuric acid (66° B.) 60;
and potassium bichromate, 100. Although it is hardly possible that any
hydration takes place, it is found advisable to soak the zinc oxide in
water for 24 hours before the other reagents are added, the sulphuric
acid after dilution being added gradually. Great care must be taken
that the solutions are all cold, and the stirring is continuous, to
avoid the pigment being deposited in a hard sandy form. Zinc yellows
thus prepared are not liable to change during washing in a manner
analogous to the lead compounds.
Zinc chrome is not much used, partly because it is expensive, partly
because it cannot compete with the lead chromes in brilliance, depth of
colour, and body. Still, owing to the fact that it can be mixed with
sulphur pigments without change, it is often employed in the place of
the lead chromes. Pure zinc chrome is completely soluble in sulphuric
acid without any effervescence, but a slight effervescence may be
disregarded. Any residue may be put down as adulteration, and its
character can be ascertained by a few simple tests: it may be chrome
yellow, barytes, &c.
GAMBOGE.--Gamboge is a product of several trees of Eastern
Asia: viz. _Garcinia Morella_ var. β. _pedicellata_ [_G. Hanburyi_],
a native of Cambodia, the province of Chantibun in Siam, the islands
on the east coast of the Gulf of Siam, and the south parts of
Cochin China; _G. Morella_, growing in the moist forests of Ceylon
and Southern India; and _G. pictoria_, of Southern India, by some
considered identical with _G. Morella_. _G. travancorica_, of the
southern forests of Travancore and the Tinnevelly Ghâts, is capable
of affording small supplies of the pigment for local use, but not for
export.
When the rainy season has set in, parties of natives start in search
of gamboge-trees, and select those which are sufficiently matured.
A spiral incision is made in the bark on two sides of the tree, and
joints of bamboo are placed at the base of the incision so as to catch
the gum-resin as it exudes with extreme slowness during a period of
several months. It issues as a yellowish fluid, but gradually assumes a
viscous and finally a solid state in the bamboo receptacle. It is very
commonly adulterated with rice-flour and the powdered bark of the tree,
but the latter imparts a greenish tint. Sand is occasionally added.
The product from a good tree may fill three bamboo joints, each 18 to
20 inches long and 1½ inches in diameter. The trees flourish on both
high and low land. Annual tapping is said to shorten their lives, but
if the gum-resin is only drawn in alternate years, the trees do not
seem to suffer, and last for many years.
Dr. Jamie, of Singapore, who has gamboge-trees growing on his estate,
says that they flourish most luxuriantly in the dense jungles. He
considers the best time for cutting to be February to April. The filled
bamboos are rotated near a fire till the moisture in the gamboge has
evaporated sufficiently to permit the bamboo to be stripped from the
hardened gum-resin. The gamboge is secreted by the tree chiefly in
numerous ducts in the middle layer of the bark, besides a little in the
dotted vessels of the outermost layer of the wood, and in the pith. It
arrives in commerce in the form of cylinders, 4 to 8 inches long and 1
to 2½ inches in diameter, often more or less rendered shapeless. When
good, it is dense, homogeneous, brittle, showing conchoidal fracture,
scarcely translucent, and of rich brownish-orange colour. Inferior
qualities show rough, granular fracture, and brownish hue, and are
sometimes still soft. The pigment consists of a mixture of 15 to 20
per cent. gum with 85 to 80 per cent. resin. Its chief uses are in
water-colour painting, and in varnishes.
KING’S YELLOW.--A familiar name for the trisulphide of
arsenic, also known as orpiment (see p. 280).
NAPLES YELLOWS.--This group of pigments embraces several
combinations of the oxides of lead and antimony, derived from various
sources, and prepared by sundry methods. Two of the most useful formulæ
are as follows:--
(_a_) Mix 3 lb. powdered metallic antimony, 1 lb. oxide of zinc, and 2
lb. red-lead; calcine, grind fine, and fuse in a closed crucible; grind
the fused mass to fine powder, and wash well.
