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account, for the
purpose of observing the exact shade of colorations produced, as it is impossible to convey, with sufficient exactitude, an idea of the actual tint of any coloration or precipitate which may be produced.
Cochineal.-Alumina sulphate gives a bright crimson solution; on the addition of sodium or potassium carbonate a crimson precipitate falls down. Stanno’s chloride gives a dull purplish coloured precipitate; copper sulphate a purple precipitate; ferric chloride a brown precipitate; caustic soda a bluish crimson solution. Strong sulphuric acid changes the colour of the decoction to an amber colour, and, on adding water, a pale yellow solution is obtained.
Brazil Wood.--Alumina sulphate gives a red precipitate. Stannous chloride throws down a pale crimson precipitate; lead acetate a violet-rose precipitate; ferric chloride a chocolatebrown precipitate; copper sulphate a brick-red precipitate; and caustic soda forms a crimson solution. Strong sulphuric acid changes the colour of the decoction to a red-brown, and on diluting this with water a pale amber solution is obtained.
Persian Berry.--Alumina sulphate has no action. On further adding ammonia or caustic soda, a brownish-olive precipitate falls down, while alkaline carbonates throw down a brighter yellow precipitate. Stannous chloride throws down an olive-yellow precipitate, lead acetate a yellow-brown precipitate, and ferric chloride a dark olive-green precipitate. Nitric acid forms a red solution. Strong sulphuric acid forms a yellowbrown solution, and on adding water a brown precipitate falls down.
Quercitron.—Alumina sulphate throws down an ochreyellow precipitate, stannous chloride an orange-yellow precipitate, lead acetate a dark yellow-brown precipitate, copper sulphate a brownish-olive precipitate, and ferric chloride a dark olive-green precipitate. Caustic soda forms a dark yellowbrown solution, from which acids throw down a yellow-brown precipitate. Strong sulphuric acid forms a brownish-yellow solution, from which, on adding water, a dark brown precipitate falls down.
Fustic.—Alumina sulphate, stannous chloride and lead acetate throw down orange-yellow precipitates; the first is bright, the last rather duller than those with the other two salts. Copper sulphate throws down a dull yellow precipitate, and ferric chloride a dark olive-brown precipitate. Strong sulphuric acid forms a brown-yellow solution, from which, on diluting with water, a brown precipitate falls down.
Annatto. — Alumina sulphate throws down a brownish precipitate, stannous chloride an orange-red precipitate, ferric chloride a red-brown precipitate, while strong sulphuric acid forms a dirty yellow-brown solution.
Turmeric. — Alumina sulphate gives a yellow precipitate, ferric chloride a brownish-yellow precipitate, and stannous chloride a yellow precipitate. Caustic soda forms an amber solution, and strong sulphuric acid a pale yellow solution.
Logwood. — Alumina sulphate changes the colour of the decoction to a crimson-red; ferric chloride throws down a bluishblack precipitate; potassium bichromate causes the formation of a black gelatinous mass after standing a short time; copper sulphate throws down a violet-black precipitate, and antimony chloride a violet precipitate; caustic soda changes the colour of the decoction to a violet, while hydrochloric acid turns it red.
Of late years the use of the coal-tar colours for the preparation of lake-pigments has grown very extensively, and now such lakepigments are largely used, especially for paper-staining, manufacture of paper-hangings, printing, lithography, &c. Ir many cases they have displaced some of the older colours, especially such as emerald-green, which are rather poisonous, and vermilion, which is expensive.
The aniline or coal-tar lakes are not true lakes, although they are prepared by precipitating a coal-tar colour with a suitable precipitating agent; they always contain another substance in addition to the lake proper, which is added for the double purpose of increasing the covering power or body of the lake as well as, in many cases, of modifying or toning down the shade ; at the same time it reduces the cost of the lake and brings it within the purchasing power of painters for use in common decorative work. Three things are required in the preparation of coal-tar colour lake-pigments-1st, the colour; 2nd, the precipitating agent ; 3rd, the base or, as it is sometimes called, the carrier.
