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.
Calcium Oxide (Quicklime), CaO = 56.—When chalk and limestone, which consist of calcium carbonate, are heated, carbonic acid is evolved, and calcium oxide, commonly called quicklime, is left. For our purposes only very pure quicklime is to be used. Its ordinary impurities are iron oxide and magnesia: the former is found in lime made from red or brown limestone; the latter in lime made from dolomitic limestone. The presence of oxide of iron is recognised by the reddish tinge of the quicklime; if magnesia be present, a small quantity of the quicklime, when mixed with a very large quantity of water, leaves an insoluble residue which consists of magnesia.
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.
Calcium Hydroxide (Slaked Lime), Ca(OH)₂ = 74.—In order to prepare slaked lime, which contains lime united with just the necessary quantity of water, the pieces of quicklime are sprinkled with water from a watering can. The water is rapidly taken up and the sprinkling is repeated until the lumps begin to fall to a fine powder; in the process the lime becomes very hot. The slaked lime is then passed through a sieve in order to separate the larger pieces of quicklime which have not been slaked; the powder must be kept in well closed packages, since it energetically absorbs carbonic acid out of the air.
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.
Calcium Carbonate, CaCO₃ = 100, is found naturally in large quantities as chalk, which consists of the skeletons of extremely small animals. By powdering and levigating, it is converted into a soft powder, which is used to lighten the shade of lakes and other colours.
Calcium Sulphate (Gypsum), CaSO₄.2H₂O = 172.—This mineral, when finely powdered, is added to some colours.
Calcium Phosphate (Bone Ash), Ca₃(PO₄)₂ = 310, is sometimes used to lighten the shade of certain colours which might be injured by calcium carbonate. It comes in large quantities as a fine, white powder from South America. Naturally, only quite white bone ash can be used; if not completely burnt it is grey, and will then impair the shade of colours with which it is mixed.
Magnesium Carbonate (Magnesia), MgCO₃ = 84, is also used as an addition to colours in order to obtain pale shades. It is most cheaply obtained by dissolving magnesium sulphate (Epsom salts) in water and adding soda solution so long as a precipitate forms, which is then washed and dried. The magnesium carbonate prepared in this way is a very fine, light powder, insoluble in water, which can be mixed with the most delicate colours without harming them. White magnesia is an extremely light powder, which may be used when it can be bought at as low a price as the above preparation.
Barium Compounds.—The raw material used for the preparation of barium pigments is either barium sulphate (barytes, heavy spar) or barium carbonate (witherite). The latter is much more rare than barytes, which is almost exclusively employed in the preparation of barium compounds. The barium compounds of particular importance for our purpose are barium chloride and nitrate.
Barium Chloride, BaCl₂.2H₂O = 244.—This salt is now a common article of trade, and can be bought at a low price. When pure it forms colourless crystals readily soluble in water. If the colour maker is able to get cheap barytes and fuel, it may be advantageous for him to prepare barium chloride himself.
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 (barium carbonate) 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 (rain or distilled). Water which contains carbonates or sulphates always gives a turbid solution by precipitating barium carbonate or sulphate.
Aluminium Compounds.
The compounds of the earth metal aluminium play a very important part in colour making, since they form beautifully coloured compounds with many organic colouring matters. Formerly alum was the only material used in colour factories for the preparation of the alumina compounds; at present aluminium sulphate is used, and when it is sufficiently pure it is the most valuable material, because it contains the greatest quantity of alumina.
Under the designation of alum only one compound, the so-called potash alum, was at one time found in commerce, but now there are other alums, which contain soda or ammonia in place of potash. These salts are of equal use in colour making to potash alum. The preference is to be given to the compound which contains the largest proportion of alumina. The chief point to be observed in connection with alumina compounds for use in colour making is that they shall be free from iron, because iron oxide, which would be precipitated out of the solution along with the colours, in consequence of its red colour would spoil the shade of the pigment.