(_b_) Grind 1 part washed antimony with 2 parts red-lead to a stiff
paste with water, and expose to red heat for 4 or 5 hours.
There are a great many modifications both in the ingredients and the
processes, and a great variety of shades in consequence.
Taken as a whole, the Naples yellows are unsatisfactory pigments, very
prone to deteriorate in impure air, and necessitating great care in
their preparation to avoid contact with iron, which turns them green.
They cover well, are fairly brilliant, and mix readily with water or
oil, but their application is declining rapidly.
OCHRES.--The large class of mineral pigments known
collectively as ochres or sienna earths possess considerable
importance, notably on account of their remarkable durability and
their reasonable price. They all consist essentially of an earthy base
coloured by oxide of iron or of manganese, or of both. Some authorities
differentiate between ochres and siennas, and ascribe the latter name
only to those earths which contain manganese, but this seems to be
an arbitrary proceeding, because the term sienna, or more properly
Siena, is derived solely from the name of the Italian province in
which these minerals are worked. They are of widespread occurrence,
both geographically and geologically, and the methods of mining and
preparing them are not subject to much variation.
Consul Colnaghi, in his report on the mineral products of the province
of Siena, says that Siena earths, known also under the names of
ochre, bole, umber, &c., are considered by some mineralogists to be
ferruginous clays, by others, minerals of iron. They are chiefly found
in large quantities in the communes of Castel del Piano and Arcidosso.
The yellow earths and bole found on the western slopes of Monte Amiata
are true lacustrine deposits found amid the trachytic rocks, of which
it is principally composed. They lie under, and are entirely covered
by, the vegetable soil. Varying in compactness and colour, they are
termed yellow earths when of a clear ochreous tint, and terra bolare,
or bole, when of a dark chestnut colour. Each deposit consists for the
greater part of yellow earth, beneath which bole is found in strata or
small veins. The mineral being very friable, its excavation is easy,
and is generally conducted in open pits.
The different qualities are separated during the process, the bole,
which has the higher commercial value, being the more carefully
treated. After the first separation the bole is further classed into
first, second, third, and intermediate qualities--_boletta_, _fascia_,
_cerchione_, &c. Its most important characteristic is termed, in
commercial language, _punto di colore_, or tint. The value of the bole
rises as its tint deepens. Thus bole of the third quality is lighter
than that of the second, and the second than that of the first. After
the third quality comes the _terra guilla_. The yellow earths, after
excavation, are exposed to the open air for about a year, by the pit
side, without classification. The bole, on the contrary, is placed in
well-ventilated storehouses to dry for about six months. This diversity
of treatment is owing to the fact that exposure to the elements
brightens the colour of the yellow earths, and raises their value,
while it would damage the bole by turning its darker tint first into
an orange yellow, and, if continued, into an ordinary yellow earth. It
also loses in compactness and crumbles up under exposure.
In addition to the _punto di colore_, the size of the pieces influences
the commercial value of the bole, which increases with their volume.
Thus the classification is _bolo pezzo_, _bolo grapolino_, and _bolo
polvere_. The yellow earths are classed as _giallo in pezzo_, _giallo
commune_, and _giallo impalpabile_, the impalpable being worth more
than the common yellow. The production of the Siena earths is estimated
at about 600 tons per annum, of which amount about 50 tons are
calcined, and the rest sold in the natural condition. The value of the
trade is estimated at from £4000 to £6000.
The European trade in these earths is very large. Rouen exports some
5000 tons yearly, and Havre about 1500 tons.
Similar deposits occur in America, where they are known as
“paint-beds,” and the earths are called “metallic paints.” A prominent
example is the paint-bed at Lehigh Gap, Carbon County, Pennsylvania,
which was originally opened as an ironstone mine. The mineral proved
valueless metallurgically, but remarkably useful as a pigment, since
it contains about 28 per cent. of hydraulic cement, which hastens the
drying and causes the paint to set without any addition of artificial
dryers, thereby making it eminently fitted for all outdoor application.
