Compounds of the Heavy Metals.
Zinc Compounds.—Zinc oxide, ZnO, and zinc sulphate, ZnSO₄.7H₂O, are the compounds of this metal used in the colour industry. Zinc oxide, which is used as a white pigment, is a powder which turns yellow when heated, and is not acted upon by sulphuretted hydrogen. Zinc sulphate (white vitriol) occurs as colourless crystals, or more frequently as greyish white crystalline masses. The freedom of zinc sulphate from iron is of particular importance; the commercial article is seldom satisfactory in this respect. In order to free commercial zinc sulphate from iron, the property of zinc hydroxide of precipitating iron oxide from neutral solutions may be employed. The zinc sulphate is dissolved in water, and ammonia added in small quantities until the precipitate of zinc hydroxide remains on stirring. When the liquid is left in contact with the precipitate and stirred up once or twice a day, if iron is present the precipitate will turn yellowish brown, owing to the separation of ferric hydroxide, and in the course of a few days all the iron will be removed from solution. The liquid should then give no blue colouration with yellow prussiate of potash.
Zinc oxide is used as a white pigment, zinc chromate as a yellow, and zinc cobalt compounds as green colours.
Cadmium Compounds.—Cadmium is a metal which possesses great similarity to zinc, with which it occurs in nature. In the preparation of cadmium compounds the metal is generally used. This is dissolved in dilute sulphuric acid, hydrogen is evolved, and a solution of cadmium sulphate obtained.
Cadmium is used in colour making only for the preparation of the beautiful cadmium yellows.
Iron Compounds.—These are of the greatest importance to the colour maker. Several, in which iron alone is the colour principle, are very valuable: ochre, rouge, Venetian red, sienna and umber, for example. Iron compounds are also used in the preparation of many colours. The most important is:—
Ferrous Sulphate (Green Vitriol, Copperas), FeSO₄.7H₂0.—This substance, which occurs commercially in a form of great purity at a very low price, is generally the starting point in the preparation of iron pigments. When pure, it forms fine sea-green crystals, with an astringent metallic taste, which are not poisonous and are easily soluble in water. After long exposure to the air, ferrous sulphate becomes covered with an ochre-coloured crust, consisting of basic ferric sulphate. The ferrous oxide contained in the green vitriol has united with oxygen and been converted into ferric oxide. The latter requires a larger quantity of acid than ferrous oxide for the formation of soluble salts, so that an insoluble basic salt is separated. The same thing occurs when a solution of ferrous sulphate is exposed to the air.
When green vitriol, or any other ferrous salt, is exposed to the action of oxidising agents, such as chlorine or nitric acid, the iron is rapidly changed into the ferric state. This transformation is of particular importance in the manufacture of certain blue pigments.
We give in a table the relation between the percentage of crystallised ferrous sulphate (FeSO₄.7H₂O) contained in a solution at 15° C. and its specific gravity:—
| Specific Gravity | Percentage of Ferrous Sulphate. | Specific Gravity | Percentage of Ferrous Sulphate. |
|---|---|---|---|
| 1·000 | 0 | 1·118 | 21 |
| 1·005 | 1 | 1·125 | 22 |
| 1·011 | 2 | 1·131 | 23 |
| 1·016 | 3 | 1·137 | 24 |
| 1·021 | 4 | 1·143 | 25 |
| 1·027 | 5 | 1·149 | 26 |
| 1·032 | 6 | 1·155 | 27 |
| 1·037 | 7 | 1·161 | 28 |
| 1·043 | 8 | 1·168 | 29 |
| 1·048 | 9 | 1·174 | 30 |
| 1·054 | 10 | 1·180 | 31 |
| 1·059 | 11 | 1·187 | 32 |
| 1·065 | 12 | 1·193 | 33 |
| 1·071 | 13 | 1·200 | 34 |
| 1·077 | 14 | 1·206 | 35 |
| 1·082 | 15 | 1·213 | 36 |
| 1·088 | 16 | 1·219 | 37 |
| 1·094 | 17 | 1·226 | 38 |
| 1·100 | 18 | 1·232 | 39 |
| 1·106 | 19 | 1·239 | 40 |
| 1·112 | 20 | —— | —— |
Yellow and red prussiate, which have been already mentioned, also belong to the iron compounds. They have been separately mentioned because the iron is contained in them in a peculiar form as a portion of an organic radical.
