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No. 6 had a most peculiar fetid odour and contained a quantity of unburnt oil ; from its appearance and composition it is evident that its only claim to the name of animal black was that it was made by the lamp-black process from bone oil.
No. 7 was a true bone-black. With these two exceptions the above analyses show that this black is made from a variety of materials.
Animal black can be used for all purposes to which black pigments are applied.
FRANKFORT OR DROP-BLACK. This black is named “Frankfort black,” because it was first prepared in the old German town; “drop-black," on account of the shape into which it is made up for sale.
Drop-black is made from a great variety of materials of an organic character, such as vine twigs, refuse of wine-making, peach stones, hop bine, bone shavings, ivory cuttings, &c. These are calcined in a closed vessel until they are thoroughly charred. The black so obtained is then ground up as fine as possible with a little water; then the mass is lixiviated to free it from soluble matters, and dried. Then it is mixed with a little glue water and made up into pear-shaped drops for sale, for which purpose they are ready when dry.
Drop-black is a black of fine texture, varying in hue from a bluish-black to a somewhat reddish-black, which is due to the different materials of which it is made ; vegetable matters yield a black of a bluish hue, while animal matters give a black of a greyish hue.
Drop-black owes its colour to carbon, the amount of which varies in different samples; it also contains some mineral matter which will vary in amount and kind according to the character of the material from which the black was prepared.
The following is an analysis by the author of a sample of drop-black:
2:333 per cent.
The mineral matter contained phosphate of lime, which showed that bones had been used in making this sample.
Drop-black is used for all purposes for which black pigments are required.
German black is a synonym for drop-black,
MISCELLANEOUS BLACKS. Besides the black pigments described above, several other substances may be briefly noticed, which have been proposed for use as pigments, or have been so used on a small scale.
CANDLE BLACK is a kind of lamp-black made on a small and extremely limited scale from the flame of a candle by holding a cold plate over it.
PRUSSIAN BLACK is made by calcining Prussian blue in a closed vessel until the residue has a black colour; this black is a mixture of carbon and oxide of iron, and usually has a brownish tinge; it possesses no advantage over lamp-black and is more expensive.
BLACK LAKE.—When a solution of sulphate of copper and bichromate of potash is added to a decoction of logwood a black precipitate falls down; this, after being washed and dried, forms black lake. Too much bichromate of potash should be avoided, as it has a tendency to turn the lake grey; the substitution of a little sulphate of iron for some of the bichromate improves the lake, while reducing the risk of adding too much of the chrome salt. Black lake is a pigment of a fine hue and texture; it is not permanent when used as a pigment, fading on exposure to light and air.
TÄNNIN-BLACKS are similar pigments to the last, and are made by adding solutions of either the sulphate or the socalled “nitrate of iron ” to solutions of tannin materials, such as sumach, divi-divi, myrobolans, &c. Their production from waste leather has been made the subject of a patent; the leather scraps are boiled in an alkaline solution, which dissolves out the tannin matter used in tanning the leather; to this solution a mixture of alum and sulphate of iron is added and the black precipitated. The tannin-blacks have a bluish hue; they have no great colouring power, and are not permanent when exposed to light and air. They have been very little used as pigments.
CHARCOAL-BLACKS.—These are made by grinding the charcoal obtained by charring soft woods. The grinding should be well done, and the black pigments should be washed with water to remove any soluble matters it may contain. These blacks are mostly sold as lamp-blacks, carbon-blacks, &c. their general properties they resemble the lamp-blacks, but are a little more granular in texture.
COAL-BLACKS have been made by grinding coal and shale, but it is doubtful whether any are now used. LEAD-BLACK is the sulphide of lead made, according to the
patent specification, by taking lead-fume (i.e., the powder which collects in the flues of lead furnaces) and boiling it for some time with a solution of sodium sulphide. The black soon forms. It is doubtful whether this black has been used on a practical scale. Its permanency is doubtful, as it is possible that it may be prone to oxidation to lead sulphate and, therefore, to decolorisation.
PRUSSIATE BLACK is the black residue (consisting mostly of carbon) which is obtained as a bye-product in the manufacture of yellow prussiate of potash. This is collected and well washed with water, when it is ready for use. It is largely used as a decolorising agent.
MANGANESE-BLACK is the oxide of the metal manganese found naturally and simply ground up very fine for use as a pigment. The great objection to its use as a pigment is its great drying properties. It is also expensive, and possesses no material advantage as a pigment over lamp-black.