Aluminium Sulphate (Sulphate of Alumina), Al₂(SO₄)₃.18H₂O = 664.—Any manufacturer who can obtain cheap china clay (kaolin) and sulphuric acid can himself prepare this compound with advantage. The apparatus used for this purpose is an iron pan containing sand, in which is placed a large earthenware dish. In this dish are put very finely ground kaolin and strong sulphuric acid, and the mixture is heated so strongly that the acid boils, evolving heavy, white vapours. It is absolutely necessary to heat in this manner in order to avoid dangerous accidents. Sulphuric acid bumps so violently on boiling that it may even break a thick earthenware dish. The use of a sand bath makes the bumping harmless.
China clay, which consists of silicate of alumina, is decomposed by heating with sulphuric acid into silicic acid and sulphate of alumina. The original milky liquid becomes more transparent during boiling, and has at last the appearance of starch paste. Kaolin contains varying quantities of silica. The quantity of sulphuric acid necessary for its decomposition can only be found by trial. The quantities are chosen so that a small amount of kaolin remains undecomposed in order that the aluminium sulphate shall contain no free sulphuric acid.
When the decomposition is finished the pan is allowed to cool and the solid mass is brought into a vat filled with water, in which it is stirred until dissolved; then the liquid is left until the jelly-like mass of silicic acid has sunk to the bottom, when the clear solution of aluminium sulphate is drawn off and can at once be used.
If solid aluminium sulphate is required—and this is to be recommended when large quantities are to be prepared—the solution is evaporated in earthenware dishes until a portion solidifies when dropped on a cold plate. The molten aluminium sulphate is then cast in prismatic blocks, which are preserved in boxes. These blocks are of a pure white colour and very crystalline; they dissolve with difficulty in cold, but readily in hot water without residue. The solution has an acid taste, even when it contains no excess of sulphuric acid. When the blocks have a yellowish tinge, this denotes the presence of iron, and the solution must be freed from this impurity. Nowadays sulphate of alumina can be obtained so cheaply that it is hardly of advantage to make it.
The Alums.
These are double salts of aluminium sulphate and potassium, sodium or ammonium sulphate. There are also other double sulphates known as alums which, in place of aluminium, contain chromium, iron, etc., but they are not of interest here. It may still be observed that all alums, whatever their composition, possess the property of crystallising together from mixed solutions, so that crystals can be obtained in which every existing alum is contained.
The potassium, sodium and ammonium aluminium alums are used in colour making.
Potassium Aluminium Alum, KAl(SO₄)₂.12H₂O = 474.—This is the substance commonly called alum. Like all alums it crystallises in fine octahedral crystals, which at first are quite transparent, but slowly effloresce in the air and become covered by a white powder. It dissolves with difficulty in cold, but readily in hot water. The solution has at first a sweet taste, with an astringent after-taste.
Alum comes into the market in different forms, of which the following are the most important: as crystallised alum, in the form of large crystals united together; as alum meal, a coarse crystalline powder obtained by rapidly cooling and stirring a hot alum solution. On account of the larger surface this form dissolves more quickly than the large crystals. Roman alum is the name of a variety chiefly imported from Italy; it owes its reputation to its great purity—it contains a very small quantity of iron.
In order to prepare alum quite free from iron from the ordinary alum containing iron, it is recrystallised, that is, as much as possible is dissolved in boiling water and the solution quickly cooled with continual stirring; the small crystals so obtained are then washed with cold water. The residual saturated solution of alum, which contains the greater part of the iron, can be used for the preparation of those colours which are not injured by the presence of iron.