Ferrous Chloride, FeCl₂, may sometimes be used instead of green vitriol. When iron is dissolved in hydrochloric acid hydrogen is given off and a solution of ferrous chloride is obtained; but when rouge is dissolved in the same acid, ferric chloride is formed. When iron is dissolved in nitric acid, in consequence of the oxidising properties of this acid a ferric salt is obtained. Iron forms two series of salts: in the ferrous compounds the iron is in the same form as in green vitriol and the corresponding salts; in the ferric compounds the iron is contained in a higher state of oxidation. By powerful oxidising agents, as nitric acid or chlorine, ferrous compounds are converted into ferric.
Manganese Compounds.—Manganese (Mn) is a metal whose compounds show great similarity with those of iron, like which it forms two oxides (also others), manganous oxide (MnO) and manganic oxide (Mn₂O₃). The salts of manganous oxide are not oxidised in the air like those of ferrous oxide.
The raw material used in the preparation of manganese compounds is the mineral pyrolusite, which is manganese dioxide (MnO₂).
Manganese sulphate (MnSO₄) forms rose-red crystals containing varying quantities of water. The residues from the preparation of chlorine can be used as the material for the preparation of colours. According as pyrolusite and hydrochloric acid or pyrolusite, salt and sulphuric acid are used for this purpose a solution of manganous chloride or sulphate is obtained.
Manganese compounds have but a restricted use in colour making.
Nickel Compounds are generally coloured green, but they are not used as pigments.
Cobalt Compounds.—Among these are found many important pigments. All cobalt compounds are coloured; in beauty and variety of shade they can only be compared with those of chromium. In properties cobalt is very similar to iron and nickel.
The form in which cobalt is used in preparing colours is either cobalt nitrate, Co(NO₃)₂.6H₂O, or cobalt chloride, CoCl₂.6H₂O. Both salts are articles of commerce, but generally they are so dear that it is more profitable for the colour maker to prepare them direct from the cobalt minerals. A simple method for preparing cobalt compounds from the ores is therefore given. The most important cobalt ores are speiss cobalt, a compound of cobalt and arsenic, and cobalt glance, a compound of cobalt, arsenic and sulphur. The former mineral often contains only small quantities of cobalt, and it is advisable for our purposes to use cobalt glance, which contains from thirty to forty per cent. of cobalt. This mineral is first roasted, that is, is heated with a plentiful air supply, by which means the arsenic is driven off. On account of the poisonous nature of the arsenic vapours the roasting must be conducted in a furnace with a very good draught.
Under the name of zaffre, roasted cobalt ores come into commerce. These may be used in the preparation of cobalt compounds, by which means the operation of roasting is avoided. According to the quality of the ore which has been used to obtain zaffre, it contains a very varying proportion of cobalt. The varieties richer in cobalt must be used; they are technically known by the mark FS, or FFS (the best).
The roasted cobalt ores or zaffre are treated with fused acid potassium sulphate, when the salts of iron and manganese are decomposed, whilst cobalt and nickel sulphates remain unchanged. In a Hessian crucible are melted 300 parts of acid potassium sulphate, and 100 parts of the powdered zaffre are gradually added, mixed with one part of green vitriol and one part of saltpetre; the mixture is heated so long as sulphuric acid escapes. The mass is then boiled with water and the red solution treated with sulphuretted hydrogen so long as a precipitate is formed; this may contain copper, manganese, and bismuth. After filtering, soda is added to the boiling liquid; cobalt carbonate is precipitated, which can be converted into nitrate or chloride by solution in the corresponding acid. If cobalt sulphate is required, the solution, after treatment with sulphuretted hydrogen, need only be evaporated to crystallisation, when the sulphate separates in fine red crystals. Cobalt nitrate and chloride are very soluble in water; to obtain them their solutions must be strongly evaporated and quickly cooled whilst stirring. The crystals of cobalt nitrate and chloride absorb moisture from the air and deliquesce; they must be kept in glass vessels with well-ground stoppers.