Along the outcrop of the paint, the beds are covered by a cap or
overburden of clay, and by the decomposed lower portion of the
Marcellus slate, which is 50 feet thick at the Rutherford shaft.
Beginning with the Marcellus slate, the measures occur in the following
descending order:--
_a._ Hydraulic cement (probably Upper Helderberg), very hard and
compact.
_b._ Blue clay, about 6 inches thick.
_c._ Paint-ore, varying from 6 inches to 6 feet in thickness.
_d._ Yellow clay, 6 feet thick.
_e._ Oriskany sandstone, forming the crest and southern side of the
ridge.
East of the Rutherford shaft the sandstone forms the top-rock of the
bed. This is due to an overthrow occurring between the Rutherford
tunnel and shaft.
The paint-bed is not continuous throughout its extent. It is faulted
at several places; sometimes it is pinched out to a few inches and
again increases in width to 6 feet. A short distance south of Bowman’s
there is a fault striking; north-east in the Marcellus slate, which has
produced a throw of about 200 feet. The measures dip from 10° to 90°.
The dip at the Rutherford shaft is about 79° south, whereas at the
tunnel it is 45° north. The ore is bluish-gray, resembling limestone,
and is very hard and compact. The bed is of a lighter tint, however,
in the upper than in the lower part, and this is probably due to its
containing more hydraulic cement in the upper strata. The paint-ore
contains partings of clay and slate at various places.
At the Rutherford shaft there are fine bands of ore, alternating with
clay and slate, as follows--Sandstone (hanging-wall), clay, ore, slate,
ore, clay, ore, clay, ore, slate, ore, cement, slate (foot-wall).
These partings, however, are not continuous, but pinch out, leaving
the ore without the admixture of clay and slate. Near the outcrop the
bed becomes brown hematite, due to the leaching out of the lime and to
complete oxidation. Occasionally, streaks of hematite are interleaved
with the paint-ore. In driving up the breasts, towards the outcrop, the
ore is found at the top in rounded, partially oxidised and weathered
masses, called “bombshells,” covered with iron oxide and surrounded by
a bluish clay. In large pieces the ore shows a decided cleavage.
The method used in mining is a variation of panel-work. Nearly the
same system of working is employed by all of the companies who have
developed their mines either by means of tunnels or shafts. Tunnels are
preferred whenever equally convenient, because they involve no expenses
for pumping and hoisting machinery, fuel, repairs to machinery, &c.
The following description of the operation of the Rutherford mines is
typical of all the workings in the vicinity.
The Rutherford tunnel is 6 feet high and 600 feet long. The gangways
are driven along the foot-wall of the cement side, 6 feet high, and
are heavily timbered and lagged at the top and on the clay side. The
sets of timbers are 3½ feet apart, and usually of 9-inch timber. The
width at the top is 3½ feet, with a spread of 5 feet at the bottom, the
extra width being cut from the clay. Where the cement-rock is firm,
the collar is hitched 6 inches into it and supported by a leg on the
clay side. The cost of the timber is 54 cents (2_s._ 3_d._) per set,
including the lagging. The monkey gangway, which carries the air along
the top of the breast from the air-shaft, is 2½ feet high, 1½ feet wide
at the top, with a spread of 2½ feet at the bottom. Wooden rails with a
gauge of 18 inches are spiked to the cross-ties.
The gangway is not driven continuously, but after being driven about 55
feet on either side of the shaft, the breasts are started 25 feet from
the shaft, a pillar being left to protect it. The breast is then opened
up to the face of the gangway, and when one ore-breast is worked out,
the gangway is driven ahead about 30 feet, and a new breast is opened
and worked out before commencing a third. The air-hole is first driven
to the surface, then the breast is opened to its full width of 6 feet.
