Read Ebook: The manufacture of mineral and lake pigments by Bersch Josef Wright A C Arthur Columbine Translator

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Carbonate of lime and common salt occur in small quantities in every well water. The colour maker must do the best he can with such a water; its use will not particularly harm the shade of the colours prepared with it if the amount of the impurities is not very large. Water containing much iron is practically useless; the oxide of iron would injure the colours so much that it would not be possible to obtain brilliant shades. Water from wells in the neighbourhood of deposits of turf or cemeteries often contains considerable quantities of organic substances which act injuriously on the shade of pigments; such water should not be used in colour making.

The impurities in a water are more or less harmful according to the purpose for which it is to be used. Sulphate of lime is generally more injurious than carbonate of lime, since the precipitates which the latter causes in solutions of the salts of certain metals can be prevented by the addition of acids. This is not the case with sulphate of lime; when lead or barium salts are dissolved in water containing this substance, a precipitate of lead or barium sulphate is obtained, which is insoluble.

In dealing with the salts of costly metals, such as mercury or silver, it is better to dissolve them in distilled water, or, at least, very pure rain water. The rain water which runs from zinc or well tiled roofs is generally very pure; for practical purposes it may be regarded as free from carbonate and sulphate of lime, sulphuretted hydrogen and common salt. The colour maker should take care to obtain as much of this pure water as possible by erecting large rain-water tanks.

The less impurity a water contains the more useful it is for our purpose. After rain water soft river water is the best, and after this the softer well waters. All mineral waters distinguished by a high content of salts or gases are quite useless for colour making; for this reason sea water is disqualified.

An accurate analysis of a water is much too complicated for the manufacturer; it is sufficient for him to convince himself of the absence of certain substances. Water which, some time after the addition of a little tannic acid solution, acquires a clear green or a bluish to black shade contains much iron, and is useless. Water which coagulates a large quantity of a solution of soap in alcohol is very rich in carbonate or sulphate of lime. In order to decide approximately in what relative proportion these salts are present, a solution of barium chloride is added to the water so long as a precipitate forms. If this disappears completely on the addition of nitric acid, the water contains only carbonate of lime; if it only partly dissolves, sulphate of lime is also present. The presence of chlorine is shown by a considerable turbidity on acidifying the water with nitric acid, boiling and adding silver nitrate. If the precipitate obtained on the addition of a lead salt is not pure white, but discoloured, the water contains sulphuretted hydrogen, which has formed black lead sulphide. In order to test the water for organic substances, about a litre is evaporated to dryness in a porcelain dish, and the residue heated to redness; if it turns brown and black, and possibly gives off a smell of burnt feathers, the water contains much organic matter.

Pure water is coloured permanently red by a solution of potassium permanganate; but if it contains organic matter, the solution is decolourised after some time and a brown precipitate is deposited at the bottom. From the amount of this precipitate an idea of the quantity of organic matter present may be obtained.

It is only necessary to be very scrupulous concerning the quality of the water when it is to be used for the solution of salts or the extraction of dye-woods. For washing precipitates, which requires a large volume of water, there can generally be used, without detriment, water containing much lime, but it must be free from iron and sulphuretted hydrogen. The latter is particularly harmful to most of the lead colours, which would lose in beauty by washing with water containing this substance.

It is hardly necessary to say that the water used in colour making must be quite clear. Muddy river water must in every case be completely freed from the solid particles contained in it, either by settling or by filtering. Filters filled with well washed sand give good results for this purpose.

Formerly chlorine was exclusively made in lead apparatus, because this metal is one of the least readily attacked. When such an apparatus is used for the first time a layer of lead chloride is formed, which, like a varnish, protects the metal beneath from further attack. Fig. 1 represents an apparatus formerly employed for the preparation of chlorine in chemical works. In the upper part of the pear-shaped vessel, K, there are four openings, two of which, D and C, are provided with water lutes. This means that the opening is surrounded by a moat containing water, into which the rim of the cover dips, thus making a joint. Through the middle opening goes the axle of the stirring apparatus, R; in the fourth is a lead safety funnel, J. Solid materials are introduced through D, liquid through J; the tube C carries away the chlorine formed; the tube A, furnished with a stop cock, can draw off the fluid contents of the apparatus.

Since lead melts at low temperature, the apparatus cannot be heated over the fire without danger, therefore it is surrounded by an iron jacket, W, which is filled with water, or else the apparatus is heated by steam introduced into W. Larger quantities of chlorine are more conveniently prepared in an apparatus, of similar structure, made of stone or earthenware, which have the advantage over lead that they are not at all attacked by chlorine.

Pyrolusite and hydrochloric acid are now generally used for the preparation of chlorine, because the solution of manganese chloride, left at the end of the operation, is valuable.

If all the chlorine made in one operation is not at once required for the manufacture of a colour, it can be utilised by sending it into a box filled with slaked lime, which is converted into chloride of lime or bleaching powder. The liquid run away from the apparatus at the conclusion of the operation contains manganese and sodium sulphates, or manganese chloride as the case may be, and can be used for the preparation of manganese pigments.