The cobalt compounds which are to be used in colour making must be free from iron, nickel and arsenic, which would detract from the cleanness of the colours. If the precipitate produced by soda contains iron it is mixed with excess of solution of oxalic acid, and after a few hours the cobalt oxalate is filtered from the liquid, in which all the iron is dissolved. The cobalt oxalate can then be converted into nitrate or chloride by treatment with nitric or hydrochloric acids.
These salts form the material for the preparation of the cobalt compounds, a large number of which are used as extremely durable red, blue and green pigments; several of them, such as cobalt blue, cannot be exactly replaced by other pigments. On account of the industrial importance of the cobalt colours, these directions for the preparation of the soluble cobalt salts from the ores have been given with some detail. The preparation of the cobalt colours will be given in extenso later on.
Chromium Compounds.—As the name indicates, this metal yields numerous coloured compounds (χρῶμα, colour); in fact, only coloured chromium compounds are known, and the colours are most varied—yellow, green, red and violet. On this account the chromium compounds are among the most important used in colour making; a great number of colours are prepared by their aid. Chrome ironstone, as we have already stated, is the raw material for the preparation of chromium compounds. From it potassium bichromate is made on a large scale in special works, so that no colour maker is compelled to prepare chromium salts himself.
When the chromium pigments contain no metal blackened by sulphuretted hydrogen, they have the desirable property of being unaltered by the atmosphere. Like the cobalt compounds, they are distinguished by their great stability when heated; on this account, they have a large use in porcelain painting.
Molybdenum, Tungsten and Vanadium Compounds on account of their cost have a very limited use as pigments. Molybdenum compounds are obtained from molybdic acid; compounds of tungsten from the metal; and those of vanadium from ammonium vanadate.
Antimony Compounds can be used in the preparation of several pigments, but, on account of their behaviour towards sulphuretted hydrogen, the pigments cannot be regarded as really permanent, and their use is generally diminishing. The so-called antimony vermilion is the only antimony compound at all extensively employed.
Bismuth Compounds possess properties very similar to those of antimony. Only one bismuth preparation is used as a pigment, and this is very sensitive to the action of sulphuretted hydrogen, being changed into black bismuth sulphide.
Tin Compounds are employed in two ways: some are themselves colours, such as stannic sulphide (mosaic gold); others, themselves colourless, are used in making pigments, as stannous and stannic chlorides.
Stannous Chloride, SnCl₂.2 H₂O, is obtained when tin is dissolved in hydrochloric acid, hydrogen being evolved. Stannic Chloride, SnCl₄, is formed when tin is dissolved in a mixture of hydrochloric and nitric acids (aqua regia).
Tin compounds have, similarly to aluminium salts, the property of forming coloured insoluble compounds (lakes) with many organic colouring matters. Their use for this purpose is extensive.
Arsenic Compounds formerly played an important part in colour making. They were used in the manufacture of a large number of pigments, very beautiful but extremely poisonous. At the present time, we can, fortunately, entirely dispense with arsenic in colour making; the arsenic colours can be replaced by others equally handsome and less, or not at all poisonous. The most important of the arsenic compounds is the trioxide As₂O₃, or Arsenious Acid, commercially known as white arsenic. This substance is obtained in large quantities as a by-product in the extraction of several metals. It forms masses which are either glassy or have the appearance of porcelain. Freshly sublimed arsenic trioxide is glassy; this form gradually changes into the porcellaneous variety, it dissolves with difficulty in water. A strong solution can only be obtained by boiling for many hours.
The compounds of arsenic with sulphur, formerly extensively used as pigments, have now almost fallen into disuse.
Lead Compounds belong to the substances most largely used in making colours. Unfortunately all lead colours have two very important drawbacks. They are all very poisonous and at the same time extremely sensitive to sulphuretted hydrogen, so that they are very considerably altered by the action of the small quantities of that gas contained in the atmosphere of an ordinary dwelling. A striking example of this is seen in the lead paints used on the doors of water-closets. The paint, at first pure white, becomes gradually darker, and at last almost black, the lead compound having been changed into black lead sulphide.
On account of this great sensitiveness of lead compounds, it would be better if they could be excluded from the list of colours. Great care must be taken not to mix lead compounds with others which contain sulphur; a discolouration of the mixture would be the inevitable result in a very short time.
