ALUMINA. Aluminium hydroxide is the most important carrier, and is nearly always present in the best lakes. It is used as such, or produced at the same time as the colouring matter is precipitated. In a nearly pure state, or at the most mixed with a little artificial barytes—which is generally precipitated at the same time—it serves as carrier for most printing inks, and then forms the principal part of the base.
On the other baud, good lakes for wall and fancy papers contain more artificial barytes than alumina, and therefore both bodies are precipitated at the same time as the colouring matter. An addition of artificial barytes must generally be given, or else the quantity of sodium sulphate formed when alumina is produced must be increased by a certain addition of crystallised or calcined Glauber salt.
The nature of, and suitable method of producing, the alumina used in lake-making are therefore of the greatest importance; and although good commercial alumina is quite suitable, its preparation in the works is often s matter of necessity, the former article being seldom in such a fine state of division as that obtained by direct precipitation. Moreover, in many case: the whole operation is based on the simultaneous precipitation of the colouring matter and alumina with or without artificial barytes (or other bodies).
Aluminium hydroxide, Al,(OH)„, generally called "alumina," is always produced when soluble aluminium salts are treated with caustic alkalies of alkali carbonates, the product, however, varying according to the salt used. Alumina produced by the aid of caustic alkali—caustic soda potash— is always a slimy or gelatinous mass, whether prepared by the cold or by the hot method. When dry it is very hard and horny. I dissolves in an excess of the precipitating agent, and is therefore of no use for lake-making.
For many purposes (referred to later on) a solution of alumina in caustic soda, the so-called sodium ahtminale, is used, and is sold in the solid form.
Alumina precipitated by alkali carbonates, however, has quite different properties ; it is insoluble in excess of the precipitant, and is less mutinous, even from dilute cold solutions, than that made from caustic alkalies. When produced at a high temperature, and from not too concentrated solutions, it is opaque, pure white, and in the dried state is soft and powdery.
The raw materials for manufacturing alumina are aluminium sulphate, alum, and calcined or crystallised soda. The chemical changes occurring in the reaction are briefly as follows : The sulphuric acid of the aluminium salt combines with the sodium of the carbonate, and insoluble alumina is preci pitated on addition of water, whilst carbonic acid escapes with effervescence* and Glauber salt remains in solution. The formation of the alumina pro ceeds gradually and by stages, basic aluminium sulphate being first formed, and then progressively deprived of its sulphuric acid by the further addition of sodium carbonate, until when boiled with an excess of the latter the loss of acid is complete.
Practically the precipitation is effected at or very near the boiling-point. A boiling solution of alum or aluminium sulphate is treated with a hot solution of soda in a very large tank, with constant stirring, until no further escape of carbonic acid can be observed on continued boiling and addition of soda solution. The liquor in the tank will then have a slightly alkaline reaction.
The product must be washed to recover the perfectly pure alumina, but as the sodium sulphate is very obstinately retained by the alumina precipitate, unless very large tanks can be used, a frequent change of water is necessary, until barium chloride produces no precipitate, or only a slight whitish mist, with the washings. Boiling and rapid stirring will greatly accelerate the troublesome washing process, especially when hot or boiling water is used.
The temperature and concentration of the solutions greatly influence the nature of the resulting aluminium hydroxide, as does also the presence, after the precipitation, of an excess of one of the two applied salts. If very hot or strong solutions be used for the precipitation the alumina is thrown down in a lumpy or granular state, owing to the too violent liberation of carbonic acid, whilst with very dilute and cold solutions it is obtained in a gelatinous, transparent state, of little covering power when dried. In this state washing is a very difficult process. The basic aluminium sulphate first formed settles very slowly, and therefore when very dilute and cold saline solutions are used a large quantity of soda must be added before the carbonic acid begins to come off and aluminium hydroxide is formed. This is a very important point in the manufacture of many lakes from basic coal-tar colours.
Except when the colouring materials are very sensitive to alkalies, an ex cess of soda in the precipitating process is usually no disadvantage unless for reasons of economy. Au excess of aluminium sulphate, however, causes the formation of crystalline double compounds of the hydroxide with the excess of sulphate, and renders the precipitate granular, rough, and unsuitable, especially for lakes in paste form. In view of these circumstances, it is desirable to adjust the proportions of the saline matters so that after the precipitation neither aluminium sulphate nor soda is present in excess.