For the sake of uniformity, it is urgently to be desired that all manufacturers who use the hydrometer to estimate the content of a liquid in ammonia, potash, soda, hydrochloric, sulphuric, nitric acids, etc., should employ simple specific gravities. This is desirable, because the percentage strength of a solution, corresponding to the specific gravity, can be at once accurately found from tables. On these grounds, in the present work, we have restricted ourselves to tables showing simply the specific gravities of solutions and the corresponding composition.

ACIDS.

In colour making many acids are used for the solution of metals, the production of precipitates, for oxidations and so forth. Commercial acids, especially inorganic acids, generally contain not inconsiderable quantities of impurities which are injurious in the manufacture of many colours.

Ordinary hydrochloric acid is a solution of hydrochloric acid gas in water. The strongest acid contains 42?85 per cent. of the gas, and has the specific gravity 1?21. The following table gives the strengths of acids of various specific gravities:--

When sulphuretted hydrogen is required the basket is lowered by the screw, S, until it dips in the liquid; according as the basket dips more or less into the liquid a fast or slow current of the gas is obtained. When the gas is no longer required the basket is raised out of the liquid, and the evolution of gas at once ceases. The funnel, T, serves for the introduction of the liquid, the tap, H, for drawing off the iron sulphate solution, which can be used with advantage for the preparation of fine iron colours. The apparatus should not be opened so long as sulphide of iron remains in the basket.

Since the action of nitric acid chiefly depends on its oxidising properties, which are possessed by both kinds, it generally does not matter which is used. The usual impurities are chlorine and sulphuric acid; the presence of the first is shown by silver nitrate solution, of the latter by barium chloride, in each case added after diluting. When the acid is used for oxidations these impurities do not interfere, but nitric acid containing chlorine cannot be used to dissolve silver, because the chlorine would form insoluble silver chloride.

The strength of nitric acid is gauged by its specific gravity as given in the table.

ORGANIC ACIDS.

The organic acids which are important in colour making are acetic, oxalic and tartaric acids.

The strength of a solution of acetic acid cannot be found by a simple estimation of specific gravity, since the density does not increase with the percentage of acetic acid. If an accurate estimation of the strength of acetic acid is required, it must be obtained by neutralising the acid with an alkali by a process of volumetric analysis.

For practical purposes, where it is generally known whether a very strong or a more dilute acetic acid is under consideration, the following table, showing the connection between specific gravity and percentage strength, is sufficient.

METALLIC COMPOUNDS.

ALKALIS.

The compounds of the alkali metals, potassium and sodium, play a considerable part in colour making. Formerly the potassium compounds were in general use, but the sodium compounds are at present obtainable at a much lower price, and in most cases they can be used equally well. Thus, in colour making, sodium compounds are chiefly employed. The cyanogen compounds are an exception; their potassium compounds are used exclusively.

Pure potash forms crumbling lumps with a slight yellow or bluish grey tinge, rapidly absorbing moisture from the air, and in time completely liquefying. The yellowish tinge is caused by oxide of iron, the bluish by manganese compounds. The so-called calcined potash has been strongly heated, and thus all organic substances contained in it have been destroyed.

Potashes are in no way pure potassium carbonate; they contain a mixture of all those salts which are found in plants--potassium sulphate and chloride, small quantities of silicic acid, etc. These impurities are rarely harmful, still it is generally necessary to know the percentage of pure potassium carbonate contained in potashes.

Although at present in commerce the strength of potashes is frequently guaranteed, it is still desirable to estimate the strength. It is sufficient for practice to allow a small quantity, say 100 grammes, to stand with an equal quantity of very cold water for some hours, then to filter and pour a similar quantity of water over the residue on the filter. The weight of undissolved substance subtracted from 100 gives with sufficient accuracy the weight of pure potassium carbonate contained in 100 parts of potashes. This method is founded on the fact that potassium carbonate dissolves readily even in cold water, but the other salts with difficulty. This procedure can also be used to obtain pure potassium carbonate from crude potashes; it is only necessary to filter and evaporate to dryness the solution obtained by pouring very cold water on crude potashes.

With this object 11 parts of potash, contained in a tub with an opening at the bottom, are mixed with 100 parts of cold water. Two hours later, the clear solution is run off into a clean iron pan, in which it is heated to boiling. To the boiling solution is added milk of lime prepared from water and 3?5 parts of quicklime. After the liquid has boiled a few minutes, a small portion is filtered and hydrochloric acid added to the clear filtrate; if no effervescence occurs, then all the potassium carbonate is converted into caustic potash. Should effervescence occur, milk of lime is added until a new portion no longer effervesces on the addition of hydrochloric acid. Then the pan is covered with a well-fitting lid, and the cooled liquid, if not required for immediate use, preserved in well-corked glass bottles.