The oxides of lead and a number of its salts are themselves pigments, for example, litharge, PbO, red lead, Pb₃O₄, and white lead (basic lead carbonate). These pigments are prepared on a large scale in particular works. At this point only those lead compounds will be mentioned which are generally used for the preparation of other lead pigments; they are: lead sulphate, nitrate, acetate and chloride.
Lead Sulphate, PbSO₄, is formed when sulphuric acid or the solution of a sulphate is added to the solution of a lead salt; so obtained it is a white crystalline powder insoluble in water. This substance is generally not made in colour works, but is purchased from chemical works or dye houses, of which it is a by-product. In this form (lead bottoms) it is generally not sufficiently pure, but contains admixtures of sulphuric acid or aluminium salts, from which it is freed by washing. The lead sulphate is stirred up in water, the heavy precipitate allowed to settle, the wash water drawn off, and after repeating this process until the wash water no longer shows an acid reaction, the purified precipitate is dried. In this condition it is a heavy, white powder, and can alone be ground into paint. But on account of its crystalline nature, which reduces the covering power, such use is inadvisable.
Lead Nitrate, Pb(NO₃)₂.—This very important compound may be bought, but it is advisable to prepare it in the works. Water is placed in a wooden tub, then half the volume of nitric acid is added, and finely powdered litharge gradually stirred in, the liquid being kept in constant movement. When it is seen that the litharge is only slowly dissolved, the liquid is well stirred after each addition of litharge and then tested by litmus paper. When this is no longer reddened, the nitric acid is completely saturated, and the liquid contains only lead nitrate in solution. It is allowed to stand until the insoluble portions have settled, and then drawn off into another vessel where crystals of lead nitrate separate in a few days. If the salt be required in solid form, the solution may be evaporated in earthenware dishes; generally the solution is used as it is obtained.
Pure lead nitrate forms white crystals which are not particularly soluble in water, 1 part requiring 2 parts of water at the ordinary temperature. Lead nitrate is decomposed on heating, like all nitrates, and litharge remains. The solution of this salt is used in the preparation of those lead pigments which are obtained by precipitation, for example, chrome yellow.
Lead Acetate, Pb(C₂H₃O₂)₂.3H₂O.—The compounds of lead with acetic acid are of great importance. Two of these are to be considered: neutral lead acetate, commonly known as sugar of lead, and basic lead acetate. It may be advisable to manufacture both these compounds, the latter always. Neutral lead acetate comes into commerce in the form of colourless heavy crystals, which are often covered by a white powder of the basic acetate; they dissolve readily in water, and the solution has a sweetish taste, hence the name “sugar of lead”. Frequently the solution is very turbid; this is caused by the carbonates contained in the water. The turbidity may be removed by the addition of a little acetic acid. It is only economical for the colour maker to prepare sugar of lead when he can obtain cheap raw materials, lead or litharge and vinegar. Pyroligneous acid may also be used if it is colourless, its odour being without importance for this purpose.
The best method for preparing lead acetate from litharge is to place the vinegar in a tub and hang in it a strong linen bag filled with finely ground litharge. The tub is kept covered for a few days and its contents then tested with red litmus paper. When this is turned blue, the liquid is drawn off and vinegar gradually added whilst stirring, until blue litmus paper is just turned red. In this process, after neutral lead acetate has been formed, more lead oxide is dissolved, and the liquid thus acquires an alkaline reaction. The further addition of acetic acid reconverts the basic salt into neutral lead acetate.
Lead acetate solution may, with advantage, be prepared directly from metallic lead. For this purpose, lead is granulated by melting and pouring in a thin stream into cold water, where it solidifies in irregular pieces. This is done in order to give the lead as large a surface as possible. Three high narrow tubs placed one above the other so that liquid may flow from the highest to the middle, and from this into the lowest, are filled with granulated lead. Vinegar is placed in the uppermost vessel to cover the lead; after twenty-four hours it is allowed to flow into the middle, and after a further twenty-four hours into the lowest tub. In this way, a solution of basic lead acetate is formed, to which the necessary quantity of acetic acid is added to bring it into the neutral condition. If crystalline lead acetate is required, the liquid is evaporated down and quickly cooled with stirring, so that small crystals are formed. Generally, however, evaporation is unnecessary, since lead acetate is always used in solution in preparing colours.