To obtain a pure white alumina, free from other combinations, it is of course important that pure raw materials should be used—viz., the best commercial qualities : perfectly dry powdered, pure white aluminium sulphate, with the largest possible content of aluminium oxide, (the maximum being about 16 per cent.). Poorer brands of sulphate, especially those sold in lumps contain only 11 to 15 per cent. of oxide, owing to the higher content of moisture. Though the presence of iron in the aluminium sulphate is not so prejudicial fur most lake pigments as is generally believed, only brands with a minimum of iron should be used.
Aluminium sulphate perfectly free from iron is seldom obtainable at a reasonable price, but that containing per cent. of iron is easily pro curable, and is suitable even for manufacturing the alizarine and azarine lakes, which are very sensitive to iron salts. For making alumina absolutely free from iron, alum is preferable, the commercial salt being practically free from iron, though, of course, it is dearer than aluminium sulphate. Alum is also more suitable for making finer azo colour lakes for printing pur poses, for which a higher price can be obtained, the precipitates obtained from alum and barium chloride generally giving softer and brighter tints and lakes of better covering power and intensity than the corresponding aluminium sulphate lakes.
Ammonia soda is the most suitable commercial salt, though crystal soda is used in some cases. Of course, the purest kinds must be chosen, and the crystals must be quite free from Glauber salt.
It is only in very few cases that alumina is used alone as a carrier for lake pigments ; more frequently it is precipitated in well defined quantities directly upon other bodies, like artijicial barytes, barytes, lead sulphate, china clay, red lead, &c. The process is the same in all cases. Me carrier, mixed with water, is passed through a sieve into a precipitating tank, the hot solution of aluminium sulphate is run in, and finally the solution of soda is slowly added, with continual stirring. By this simple method a most intimate mixture of the hydroxide and the other bodies forming the carrier is obtained, the dissolved aluminium sulphate penetrating, so to speak, the individual particles of these bodies, and these are covered by the deposited alumina the more completely in proportion to the degree of dilution maintained. If lakes made from bodies so mixed are to be used in the dry state and alumina is not predominant in the mixture, its physical character has usually little influence on that of the dry lake. In such case the hydroxide can be precipitated with strong or dilute, cold or hot saline solutions, unless the shade of the lake is likely to suffer. The carrier made from such mixtures is washed in the ordinary way, and, on account of the high specific gravity of the materials, is effected more rapidly than with alumina alone.
A great many carriers, however, are not washed before being coloured, the Glauber salt being converted into artificial barytes by an addition of barium chloride, and utilised as a further component of the carrier. This method is the more practical since it gives a better fixation of the acid coal tar colours, most of which are completely precipitated by barium chloride.
A special method, suitable for many acid coal-tar colours, in decomposing aluminium sulphate with milk of lime or levigated chalk. Besides aluminium hydroxide, this treatment produces a very light and voluminous form of gypsum, which is partly reconverted into artificial barytes when the colouring matter is afterwards precipitated with barium chloride. Lakes obtained in this way are said to be specially soft, and often specifically lighter than usual.
For certain lakes lead sulphate is prepared in the same way, lead acetate or nitrate being used as the precipitating agent instead of barium chloride. When the coal-tar colour—for instance, eosine, erythrosine, phloxine, rose Bengale—can be precipitated by these salts, it is fixed at the same time as the lead sulphate, in the same way as mentioned in the case of barytes. In some cases, and especially when these lead salts are not employed as lake formers, the lead sulphate is added direct to the alumina, for the sole purpose of increasing the covering power.
A more difficult process, on account of liability to alteration, is the pre cipitation with arsenic, phosphorus, stannic acid, or resin, combined with the precipitation of alumina, in which the corresponding alumina salts are formed in addition to aluminium hydroxide, and act as direct, slightly acid lake-formers.
Arsenious Acid, H,AsO,, obtained by dissolving arsenic trioxide, As,O, (ordinary arsenic), in water, is too easily decomposed to be suitable for lake making, and therefore the more stable sodium salt obtained by dissolving arsenic in a solution of soda is used. The concentration of the soda solution and the duration of boiling play a considerable part. As a rule the solution of arsenic gives the better results in proportion to its degree of dilution and the longer it is boiled ; but it is useless to boil longer than one hour, and thirty to forty minutes will generally he sufficient. The resulting solution of arsenic is, with few exceptions, added slowly and with continual stirring to a hot solution of aluminium sulphate, containing the colouring matter in solution. The reverse procedure is adopted with certain basic colouring matters, the solution of arsenic, cooled to about 50° C., eventually diluted with cold water, being precipitated by the hot solution of alumina, and the solution of the colouring matter added afterwards. In this case the latter must be fixed by other precipitating agents, but the lake is particularly soft and smooth, settles down quickly, and is easily washed.