The strength of a caustic potash solution can be found by means of a hydrometer. The following table shows the relation between the specific gravity of a solution and the percentage of caustic potash it contains:--

Potassium bichromate is unaltered in air; it dissolves easily in water, and is of great importance in the preparation of many colours, in particular chromium oxide and the lead pigments. The commercial salt generally contains potassium sulphate, with which at times it is intentionally adulterated. The adulteration is detected by dissolving in water, adding half the volume of pure hydrochloric acid, and cautiously and carefully dropping in spirits of wine. A rapid action takes place, which is only assisted by warming when necessary. The red liquid changes to emerald green. If barium chloride be now added, a white precipitate is obtained in the presence of potassium sulphate.

The behaviour of yellow prussiate towards iron salts is noteworthy. With ferrous salts, for example green vitriol , it gives a white precipitate which gradually turns blue in the air; with ferric salts, for example ferric chloride , it at once gives a blue precipitate.

Pure potassium ferricyanide forms beautiful dark red crystals, which readily dissolve in water. The solution gives a blue precipitate with ferrous salts, but only a brown colouration and no precipitate with ferric salts. Both yellow and red prussiate are used in the preparation of several much used colours, for Prussian and Chinese blues, and several others. All cyanogen compounds, with the exception of yellow prussiate, are extremely poisonous. The following table gives the solubility of potassium ferricyanide at different temperatures:--

In the retail trade a form of soda is found which is adulterated with very large quantities of Glauber's salt. This is recognised by the different form of the crystals. Manufacturers sell soda stating its strength. The colour maker should only buy with this guarantee.

Caustic soda and caustic potash have similar properties; they have a corrosive action on the skin, readily unite with carbonic acid from the air, and separate the heavy metals from their solutions in the form of hydrated oxides.

The strength of caustic soda solutions is given in the following table:--

Besides soda and caustic soda, few soda salts are used in colour making. However, sodium nitrate , NaNO?, is frequently used instead of ordinary saltpetre, from which it differs in being deliquescent.

SALTS OF THE ALKALINE EARTH METALS.

The metals which are known as the alkali earth metals have much similarity with the alkali metals in their compounds, with the difference that their alkalinity is much less, and that their salts are much less soluble in water. For colour making the compounds of three of these metals--calcium, barium and magnesium--are used.

CALCIUM COMPOUNDS.

The most important calcium compounds are lime and carbonate and phosphate of lime. Carbonate of lime is used as a pigment, and will be dealt with in detail among the mineral colours; it will be but briefly described here.

When quicklime and water are brought together they unite very energetically and form calcium hydroxide or slaked lime. According to the quantity of water used for slaking, either dry slaked lime, lime paste, or milk of lime is produced, all of which find a use in colour making.

If so much water is added to the lime that a homogeneous wet mass is formed which can be readily moved with a shovel, one has then lime paste, which can be conveniently kept in pits as the masons do; it may be stored in this way for many months without appreciable alteration, still it is better to keep it covered. To prepare milk of lime, so much water is used in slaking that a milky liquid is formed, or the lime paste is mixed up in the proper quantity of water. Slaked lime dissolves in 700 to 800 parts of water; on standing, the undissolved slaked lime settles to the bottom of the milk of lime: thus it is better to prepare milk of lime immediately before use, and to stir it well to prevent the settling of the solid particles.

Slaked lime in one of its forms is often used instead of the more costly caustic soda in order to precipitate metallic oxides from their salts.

At times one finds a too strongly burnt lime, so-called "dead-burnt" lime, which is very slowly slaked by water. Such quicklime is slaked by allowing it to lie in water for days, or by means of hot water, which accomplishes the slaking more quickly.

To prepare barium chloride from barytes, the latter is very finely ground and levigated, intimately mixed with coal, and the mixture subjected to a very high temperature, when barium sulphide is formed, which is dissolved by washing out the mass with water and converted into barium chloride by adding hydrochloric acid, sulphuretted hydrogen being evolved.

The best method is to mix 4 parts of barytes with 1 part of bituminous coal and so much coal-tar that a plastic mass is formed, which is well kneaded and made into small cylinders 3 centimetres in diameter and 10 centimetres long. These cylinders are placed in layers in a cylindrical furnace with a good draught, which contains at the bottom a layer of coal 15 to 20 centimetres thick, then a layer of the cylinders, then again coal, and so on until the furnace is full. The lowest layer of coal is lighted, and the whole burnt at a bright red heat, when the barium sulphate is changed into sulphide. Hydrochloric acid is poured over the residue and the insoluble part, consisting chiefly of unaltered barytes, is used for the next operation.

Witherite can be converted into barium chloride in a very simple manner. Hydrochloric acid is added, in which it dissolves with the evolution of carbonic acid. The solution is allowed to stand twenty-four hours with excess of witherite; the whole of the dissolved iron is thus precipitated. The solution is then filtered, evaporated down and left, when pure barium chloride crystallises out.

Barium chloride and all soluble barium salts should only be dissolved in pure water . Water which contains carbonates or sulphates always gives a turbid solution by precipitating barium carbonate or sulphate.

ALUMINIUM COMPOUNDS.

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