1st. THE COLOURING MATTER.—The dyestuffs prepared from coal-tar products may be divided into 16 groups according to their chemical composition and relations ; but for the present purpose they may be divided into three groups differing from one another in the character of the methods used in preparing lakes from them. Although something like 300 different coal-tar colours are to be found in the market, yet,
for various reasons which will be pointed out later on, inany of these cannot be used in the preparation of lake-pigments.
1. Basic Colours. These dyestuffs are salts of certain complex organic bases prepared, in the first instance, from aniline, toluidine, &c. They include such dyestuffs as magenta, brilliant green, auramine, aniline-blue, methyl-violet, Bismarck brown, benzoflavine, rhodamine, &c. These colouring matters form, with certain weak organic acids, such as tannic acid, picric acid, &c., coloured precipitates, and it is on this property that their use in the preparation of lake-pigments is based.
(a) Colouring matters precipitated by tannic acid.--All the basic coal-tar dyestuffs are precipitated by tannic acid, but the following are of special value as lake-pigment makers :magenta, safranine, auramine, aniline-blue, brilliant green, methyl-green, new green, Nile blue, rhodamine, phosphine, methyl-violet, Paris violet, Hofmann violet, Bismarck brown, chrysoidine, quinoline-yellow, benzoflavine, pyronine G. The combination between the tannic acid and the colouring matter is a chemical one and takes place in definite proportions; but, as these dyestuffs are rarely sold pre, only approximate quantities can be given in any recipes for their transformation into lake-pigments. There is one point to be noticed in connection with the precipitation of these basic colours with tannic acid, which is that the precipitated lake has the property of carrying down with it some of the colouring matter in a free form.
It is best to use tartar emetic along with the tannic acid in making the lakes, so as to form a double combination of the tannic acid with the colouring matter on the one hand and with the antimony of the tartar emetic on the other, which has a material intluence on the complete precipitation of the dyestuff and on its fastness to light and air.
One fault of the tannic lakes is that when used as oil colours they do not dry well or completely; the cause of this lies with the tannic acid, which exerts a retarding action on the drying of linseed oil, and, if present in large proportions, will almost completely prevent the oil from drying. Hence tannic acid lakes cannot be used with satisfactory results as oil colours, but for all other vehicles they are quite satisfactory. The precipitation of the lakes is more complete if sodium acetate be added; this acts by neutralising the hydrochloric acid which is liberated from the colour by the action of the tannic acid and which exerts a solvent action on the colour lake, the free acetic acid formed from the sodium acetate having no such action. The tartar emetic has a similar action (see below).
(6) Colouring matters precipitated by picric acid.-This subgroup includes auramine, night-blue, methyl-blue, methyl-green, brilliant green, safranine, and a few others. Picric acid being itself a yellow dyestuff, it is ohvious that the shade of the resulting colour lake must be modified thereby; thus, the greens which are naturally of a blue shade and are so precipitated by tannic acid, are thrown down by picric acid of a yellow-green tone, while the reds take a yellower tone, and the blues become greenish. The picric acid lakes may be used as water-colours, but they cannot well be used as oil-colours, for the oil does not dry properly; they thus resemble the tannic acid lakes, but, if anything, the defect is stronger.
2. Acid Colouring Matters.—The term “ acid ” as here used has special reference to the fact that the colours comprised in this group are dyed on to wool or silk from baths containing a small quantity of free acid and not to the fact that the colours themselves are of an acid character, although many of them are.
Some of them are derived from the basic colours by a process of sulphonation, by which means they are rendered capable of dyeing wool or silk from acid baths, which, previously, they were not capable of doing. The great majority of the acid colours are what are called azo-colours, that is, bodies characterised by containing a group of two nitrogen atoms, -N=N-, combined with some organic radicle of a basic or phenolic character; in some colouring matters there is more than one such group. Other dyestuffs belonging to this group are nitro-derivatives, usually of phenols.