In both methods, however, certain precautions must be taken. The correct proportion of arsenious acid and soda is very important, an excess of arsenic injuriously influencing the physical properties of the lake even a long time after use, besides being uneconomical, and under certain circum stances prevents the absolute fixation of certain of the basic coal-tar colours.
The necessary quantity of arsenic depends, therefore, on the capacity of the arsenious acid to form a lake with the colouring matter in question, and differs with each group of the basic colouring matters.
Of no less importance in the precipitation with arsenic is the alkaline or acid character of the solutions. If the precipitation is effected in a strongly acid solution, the precipitate is mostly very fine, settles down slowly, and cannot be easily washed, on account of its gelatinous and sludgy condition, whilst the finished lake very obstinately retains the adhering water, is difficult to press, dries badly, and darkens in colour on drying, thus losing its warmth and brilliance of tone. From alkaline solutions the lake comes down in a more voluminous and softer condition, but the free alkali either destroys most of the basic coal-tar colours in a very short time or prevents suitable fixation—sometimes in a very disagreeable manner. In the most favour able case alkaline precipitation is as unreliable as the acid method, anti therefore a neutral or even slightly acid character of the solution is pre ferred, being, moreover, not difficult to obtain by an accurate calculation of the quantity of the materials and by a careful supervision of the process.
The chief of the chemical auxiliaries added to the solutions for the pur pose of regulating the decomposition—viz., to produce and maintain neutrality, or even to reduce the acid character—is alkaline sodium acetate, the effect of which is probably explicable as follows : The greater affinity of the sulphuric acid for the alkali sets up a reaction between the aluminium sulphate and sodium acetate, an excess of aluminium acetate being pro duced, which has less influence on the precipitation.
Precipitations based on a combination of alumina with aluminium phos phate as a carrier, in which the latter acts at the same time as lake-former, are generally performed by adding a solution of acid sodium phosphate, treated with soda, to a solution of aluminium sulphate, already containing the colouring matter. The addition of soda to the solution of sodium phosphate must be done in certain definite proportions, in order to obtain a perfectly neutral precipitation. From an acid solution the precipitate would fall down in a gelatinous state, whereas with an excess of alkali a quantitatively com plete precipitation is impossible. Here again the concentration and tem perature of the solutions are of special importance. If the liquids are too dilute or too cold, the resulting lake has very little covering power, dries exceedingly slowly, on account of obstinately retained moisture, and is liable to form afterwards a vitreous and poorly coloured mass. Precipitated from over-concentrated solutions, the dried lake is softer, but the precipitation of the colouring matter is irregular, and the dry lake streaky and spotty on the surface of fracture. Hence the precipitation is preferably effected with 10 per cent. solutions and at boiling heat, the liquors being stirred continually.
Alumina-tin compounds are now seldom used as carriers for lakes, stannous chloride—" tin salt "—and stannic chloride being the only salts employed, though stannic acid and metastannic acid are sometimes taken into con sideration. As a rule lakes obtained by precipitation with tin compounds are very liable to variation, and the results of one and the same process are seldom uniform, especially with regard to intensity and shade. This uncer tainty may explain why tin compounds are now shunned as precipitating agents. On account of its powerful reducing action, stannous chloride is rarely used as a direct precipitating agent, the lakes obtained being rarely bright and full, but needing correction with other agents, such as tannin. As a raw material for stannates, it gives better results, however, especially with certain green and blue basic coal-tar colours.
The effect of stannic chloride, however, is based on its decomposition, in an alkaline solution, to tin hydroxide, which, at the moment of precipitation, throws down the dissolved colouring matter as a tin lake, if the solution is not too strongly alkaline in character to produce a lake. These precipita tions are of no practical value, and are now only used with certain vegetable colouring matters, especially Persian berries.
Tin combinations are generally introduced into the alumina carriers by mixing the salt with aluminium sulphate or soda, according to the nature of the colouring matter in question, the corresponding aluminium-tin compound being formed and precipitated at the same time as the alumina.
In certain cases nitro-, azo-, acid-, and resorcine colouring matters should be precipitated in presence of alumina by lead hydroxide, zinc, or the alkaline earth metals. In this process the solution of colouring matter is mixed with alumina, and the metal or hydroxide of the alkaline earth metal is added in a freshly precipitated, washed state, mixed with water. Many lakes obtained by this process are specially fast to water and alcohol.