If the lead acetate solution be not colourless, which is generally the case when coloured acetic acid is used, the defect may be removed by stirring a little bone black into the liquid and filtering after twenty-four hours, when a completely colourless solution is obtained.
It is always necessary to know exactly how much lead acetate is contained in the solutions prepared by these processes. The lead or the litharge is therefore weighed and the volume of the lead acetate solution measured. One hundred parts by weight of crystallised lead acetate are obtained from 62·54 parts of lead.
Lead acetate solutions must be kept in well-covered vessels. The carbonic acid of the air will turn the liquid turbid. The turbidity may be removed by the addition of acetic acid.
In the following table is given the percentage of crystallised lead acetate contained in solutions of different specific gravities:—
| Specific Gravity | Crystallised Lead Acetate per cent. | Specific Gravity | Crystallised Lead Acetate per cent. |
|---|---|---|---|
| 1·0000 | 0 | 1·1869 | 26 |
| 1·0064 | 1 | 1·1955 | 27 |
| 1·0127 | 2 | 1·2040 | 28 |
| 1·0191 | 3 | 1·2126 | 29 |
| 1·0255 | 4 | 1·2211 | 30 |
| 1·0319 | 5 | 1·2303 | 31 |
| 1·0386 | 6 | 1·2395 | 32 |
| 1·0453 | 7 | 1·2486 | 33 |
| 1·0520 | 8 | 1·2578 | 34 |
| 1·0587 | 9 | 1·2669 | 35 |
| 1·0654 | 10 | 1·2768 | 36 |
| 1·0725 | 11 | 1·2867 | 37 |
| 1·0796 | 12 | 1·2966 | 38 |
| 1·0867 | 13 | 1·3064 | 39 |
| 1·0939 | 14 | 1·3163 | 40 |
| 1·1010 | 15 | 1·3269 | 41 |
| 1·1084 | 16 | 1·3376 | 42 |
| 1·1159 | 17 | 1·3482 | 43 |
| 1·1234 | 18 | 1·3588 | 44 |
| 1·1309 | 19 | 1·3695 | 45 |
| 1·1384 | 20 | 1·3810 | 46 |
| 1·1464 | 21 | 1·3925 | 47 |
| 1·1544 | 22 | 1·4041 | 48 |
| 1·1624 | 23 | 1·4156 | 49 |
| 1·1704 | 24 | 1·4271 | 50 |
| 1·1784 | 25 | —— | —— |
Basic Lead Acetate, Pb(C₂H₃O₂)₂.2PbO.—This salt may be regarded as a compound of neutral lead acetate with lead oxide. It is obtained by digesting vinegar with excess of litharge or with metallic lead, and also by treating lead acetate solution with litharge so long as the latter is dissolved. In the method last given, 100 parts of sugar of lead require about 118 parts of litharge to produce a saturated solution of basic acetate. The solution of this compound is alkaline; it turns red litmus paper blue. When exposed to the air, a turbidity is quickly produced owing to the separation of lead carbonate. In one white lead process basic lead acetate is the starting point of the manufacture.
Lead Chloride, PbCl₂, is seldom used in making colours. It may be prepared by stirring powdered litharge in common salt solution until the powder appears white. This, when washed, constitutes basic lead chloride. On adding hydrochloric acid to the washed mass until the liquid remains acid, lead chloride is obtained in the form of crystalline needles, which are very little soluble in cold, but more easily in hot, water.
Like any other soluble lead salt, lead chloride may be used in the precipitation of colours, but is seldom employed on account of its small solubility. Basic lead chloride was, at one time, used as a white pigment, and after melting, by which it is turned yellow, as a yellow pigment; it is no longer in use for these purposes.
Copper Compounds.—These are generally green or blue, and have an extended use in the production of colours. The metallic copper which is used in the preparation of colours is of the ordinary commercial quality. The impurities which it contains are generally so small in quantity that they are without importance for our purpose.
Copper Sulphate (Bluestone, Blue Vitriol), CuSO₄.5H₂O.—This is the commonest of the commercial copper salts, and on that account deserves our especial attention. It forms large sky-blue crystals, which effloresce slightly in the air, possess an unpleasant metallic taste, and are poisonous, like all soluble copper compounds.