Blacks have been proposed to be made from other materials by mixing ochres with peat and similar carbonaceous materials, and calcining the mass in closed vessels. Spanish black is a name given to a black made from cork shavings. The use of the black sulphide of iron was patented by Claus in 1882, but it has never come into practical use. Aniline-black has been proposed to be used as a pigment, but its great expense is against its practical use for this purpose. It is made by dissolving aniline hydrochlorate in water, and adding to it a solution of potassium bichromate acidified with sulphuric acid. The black precipitate, which rapidly forms, is collected and washed.
LAKES form a class of pigments of considerable use in painting. They were among the pigments used by the early Italian painters, from whom their use has descended to the present time. Pliny gives some account of then, and from this description the origin of the name “lake” can be gathered. The early Italian dyers for certain colours used what was known as lac,” which was either the product now known under this name, or an analogous body. This lac requires the aid of tin and alumina compounds before the colour can be developed and fixed on the fabric which has to be dyed ; during the process of dyeing some of the colouring matter of the lac combines with some of the tin and alumina to form an insoluble body, which forms a kind of coloured scum on the top of the dye-vat; this substance, known to the Italian dyers as “lacca,” was collected, dried, and sold to artists. In the same way other laccæ were obtained when other natural dyestuffs were used ; gradually methods of preparing these laccæ were discovered, by which they could be obtained direct from the dyestuffs themselves, without the necessity of troubling the dyer, and thus has arisen the preparation of the lakes, which name can be readily traced to the laccæ of the Italian dyers.
Lakes may be defined to be compounds of an organic colouring principle with a metallic body. The organic colouring principle may be obtained (as it was in the early times, and until very recent years) from natural colouring matters, such as lac, cochineal, Persian berries, fustic, Brazil wood, sapan wood, &c.; or it may be derived from the coal-tar colours, a source which has only lately come into prominence for lake-making, but which promises in the future to supplant the natural colouring matters for this purpose, as they very nearly have done for dyeing textile fabrics.
The colouring principle of most natural colouring matters is of an acid or phenolic character, and will combine with bases, such
as tin, alumina, iron, lead, antimony, &c., to form coloured bodies which are insoluble in water; as a rule, the affinity between the two bodies is so great that the lake is precipitated when a solution of a metallic salt is added to one of the colouring matter. Theoretically, a lake should be a compound of the colouring principle and the metallic base combined in equivalent proportions; but, practically, such a lake does not exist ; usually, the base largely predominates. This excess is sometimes accidental, but often purposely made, the object being to modify the shade of the lake, as is the case with Dutch pink, rose pink, and one or two others. Then, again, in some lakes there may be small traces of the colouring principle carried down mechanically with the lake during the process of making.
Lakes are usually made by preparing a decoction of the colouring matter, and then adding to this a solution of the base ; as a rule, the lake forms almost at once ; at oth times, the addition of a small quantity of a solution of carbonate of soda is sufficient to throw down the lake. By preparing alkaline solutions of the colouring matter the lake is thrown down at once on adding the solution of base ; this method is not always applicable, as the alkali sometimes affects the shade of the resulting lake, as in making alizarine-lakes.
The colouring matters, or rather their colouring principles, may be divided into two groups.
One includes such substances as fustic, Persian berries, and cochineal, which may be called substantive, or, as some writers name them, “mono-genetic,” colours, as the colour does not depend upon the mordant or base used; thus Persian berries will give a yellow lake with either alumina, tin, or lead, although there are some minor differences in the tint or shade of the yellow so produced. The other group may be called adjective, or “poly-genetic,” colours. It comprises substances like alizarine, fresh logwood, &c., in which the colour is only developed when the colouring principle is combined with a base, and differs with the base used; thus alizarine, when combined with alumina gives a red, while with iron it gives a deep dull violet; again, logwood with antimony gives a violet, with iron a blue-black, and with chrome a deep blue.
All lakes should be quite insoluble in any vehicle, such as water, oil, turpentine, or spirit, used to make them into a paint; on the other hand, a true lake is always more or less transparent when used as a pigment, and lakes are, therefore, mostly used as covering or glazing colours to modify the tint of an under coat of paint, and to obtain effects which are not obtainable with opaque pigments. Some lakes are rendered nearly opaque by