These colouring matters are precipitated from their solutions by acids and metallic salts (such as lead acetate, barium chloride, alumina sulphate, alum, zinc sulphate, magnesium sulphate, &c.)
Naturally, they vary much in the degree or ease with which they are precipitated. Some are thrown down by all the agents above noted; others, again, are only precipitated by one or two of the salts. Others, again, require the solutions to be slightly alkaline before they are precipitated. The following lists, showing how these acid dyes are precipitated, will illustrate this point.
(a) Colours precipitated by lead acetate.—Lead acetate will precipitate a very large number of the coal-tar colours belonging to the acid
group; in some cases the precipitated lake is of sufficient good colour to be of service in making lake pigments, in other cases it is not so. To enter into full details with regard to all the colours would take up more room than can well be spared in this book, but the following list and the notes appended thereto
will be found of use :Scarlet BB (lake of a poor shade, not serviceable), mandarin G, orange G, orange IV (the lake is of a brownish shade, and not useful), fast scarlet 3R (very good lake), acid mauve B, yellow N (fine shade of colour), croceineorange, fast red T, alkaline blue, citronine A, tropæoline 00 (lake of a very pale colour, not useful), citronine o, methylyellow, new yellow, scarlet G, scarlet R, ponceau 2G, fast violet (the lake is but of a poor shade), orchil brown B, fast red A, azo-yellow, double brilliant scarlet 2R, croceine 3B, quinolineyellow, crystal scarlet 6R, phosphine, scarlet 00, scarlet GT, vermilline scarlet KK, Bordeaux S (very good shade of colour lake), all the eosine colours, benzoazurine R, benzoazurine 3G (gives rather a poor shade of lake), diamine-blue B, Titan pink (gives a dark crimson colour lake), Congo 4R,* chrysamine (gives rather a dark brownish yellow lake), and chrysophenine (a fine precipitate).
(b) Colours precipitated by barium chloride.—Scarlet BB, ponceau, orange G (gives a fine bright colour-lake, very useful), orange IV, fast scarlet 3R (a fine bright colour-lake), acid mauve B, claret-red B, yellow N, croceine-orange, croceine-scarlet, fast red T, alkali-blue, Victoria blue B, citronine O, methyl-yellow, new yellow, scarlet G, ponceau 2R, scarlet R, resorcin-yellow, fast violet, orchil-brown B, fast red A, azo-yellow, double brilliant scarlet 2R, Indian yellow, crocein 3B, quinoline-yellow, crystal scarlet 6R (not so good a lake as with lead acetate), naphthol-yellow S, phosphine, scarlet 2R, scarlet GT, vermillinescarlet KK, scarlet 2RJ (gives a very fine lake), scarlet 3R, benzoazurine R, benzoazurine G, benzoazurine 3G, diamine-blue B, Congo 4R, chrysophenine (gives a good lake), chrysamine (precipitates as a brownish lake).
(c) Colours precipitated by alumina sulphate.—Scarlet BB (the lake has a brownish tone), mandarin G, Orange IV, fast scarlet 3R, croceine-orange, alkali-blue, Victoria blue B, citronine 0, scarlet G, fast violet (but a poor shade of colour-lake), orchil-brown B, fast red A (but a poor shade of lake), azo-yellow, Indian yellow, scarlet GT, benzoazurine R, benzoazurine G (a good precipitate), benzoazurine 3G, Titan pink, Congo 4R, chry
* The red colouring matters, which, like Congo-red and benzopurpurine, are derived from benzidine, tolidine and stilbene, are precipitated by lead acetate and other precipitants, but, in consequence of their being sensitive to acids in a great degree, the resulting colour-lake is liable to be much affected in colour by acid precipitants. Thus, Congo-red will give a blue lake instead of a red, and the others are more or less similarly affected ; therefore they are practically useless for making lake-pigments.