Copper sulphate comes into commerce in a very pure form, but some qualities contain zinc sulphate or ferrous sulphate. The presence of zinc may be detected most easily by boiling the solution with excess of caustic soda, when copper oxide separates as a black powder, whilst zinc oxide remains dissolved. When sulphuretted hydrogen is passed through the liquid, a white precipitate of zinc sulphide is formed.
Iron is detected by passing sulphuretted hydrogen through the solution so long as a precipitate is formed, allowing the liquid to stand in a covered vessel, pouring it off from the precipitate, adding nitric acid, boiling and adding a solution of potassium ferrocyanide; a blue precipitate denotes the presence of iron.
Copper sulphate is rarely found which is quite free from iron and zinc. If these impurities are present in but small quantity, the zinc not exceeding 1 per cent. and the iron at most 0·5 per cent., the copper sulphate may be regarded as sufficiently pure for our purposes.
It may be here remarked that copper sulphate obtained from mints is generally of great purity, and hence particularly adapted for colour making.
When copper sulphate and other copper salts are dissolved in water, pale blue flocks of copper carbonate generally separate. This is due to the carbonate of lime contained in the water. An addition of a few drops of sulphuric, nitric or hydrochloric acid suffices to prevent this separation.
Copper Nitrate, Cu(NO₃)₂.6H₂O.—This salt may occasionally be obtained in colour works as a by-product. When nitric acid is poured over copper, there follows a copious evolution of nitric oxide, which produces brown fumes of nitrogen peroxide in air. Nitric oxide may be used to convert ferrous into ferric salts, a transformation required in making Prussian blue. In working in this way, nitric acid is poured over copper contained in a vessel provided with a delivery tube for the gas. The blue solution is at once used. Pure copper nitrate forms fine blue crystals, which very readily deliquesce in the air. The solution is therefore generally used as it is prepared.
Copper Acetate, Cu(C₂H₃O₂)₂.H₂O.—Copper is readily attacked by acetic acid. A number of salts are formed, of which some are used as pigments. For our purpose it will be sufficient to describe the manufacture of verdigris; few colour makers prepare any other copper acetate. A solution of this salt is most simply prepared in the following manner: Slaked lime is stirred with strong vinegar, and the solution left in contact with the excess of lime so long as it has a weak acid reaction. The solution, which contains acetate of lime, is poured into a solution of copper sulphate so long as a precipitate of sulphate of lime is formed. When this has been separated from the liquid, the latter is ready for further treatment. It contains only a very small quantity of dissolved sulphate of lime, which is not harmful in the preparation of colours.
In addition to the copper compounds mentioned here, several others were formerly used as pigments, or in the preparation of pigments which are no longer employed, because copper compounds of good colour can be obtained in a cheaper manner.
The same precautions should be taken in the use of copper colours which were mentioned for lead pigments; copper compounds are equally sensitive towards sulphuretted hydrogen, by which they are gradually discoloured.
Mercury Compounds.—Mercury forms compounds which, vermilion in particular, are used as pigments, and others which are used in the preparation of pigments. In many cases metallic mercury is the starting point in the preparation of the mercury compounds. The compounds commonly known as calomel and corrosive sublimate are also used.
Mercurous Nitrate, HgNO₃.—Nitric acid acts upon mercury in a manner differing according to its strength, and according to whether the mercury or the nitric acid is used in excess. In order to prepare mercurous nitrate, acid free from chlorine must be diluted at least with four times its volume of water, and the mercury must be in excess. On warming, the mercury is gradually dissolved, and, on cooling, the solution deposits colourless crystalline needles of the salt. A further crop of crystals is obtained after evaporating the solution.
When the action of nitric acid is over, the solution must be at once separated from the excess of mercury to prevent the formation of basic salts. If the salt has been properly made, it is completely soluble in water, but if a lemon yellow precipitate is formed on dissolving, the nitrate contains a basic salt, which can only be dissolved by warming and adding more nitric acid.
Mercuric Nitrate, Hg(NO₃)₂, is most simply obtained by warming mercury with very strong nitric acid. The heating must be continued until a test portion of the solution no longer gives a precipitate with hydrochloric acid. When this solution is evaporated, nitric acid is given off, and a salt crystallising in white needles is obtained, which dissolves in water with the separation of a yellow basic salt. It is therefore better to use the hot solution, which contains a little free acid, without evaporating.