The thickness of the bed of ore here varies from 4 to 6 feet, depending
upon the thickness of the partings of clay and slate. The clay and
slate are left on the bottom, which is made sloping to allow the ore to
roll down to the shute; this is 6 feet wide and 4 feet long and heavily
timbered. Small props or sprags are hitched into the cement, and wedged
with a lid on the clay side to prevent falls of rock.
The holes are drilled by hand in the clay-partings. They vary in depth
from 1 to 4 feet, and the charge of dynamite is varied correspondingly,
according to the amount of ore it is desired to throw down. The loose
ore is wedged down with crowbars and picks, and is then freed from
any adhering clay and thrown down the shute. It is there loaded into
boxes holding about half a ton each, which are pushed to the shaft on
a truck. The ore-boxes have four rings at the corners, to which are
attached four chains, suspended from the wire hoisting-rope. At the top
of the shaft the boxes are detached and placed on a truck, which is run
to the dump. Thirty cars, averaging 15 tons, are extracted in a day
of two shifts, the day-shift working nine hours and the night-shift
eleven. The pay of the miners is 5_s._ per shift. The cost of mining
the ore averages 7_s._ per ton.
The ore, as it comes from the mines, is free from refuse, great care
having been taken to separate slate and clay from it in the working
places. It is hauled in 2-ton wagons to kilns, which are situated on a
hill-side for convenience in charging. The platform upon which the ore
is dumped is built from the top of the kiln to the side of the hill.
The ore is first spalled to fist-size and freed from slate, and is then
carried in buggies to the charging-hole of the kiln.
The slate, when burned, has a light yellowish colour, which would
change the colour of the product. Figs. 30 to 32 represent a front
elevation of the kiln and two sections at right angles to each
other. The kiln is 22 feet high and 16 feet square on the outside.
The interior is cylindrical, 5 feet in diameter, with a fire-brick
lining _a_ of the best quality. The interior lining slopes from the
fire-place _b_ to the door _c_, by which the charges are withdrawn;
this facilitates the removal of the calcined ore. The casing _d_ is of
sandstone, 5½ feet thick, and tied together with the best white-oak
timber _e_. When charged, a kiln holds 16 tons of ore, and the kiln
is kept constantly full. The heat passes from the fire-places _b_--of
which there are two, placed diametrically opposite each other--through
a checker-work _f_ of brick into the centre of the charge. The charge
enters at _g_ and is withdrawn by a door _c_ in the front wall, 2 feet
long and 18 inches high. The ashpit is at _i_. The fire is kept at a
cherry-red heat, and about one cord of wood is burned every twenty-four
hours.
The kiln works continuously, calcined ore being withdrawn and
fresh charges made without interruption. The ore is subjected for
forty-eight hours to the heat, which expels the moisture, sulphur and
carbon-dioxide. About 1½ tons of calcined ore are withdrawn every three
hours during the day. The outside of the lumps of calcined ore has a
light brown colour, while the interior shows upon fracture a darker
brown. Great care is necessary to regulate the heat
[Illustration: Figs. 30, 31, 32.--RUTHERFORD AND BARCLAY KILN.]
so that the ore is not over burned. When this happens, the product has
a black scoriaceous appearance, and is unfit for the manufacture of
metallic paint, as it is extremely hard to grind.
The calcined ore is carried from the kiln in wagons to the mill, where
it is broken to the size of grains of corn in a rotating crusher. The
broken ore is carried by elevators to the stock-bins at the top of the
building, and thence by shutes to the hoppers of the mills, which grind
it to the necessary degree of fineness. Elevators again carry it to the
packing-machine by a spout, and it is packed into barrels holding 500,
300, or 100 lb. each.
_Characters._--Ochres owe their colour to hydrated oxide of iron,
besides which body they contain clayey matter (silicate of alumina),
earthy matters, barytes, carbonate and sulphate of calcium, &c.,
dependent upon the locality from whence they are obtained; thus
Derbyshire ochres contain mostly calcareous earthy matters, barytes,
gypsum, &c., while Oxford ochres and French ochres contain clayey
matter; Welsh ochres are variable, and usually contain a good deal of
silicious matter.