The solubility of alum in water varies greatly at different temperatures. The table gives the weight of alum dissolved by 100 parts of water at different temperatures.
| Temperature. | Crystallised Alum. | Anhydrous Alum. |
|---|---|---|
| 0° C. | 3·90 | 2·10 |
| 10° ” | 9·52 | 4·99 |
| 20° ” | 15·13 | 7·74 |
| 30° ” | 22·01 | 10·94 |
| 40° ” | 30·92 | 14·88 |
| 50° ” | 44·11 | 20·09 |
| 60° ” | 66·65 | 26·76 |
| 70° ” | 90·67 | 35·11 |
| 80° ” | 134·47 | 45·66 |
| 90° ” | 209·31 | 58·64 |
| 100° ” | 357·48 | 74·53 |
When potash alum is heated it loses water, 75 per cent. of the total at 61° C.; at 92° C. it melts completely, and all the water is lost by continued heating at 100° C. The residue is known as burnt alum.
In alum the whole acidity of the sulphuric acid is not neutralised; the solution has always an acid reaction; if soda solution is added, the escaping carbonic acid causes the liquid to effervesce. If soda solution is added until a further addition would cause a precipitate, a solution of so-called neutral alum is formed which has no longer an acid reaction. Neutral alum is occasionally required in colour making. In preparing it the soda solution must be added with great care when the liquid is near its point of neutralisation. Any addition of soda solution after this point is reached will cause a separation of alumina. This is not desirable, since it is generally only wished to precipitate the alumina in combination with colouring matters.
Roman Alum.—Under this name, or that of “cubic alum,” a variety of alum is sold, generally at a rather higher price than ordinary alum, from which it is distinguished by its crystalline form. Ordinary alum forms octahedral crystals often the size of a child’s head, but cubic alum well formed cubes.
The property of crystallising in cubes may be imparted to any alum solution by the addition of a little potash. Much so-called Roman alum is made in German works in this way. When this alum contains very little iron it is quite equal in quality to the best Roman alum, for the higher value of the latter is entirely due to its small content of iron. The alum prepared in the province of Naples is still better than Roman alum; it contains less iron.
When alum is required for the preparation of lakes of bright and delicate shades, it is indispensable to use a preparation free from iron, because the brownish yellow oxide of iron would appreciably injure the shade. Alum, free from iron, is most simply prepared by dissolving alum in boiling water, running the boiling solution quickly through a cloth, and quickly cooling with constant stirring. The alum meal prepared in this way contains much less iron than the original alum, the iron salts remaining dissolved in the mother liquor. When this alum meal is collected and cold water poured over it to free it from mother liquor, it is generally sufficiently pure to be used for any purpose, but if not, it is again recrystallised. The alum liquors containing the iron are used for the preparation of colours which are not injured by the presence of iron.
To test alum for iron yellow prussiate of potash is used, which gives a blue precipitate with ferric salts. The test is carried out by dissolving 10 grammes of alum in 1 litre of water, placing the solution in a tall, narrow cylinder standing on white paper, adding 10 to 20 drops of a saturated solution of yellow prussiate, and well stirring. On looking down through the liquid, if a distinct colouration is at once evident, the alum contains much iron, and must be recrystallised; indeed, the crystals would generally be coloured yellow. On the contrary, if the solution does not show a blue tint until after standing several days, the alum contains but a small quantity of iron, and can be used for most purposes without further purification. Alum quite free from iron is a rare commercial article; the test will generally show a feeble blue colouration. If this is not intense and no blue precipitate is deposited at the bottom, the alum is tolerably pure, and can be used in colour works. The longer the time before the blue colouration appears the poorer is the alum in iron.
Soda Alum, NaAl(SO₄)₂.12H₂O = 458, is made in some alum works. It has the greatest similarity in properties with the potash salt, but is distinguished by a much greater solubility in water and more rapid efflorescence in air. Soda alum can be bought at varying prices; that containing iron is much cheaper than that free from iron. When the latter is to be bought at a fair price it is to be preferred to potash alum, since, as we shall show later, it contains a larger proportion of alumina.