Instead of mercurous and mercuric nitrates, the corresponding sulphates may be used, but the chlorides are more frequently employed since they can be readily obtained from the makers.
Mercurous Chloride (Calomel), HgCl, is obtained pure by adding common salt solution to a solution of mercurous nitrate and washing the precipitate, which is insoluble in water.
Mercuric Chloride (Corrosive Sublimate), HgCl₂ a common article of commerce, is prepared by heating a carefully made mixture of mercuric sulphate and common salt, when mercuric chloride sublimes. It is a white crystalline mass, soluble in 13·5 parts of water at 20° C., and soluble in 3 parts of alcohol. Although all mercury compounds are very poisonous, corrosive sublimate requires particular care in handling, since its easy solubility makes it surpass all other mercury compounds in poisonousness.
The mercuric sulphate required in the above preparation is obtained by heating mercury with sulphuric acid. Corrosive sublimate can also be prepared by adding hydrochloric acid to mercurous nitrate, and heating, with gradual addition of hydrochloric acid until a clear solution is formed, from which mercuric chloride crystallises on cooling.
Silver Compounds.—Silver nitrate, AgNO₃, is the only one of importance here. It is obtained by dissolving silver in nitric acid, when a blue solution is obtained because commercial silver contains copper, evaporating the solution to dryness, melting the residue, and keeping it molten until all the copper nitrate is decomposed. This point is recognised when a small portion of the melt dissolved in water does not give a blue colouration with excess of ammonia. Fused silver nitrate forms a white crystalline mass readily soluble in water and turning black when exposed to light, like many other silver compounds.
Gold Compounds.—Gold is now very little used in preparing colours. The compound used for this purpose is gold chloride, AuCl₃, which is obtained by heating gold with hydrochloric acid, and adding nitric acid in small quantities until all the metal is dissolved. By careful evaporation of the yellow solution gold chloride is obtained in brownish yellow crystals, which easily dissolve in water.
The compounds of molybdenum, vanadium and uranium are less used than those of gold, yet these metals find a special use in the preparation of colours for porcelain painting and for colouring glass, for which they are of great importance. The preparation of their compounds from the raw materials is complicated and not remunerative to the colour maker; they should be obtained from chemical works.
In the foregoing, the most important compounds of inorganic origin used in making colours have been briefly described, less in order to teach the methods for their preparation than to give the manufacturer the means of learning their properties. Since the great development of chemical industries during the last decades it is more advantageous for the colour maker in most cases to draw his supply of these substances from works of good reputation than to make them himself; only in the case of a few substances, which are sold at unreasonably high prices, will it be profitable for him to prepare them himself.
CHAPTER VI.
THE MANUFACTURE OF MINERAL PIGMENTS.
By mineral pigments we understand those which consist of compounds of metals with elements such as sulphur, chlorine and iodine, or with compound radicals, such as cyanogen, or of metallic salts.
Looking at the classification of mineral pigments from the chemical standpoint, a grouping according to the constituents would appear preferable, and we should have groups of colours consisting of metallic oxides, sulphides, salts, etc. In such a division of pigments, according to their constituents, no regard would be taken of the colour of the pigment. The white zinc oxide would be placed in the same group with yellow lead oxide and red lead oxide; and red mercury sulphide and yellow cadmium sulphide would fall into the same group of the sulphur compounds.
The chemical composition of a pigment is of less importance to the colour manufacturer than its shade, and it therefore appears to us more reasonable to prefer to classify mineral pigments according to similarity of colour. We shall therefore place all those mineral pigments together which possess the same colour.
Common usage differs from the scientific in the description of colours. In the physical sense, yellow, red and blue are the so-called “simple colours,” between which lie orange, green and violet as “mixed colours”. Physics knows no white colour and no black colour, but describes white as a mixture of all the simple colours and black as the absence of colour. A grey or brown shade, produced by different mixtures of simple colours, is just as little known in the scale of colours as white or black.
