Lake pigment

“ There is now no painter among civilised races to whom it would occur to prepare his own colours; the chemical works provide them for him at a low price and in such a condition that they can be immediately used for painting. The Italian painters prepared the highly prized blue pigment, ultramarine, by laborious toil from the costly lapis lazuli; to-day, this same colour, more beautiful and deeper in hue, is made by several works, and sold at a price which bears no comparison with that of the colour obtained from the mineral. ”

“ It is treated with a boiling solution of alum and filtered whilst hot; madder lake is then precipitated by cautious additions of soda, the first portions precipitated being the best.
Madder Carmine is not often found in commerce; it consists of the almost pure lakes of alizarin and purpurin. The process by which this valuable pigment is produced is based upon the great stability of the madder colouring matters. The following is the process on the large [Pg 377] scale: Good madder, very finely ground, is spread out in small heaps in a room, the temperature of which is about 16° to 18° C. ”

“ The pigments known as mineral or Brunswick blue are the least valuable, they have generally a less deep colour and are quite without the metallic lustre which distinguishes Chinese blue and the better varieties of Prussian blue. Mineral blue often contains very large quantities of alumina, chalk, or even barytes; the addition of the last is to be regarded as irrational, since it makes the pigment conspicuously heavy. ”

“ The knavish practices to which the pigment has been subjected, have acquired it an ill-fame both with authors and artists. Vermilion has been charged with fading in the light, and with being blackened by impure air; but it was the custom to crimson the colour by means of lake, or tone it to a scarlet hue by red lead. With pigments as with persons, evil communications corrupt good manners—a motto that might be written with advantage on every palette. ”

“ Similarly the dyer proceeds to dye black upon a deep blue basis of indigo, with the ruddy colour of madder and the yellow of quercitron, &c.
Some of the best blacks and neutrals of the painter are those formed with colours of sufficient power and transparency upon the palette. Prussian blue and burnt lake afford a powerful though not very durable black; and compound blacks in which transparent pigments are employed will generally go deeper and harmonize better with other colours than any original black pigment alone. ”

“ It can be used as a water colour, but not in oil painting. This pigment differs from the ordinary yellow berry lake in colour and chemical composition; the yellow lake is a compound of xanthorhamnin with the metallic oxide, whilst sap green contains an uncrystallisable bitter substance, rhamnocathartin.
The lake is obtained from yellow berries which are not quite ripe. They are broken up and left in a warm place. The mass soon ferments; after about ten days it is pressed. Four parts of the liquid are then mixed with 0·5 part of alum and 0·5 part of potassium carbonate. ”

“ The soot of hard woods, which contain little or no resin, has a deep black, though dull hue; it is a gritty powder, and has little value as a pigment. Wood containing much resin, such as the wood of the pine, rosin, fish oil, asphaltum, in short, all bodies which are at the same time easily combustible and rich in carbon, produce on the contrary a handsome, glistening soot, which forms a valuable pigment. ”

“ The treatment may be stopped when the solution acquires a slight brownish colour. When the soot has been purified so far it has lost its brown shade and now appears as a velvety black powder, very soft and very light, and distinguished by extraordinary covering power.
It is not so easy as it appears at first sight to judge of the quality of a black pigment by its appearance. To the unaccustomed eye a pigment may appear to be of an unexceptionable black, which to the expert appears decidedly brown; only long practice can give the eye the requisite keenness. ”

“ Barium sulphate, known as permanent white, enamel white, blanc fixe, barytes white, is the only white pigment which is absolutely unaltered by exposure to the atmosphere. Lead pigments are discoloured in the course of time, and in the end turn black; bismuth white behaves in the same manner; zinc white is much more lasting, but not quite permanent.
Enamel white is really permanent; it deserves the greatest attention from the colour manufacturer, especially as it can be made by a very simple and cheap method. ”

“ What is of the greatest interest for the colour maker is the preparation of the pigment itself. The conversion of the prepared pigment into (oil or water) paint is unaccompanied by difficulties.
When we look for a practical classification for pigments, we find that there are colours which exist ready formed in nature, and others which can only be obtained by certain chemical processes, at times very complicated. ”

“ This mineral is of a peculiar green colour, and the name “Celadon green” has been universally adopted in the nomenclature of colour shades. Green earth occurs native in many places, being the decomposition product of an extensively distributed mineral, augite, crystals of which are found in many of the deposits. The green earth of Monte Valdo, on Lake Garda (Upper Italy) has been used for a very long time as a pigment. It is chiefly prepared in Verona for distribution in commerce, and from this circumstance has acquired the name “Verona green,” or “Verona earth.” ”

“ 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. ”

“ By long heating, the colour of this compound is gradually changed to a handsome green, which consists of a compound of barium and chromic oxides, and has occasional use as an artists’ colour. In order to obtain this pigment, the heating must be intense and long continued. According to the author’s experiments, it is not sufficient to heat for a short time to a very high temperature; in that way a mass is obtained of very unequal colour. The best result was obtained by spreading barium yellow in a thin, even layer in a flat porcelain dish and heating to whiteness for 10 hours. ”

“ It cannot be denied that certain manipulations impart a greater brilliancy to the product, although it is impossible to assign a physical or chemical reason; but the manufacturer will quickly be able to differentiate the valuable from the worthless in these processes. Two conditions have the greatest influence on the beauty of the pigment—the temperature at which the black sulphide is sublimed, and the complete freedom of the vermilion from excess of sulphur. ”