Ammonia Alum, (NH₄)Al(SO₄)₂.12H₂O = 453.—This compound of aluminium sulphate and ammonium sulphate is now often met with; the more expensive potassium sulphate in ordinary alum is replaced by ammonium sulphate, which is cheaply obtained from the ammoniacal liquor of the gas works.
Ammonia alum is better for our purpose than potash alum since it contains more alumina, is generally cheaper and dissolves more easily in water. Unfortunately most commercial ammonia alum contains so much iron that it has to be recrystallised before it can be used in colour works.
One hundred parts of water dissolve at different temperatures the quantities of ammonia alum given in the table:—
| Temperature. | Crystallised Ammonia Alum. | Anhydrous Ammonia Alum. |
|---|---|---|
| 0° C. | 5·22 | 2·62 |
| 10° ” | 9·16 | 4·50 |
| 20° ” | 13·66 | 6·57 |
| 30° ” | 19·29 | 9·05 |
| 40° ” | 27·27 | 12·35 |
| 50° ” | 36·51 | 15·90 |
| 60° ” | 51·29 | 21·95 |
| 70° ” | 71·97 | 26·09 |
| 80° ” | 103·08 | 35·19 |
| 90° ” | 187·82 | 50·30 |
| 100° ” | 421·90 | 70·83 |
Of the different alums, ammonia alum contains the largest and potash alum the smallest proportion of alumina. The composition of the three commonly occurring alums is given in the following table:—
| Potash Alum. | Soda Alum. | Ammonia Alum. | |
|---|---|---|---|
| Potash, K₂O | 9·95 | —— | —— |
| Soda, Na₂O | —— | 6·80 | —— |
| Ammonia, NH₃ | —— | —— | 3·89 |
| Alumina, Al₂O₃ | 10·83 | 11·20 | 11·90 |
| Sulphuric acid, SO₃ | 33·71 | 34·90 | 36·10 |
| Water, H₂O₄ | 45·51 | 47·10 | 48·11 |
Thus ammonia alum is to be preferred to soda alum and soda to potash alum, whilst the latter is used on account of its greater purity. In a colour works, in which large quantities of alum are used, it is advantageous to work with ammonia alum which is recrystallised on the works.
The alums and aluminium sulphate are the alumina compounds in ordinary use in colour works; aluminium acetate could also be used if it were to be had at a reasonable price.
Alumina, Al₂O₃ = 102, and Hydrate of Alumina.—Pure alumina, or rather hydrate of alumina, is required in the preparation of many colours. When the solution of an aluminium salt is precipitated by soda, the carbonic acid escapes with effervescence, and a gelatinous precipitate is formed which it is extremely difficult to wash clean. The precipitate, which consists of hydrate of alumina, shrinks very greatly in drying, and turns to a horny mass; when strongly heated it loses water, and becomes a white, insoluble powder of anhydrous alumina.
A variety of hydrate of alumina, heavy, and therefore easily washed, is obtained by boiling a solution of alum with a plate of zinc lying on a copper plate until all the alumina has separated. By collecting this on a filter and pouring hot water over it a number of times it is obtained quite pure.
Alumina plays a particularly important part in the manufacture of cobalt colours. When treated with a cobalt salt and heated it takes a fine blue shade.
In the foregoing those metals and their compounds have been treated which are extensively used in preparing colours without themselves forming coloured compounds. The chromates and prussiates are an exception to this. The ammonia compounds, the alkalis and alkaline earths, also the acids, are used in making many colours, although they do not contain colouring principles. The alumina compounds are in similar case. Themselves colourless, they form at the same time a carrier for the coloured compound and bring it into a suitable form for use as a pigment.
An example will explain what we mean by “carrier” of the colouring matter. Logwood contains a very handsome colouring matter which can be extracted by water. In order to be able to employ this colouring matter as a pigment it is combined with alumina, a compound insoluble in water being formed, which is called a lake. In this compound the alumina is to be regarded as the carrier of the colouring matter, which it has fixed in an insoluble form.