The colour maker follows, as we have said above, the common manner of speaking; to him white and black are equally as much colours as red and green. Besides the pure principal colours (yellow, red and blue), and the mixed colours obtained from them (orange, green and violet), colour makers distinguish many shades of each colour—lemon yellow, sulphur yellow, cherry red, blood red, violet blue, etc. For the present purpose it is of great importance to accurately distinguish the several shades, for the value of many colours is in proportion to their beauty of shade. The colour maker is often required to produce a colour of some particular shade, which he accomplishes in many cases by a suitable alteration in the process by which the colour is made, in other cases by mixing different colours, in which event chemistry is of no help to him; he must depend on the sensitiveness of his eyes to colour.
White Mineral Pigments.—We are acquainted with a great number of white or, more properly, colourless mineral compounds; they possess the property of reflecting, undecomposed, all the rays of light which fall upon them, in consequence of which they produce that impression upon the eye which we call white. According as a white substance reflects every ray of light or absorbs a portion, we see it as a brilliant pure white or, in the latter case, as a white with a grey tinge. If a white body reflects the majority of the rays of light falling on it, but decomposes a small number, we perceive a white which has a yellow, blue or red tinge.
The most valuable white for the colour maker is evidently that which reflects, unaltered, all the rays of light; it is the most brilliant, and free from every tinge of colour. The physical condition of the substance is most important. Solid substances are either crystalline, that is, possess definite shapes formed according to a regular law, or they are amorphous, that is, are composed of irregularly formed particles. Snow and white lead may serve as representatives of these two classes. Snow is composed of small colourless crystals of ice, the flat surfaces of which reflect, undecomposed, the light falling on them. The smaller are the crystals, the more pure appears to us the whiteness of the snow, and the thinner is the layer of snow required to produce the sensation of whiteness. But if the snow crystals are larger, the white appears to have a bluish tinge, and only a thick layer of snow is opaque. White lead, being an amorphous substance in a condition of very fine division, reflects the light very regularly, so that a thin layer of white lead appears quite opaque.
Among the artificial pigments, crystalline or amorphous, exactly the same conditions hold good as between snow and white lead. Of amorphous pigments a very thin layer is in most cases sufficient to make the surface upon which they are spread invisible, or, as the technical expression runs, “to cover,” whilst crystalline substances possess a smaller covering power. A striking example of this is seen by a comparison of two white pigments, white lead and “patent white” (lead oxychloride). The former is amorphous, the latter crystalline. Both are completely colourless and reflect white light, but in consequence of its amorphous condition and finer particles, white lead possesses far greater covering power than “patent white”.
Among all other colours the same rule holds. Amorphous pigments have always a greater covering power than crystalline. The smaller the crystals of the latter the greater is their covering power, so that in preparing pigments of a crystalline character care must be taken to make the crystals as small as possible.
From the above definition of white pigments it follows that an immense number must exist, since every colourless substance in a state of fine division appears white. Generally only those bodies are used which are insoluble in water, or almost insoluble, and which possess great covering power. The following may be mentioned as white pigments, only a few of which are in use: white lead, white zinc, permanent white, lead oxychloride, lead sulphate and sulphite, zinc oxychloride, lead antimoniate, antimony white, tin white, tungsten white, and in addition certain earths, pipe clay, china clay, etc. Several of these pigments are far too expensive for ordinary use, and have no advantage over much cheaper pigments except for very special purposes, such, for example, as bismuth white for cosmetics.
In general use we find very few artificial white pigments; these are lead, zinc and barium compounds. Circumstances may arise which make it expedient for the colour maker to manufacture other white pigments, for example, a demand for them, or favourable opportunities for obtaining the requisite raw materials.
The white lead pigments, of which there is a large number, as we have indicated, all have the great disadvantage that they are not permanent, that is, are changed by atmospheric influences. It is well known that lead is a very delicate reagent for sulphuretted hydrogen, with the sulphur of which it forms a black compound. Now the air, especially in towns, contains sulphur in the form of sulphuretted hydrogen or ammonium sulphide; though the quantity is very small, the fate of every white or coloured lead pigment is decided by it; after a longer or shorter time it will be discoloured, will gradually darken, and finally be turned black. In spite of this great changeableness of lead pigments they are used by artists and painters, although the majority could be replaced by more permanent pigments entirely unaltered by the atmosphere.