“ In whatever way and from whatever materials carbon is made for use as a pigment, the manufacturer must always endeavour to obtain it as pure as possible, and in a condition of the finest division; the depth of colour and the covering power depend on these two conditions. Carbon only shows a pure black colour when it is pure; if it contains relatively small quantities of impurities, it has a more or less brown shade. ”

“ But often it is necessary in the course of trade to decide rapidly the nature of a pigment or to detect the adulteration of a dear pigment with a cheaper, and for this purpose it is important to have a method of examination which can be conducted without much apparatus, and which requires no great chemical knowledge. It is quite possible to test the majority of pigments in a simple manner. Few reagents are necessary. For an examination of the mineral pigments the following are generally sufficient: hydrochloric, nitric and sulphuric acids, caustic soda solution and ammonium sulphide. ”

“ In the manufacture of pigments certain by-products are produced. These can be utilised by a manufacturer possessed of chemical knowledge, whilst they are simply thrown away by many, thus making the manufacture of the particular pigment far more expensive than when the by-product is also made valuable. Strictly speaking, there are no worthless by-products in making pigments, every liquid obtained in precipitating a colour might be further utilised. ”

“ It is most convenient to examine the pigment first for mechanical admixtures. If an effervescence follows the addition of hydrochloric acid, denoting the presence of a carbonate, this is generally calcium carbonate. White lead is rarely used, since it is very heavy and is more expensive. Gypsum and magnesia are occasionally added to lakes; they are recognised by examining the residue left on ignition. ”

“ In consequence of its transparency, a crystalline precipitate will allow the colour of the surface upon which it is spread to appear through, hence it must be laid on much more thickly than is necessary with an opaque pigment, of which a thin coating is sufficient to make the colour of the surface beneath invisible. ”

“ The yellow precipitate so obtained has no handsome colour. A far finer product is obtained in the dry way. The process is often regarded as accompanied by particular difficulties, but in reality it is quite simple. It is only necessary in preparing this pigment to take care not to raise the temperature above a certain point, otherwise a great portion of the tin disulphide will be decomposed into sulphur and tin monosulphide. To prevent the temperature from rising too high, an addition of ammonium chloride is made. ”

“ The charcoal made from hard wood, such as maple or beech, is not well suited for pigments; the lighter and more porous the wood, the more pure is generally the shade of the charcoal, and the more easily it can be ground. When, therefore, charcoal is to be used as a pigment it should be made from wood of a porous nature; the wood of the lime, black-alder and spindle tree is particularly adapted for this purpose. ”

“ It is not easy to decide which colouring matter is united with the metallic oxide, because the organic colouring matters do not give, as a rule, decided reactions. In examining pigments of this kind it is always advisable to test at the same time a pigment of known composition, and to compare the reactions of the two. ”

“ The preparation of good black pigments demands considerable practice.
There are two methods by which carbon can be made for use as a pigment. One has been indicated, the heating of organic materials in the absence of air, i.e., their dry distillation; the second, which produces [Pg 286] the best qualities of black, consists in burning substances very rich in carbon with a very small supply of air. The product of this process, commonly known as soot, is very different in external appearance from carbon made by dry distillation. ”

“ From the preceding account of the manufacture of ultramarine it will be understood that there exist only two species of ultramarine, green and blue. In commerce a large number of different shades are found, which are not to be regarded as pure ultramarine; they owe their particular shade to additions. By mixing in a white pigment, such as barytes or starch, the paler shades are obtained; by adding a small quantity of a pure red pigment a colour with a violet tinge is produced. ”

“ Turnbull’s Blue.—When a solution of a ferrous salt is precipitated by potassium ferrocyanide, a fine blue precipitate is formed, identical in physical properties with Chinese blue, but differing in chemical constitution. It is ferrous ferrocyanide, Fe3 [Fe (CN) 6] 2. The manufacture of Turnbull’s blue is expensive; the pigment has no specially advantageous properties and is therefore seldom made.
In preparing this pigment, a solution of yellow prussiate is transformed into red prussiate by passing chlorine through it so long as it is absorbed by the liquid ”

“ Scheele’s green is a bright green pigment, which resists the action of the atmosphere tolerably well. It is easily decomposed by dilute acids; when strongly heated arsenious acid is set free, and copper arsenate and arsenide are produced. Copper arsenite is the essential constituent of the pigments sold under the name of mineral green, which is generally a mixture of copper arsenite with varying quantities of mountain green. ”

“ For this purpose small porcelain crucibles are used; broken pieces of porcelain or an iron spoon may also be employed. An ordinary spirit lamp is generally sufficient as a source of heat. In some cases a higher temperature is required, which is obtained by means of the blowpipe.
The blowpipe is an invaluable instrument in the examination of mineral pigments. By means of it, almost without reagents, the nature of the pigment can generally be ascertained. The reagents necessary in using the blowpipe are soda, borax, and the solution of a cobalt salt. ”

“ It is prepared by adding calcium chloride to a solution of copper sulphate and stirring in dilute milk of lime. A green precipitate of basic copper chloride is formed; potassium carbonate solution mixed with milk of lime is then added. The latter mixture contains caustic potash, which decomposes the copper chloride, copper hydroxide and calcium carbonate being simultaneously precipitated. Different shades may be obtained; by increasing the quantity of lime the product is paler. This pigment commonly comes into the market in the paste form. ”