Crude ochres should first be assayed for actual colouring matter and
grit or refuse. This can be done by a kind of levigation method: 200
grains of the crude ochre are crushed in a mortar; the grinding must
not be too well done, or otherwise faulty results will be obtained. The
crushed ochre is put into a tall conical glass; a long glass funnel
passes to the bottom of the glass, and the whole is arranged in a large
glass basin or dish. A current of water is now caused to flow down the
glass funnel; this washes the fine particles of ochre away from the
grit, and they are carried over the sides of the glass into the dish.
Here they are allowed to settle, and are collected and weighed after
drying, an operation which gives the amount of ochre in the crude
material.
Levigated and prepared ochres can be tested for colour and covering
power, by the usual methods. These are the most important points about
ochres to which attention should be paid.
ORPIMENT.--Orpiment, king’s yellow, or trisulphide of
arsenic, is a lemon or orange-yellow coloured substance, found native
in Hungary, the Hartz, and other places. The finest samples used by
artists (golden orpiment) come from Persia. The commercial article is
artificially prepared for use as a pigment in the following way:--
A mixture of arsenious acid and sulphur is placed in an iron
subliming-pot, similar to those used in the preparation of crude
white arsenic. The mixture is then heated until the sublimate which
immediately forms upon the rings fixed above the pot begins to
melt. The proportions of the two ingredients used vary largely, the
best colours being probably produced when the mixture contains from
one-third to one-fifth of sulphur; for the lighter colours, a smaller
proportion of sulphur is employed. Orpiment made in this manner
consists of a mechanical mixture of sulphide and oxide of arsenic.
Orpiment is also employed as a dye, in the preparation of fireworks,
and in some depilatories. The native sulphide is preferred to the
artificial variety by artists and dyers, by reason of its richer
colour; but it is a colour which in reality is hardly ever used now.
Sometimes it is employed in water-colours, but as a pigment it is
worthless. If it comes in contact with white lead it is decomposed in
time, and a brown or black sulphide of lead is formed. While it endures
it is a very brilliant colour.
REALGAR.--Realgar or disulphide of arsenic, is a deep
orange-red substance, soluble in water, and highly volatile and
poisonous. It is found native in some volcanic districts, especially
in the neighbourhood of Naples; but the commercial article is made by
distilling, in earthenware retorts, arsenical pyrites, or a mixture
of sulphur and arsenic, or of orpiment and sulphur, or of arsenious
acid, sulphur, and charcoal, in the proper proportions; it has not the
brilliant colour of the native mineral, and is much more poisonous.
On a large scale, the manufacture is carried on in the following
way:--The ingredients are mixed together in such proportions that the
mixture shall contain 15 per cent. of arsenic, and from 26 to 28 per
cent. of sulphur, in order to make allowance for the volatilisation
of a portion of the latter substance. The mixture is then placed in a
series of earthenware retorts, which are charged every twelve hours
with about 60 lb.; this quantity should fill them three parts full.
These are then gradually heated to redness for from eight to twelve
hours, during which time the realgar distils off, and is collected
in earthen receivers, similar to the retorts, but perforated with
small holes to permit the escape of these gases. After the operation,
the receivers are emptied, and the crude product is remelted. This
is performed in cast-iron pots, the contents being well agitated,
and the slag carefully removed. The requisite amount of sulphur or
arsenic is then added, according to the colour of the mixture, or else
a proper quantity of realgar containing an excess of the required
constituent, and the mass is again stirred. When, on cooling, it
exhibits the correct colour and compactness, it is run off into conical
moulds of sheet iron, cooled and broken up; it is sometimes refined
by re-sublimation. The chief use of realgar is as a pigment; and in
pyrotechny in the preparation of white fires.
As a pigment it possesses the same features and faults as its close
ally orpiment.
SIENNAS.--Another name for ochres, described on p. 272.