In dyeing, which in many respects is closely allied to colour making, the property of certain metallic compounds, themselves colourless, of fixing dyes is commonly utilised, the metallic compound being called the mordant. The fabric is first prepared with the metallic compound or mordant, and the colour then formed by bringing the mordanted material in contact with the colouring matter.
The “heavy metals” form, among their numerous compounds, a great number of coloured substances, and several of them are distinguished by a great wealth of coloured derivatives; for example, copper, chromium and cobalt form coloured compounds only. Although the use of pigments derived from the heavy metals has been considerably restricted in recent years by the discovery of a series of colouring matters which replace them, yet they are now, and always will be, of very great importance in the manufacture of colours. It is, therefore, necessary briefly to describe the various metals which are used in colour making, so that the manufacturer may know what metals produce harmless colours, permanent and unaltered by the atmosphere, and what do not.
The metals are divided into two great groups, designated, according to their specific gravities, the group of the light and of the heavy metals. The light metals comprise the alkali, alkaline earth and earth metals, whose important compounds we have just described. The specific gravity of each of these metals is less than five times that of water.
The heavy metals have a specific gravity exceeding 5; they are generally divided into groups, which are known by the name of the commonest metal in the group. These are as follows:—
| Zinc group | Zinc, | Zn = 65. |
| Iron group | Iron, | Fe = 56. |
| Tungsten group | Tungsten, | W = 184. |
| Antimony group | Antimony, | Sb = 120. |
| Tin group | Tin, | Sn = 119. |
| Lead group | Lead, | Pb = 207. |
| Silver group | Silver, | Ag = 108. |
| Gold group | Gold, | Au = 197. |
| Platinum group | Platinum, | Pt = 194. |
To the zinc group belong the metals zinc, cadmium and indium, of which only the two first are of importance here. The iron group comprises iron, manganese, cobalt, nickel, chromium and uranium, all of which are used in the manufacture of colours. The antimony group contains antimony and bismuth, the latter of which is of little importance. To the tin group belong tin and the rare metals titanium, zirconium, thorium, niobium and tantalum; of these tin alone is important in colour making. In the lead group are lead and thallium; lead produces many important pigments. The silver group contains silver, copper and mercury, the two latter of which are important. Of the gold group gold alone is of interest, and its importance has been diminished by the discovery of far cheaper substances which replace it. In the platinum group, which contains platinum, iridium, rhodium, ruthenium, palladium and osmium, only platinum itself is used as a colour; it is employed in porcelain painting to produce the peculiar metallic shimmer known technically as “lustre.”
The behaviour of the compounds, of the metals mentioned above towards sulphuretted hydrogen is of the greatest importance to the colour maker, since on it depends the alterability of the pigments when exposed to the atmosphere. Many metallic compounds are unaltered by the sulphuretted hydrogen present in the air, whilst others are in a high degree affected by it and become gradually darker, so that their colour may in the end approximate to black.
The pigments containing lead, copper, mercury and bismuth are extremely susceptible to the action of sulphuretted hydrogen, by the action of which they form black compounds. Since colours which contain these metals are not permanent, but darken considerably, endeavours have been made for a long time to replace them by others not susceptible to the action of sulphuretted hydrogen. Thus it is desirable to manufacture only colours free from metals forming black compounds with sulphuretted hydrogen. For the same reason pigments which contain sulphur should not be mixed with those containing metals which form black sulphur compounds.
The rules laid down in the preceding paragraphs are of the greatest importance for the artist, for by following them he will succeed in composing a “permanent palette,” that is, containing only such colours as will not, by their composition, bring about the speedy ruin of the painting.
Although the majority of the compounds of the heavy metals are poisonous, some possess this property in an eminent degree. These are chiefly colours which contain arsenic, antimony, copper and lead. So far as it is possible, these colours should be dispensed with and harmless pigments sold in their place, though this is not always possible, since several poisonous colours cannot be replaced by innocuous ones.