in the military art, include the banners, flags, ensigns, &c. of all kinds, borne in the army or fleet. See FLAG and STANDARD.
in the Latin and Greek churches, are used to distinguish several mysteries and feasts celebrated therein.
Five colours only are regularly admitted in the Latin church: these are white, green, red, violet, and black. The white is for the mysteries of our Saviour, the feast of the Virgin, those of the angels, saints, and confessors; the red is for the mysteries and solemnities of the holy sacrament, the feasts of the apostles and martyrs; the green for the time between pentecost and advent, and from epiphany to septuagesima; the violet in advent and Christmas, in vigils, rogations, &c. and in votive masses in time of war; lastly, the black is for the dead, and the ceremonies thereto belonging.
In the Greek church, the use of colours is almost abolished, as well as among us. Red was, in the Greek church, the colour for Christmas and the dead, as black among us.
To COLOUR Strangers Goods, is when a freeman allows a foreigner to enter goods at the customhouse in his name.
COLOUR-Making, the art of preparing the different kinds of colours used in painting.
This art properly belongs to chemistry; and is one of the most curious, though least understood, parts of it. The principles on which colour-making depends are entirely different from those on which the theory of other parts of chemistry is founded; and the practical part being in the hands of those who find it their interest to conceal their methods as much as possible, it thence happens, that there is not only no distinct theory of this art, but scarce a single good receipt for making any one colour hath ever yet appeared.
The first general division of colours is into opaque Division of and transparent. By the first are meant such colours as, when laid over paper, wood, &c. cover them fully, and transparent, so as to efface any other painting or stain that might have been there before; the others are of such a nature as to leave the ground on which they are laid visible through them. Of the first kind are white-lead, red-lead, vermilion, &c.; of the latter kind are the colours used for illuminating maps, &c.
Another division is into oil-colours and water-colours; by which is meant such as are appropriated to colours, painting in oil and in water. Most of those which are proper for painting in water, are also proper for being used in oil. There is, however, this remarkable difference betwixt colours when mixed with water and with oil, that such as are quite opaque in water will become perfectly transparent in oil. Thus, blue verditer, though exceedingly opaque in water, if ground with oil, seems totally to diffuse, and will become very transparent. The same thing happens to such colours as have for their basis the oxide of tin, alabaster, or calcareous earth. The most perfectly opaque colours in oil are such as have lead, mercury, or iron, for their basis; to the latter, however, Prussian blue is an exception; for though the basis of that colour is iron, it proves quite transparent when ground with oil. In water colours, those prepared from metals, Prussian blue alone excepted, are always opaque; from vegetables or animals, transparent. Coals, however, whether vegetable or animal, are opaque both in water and oil.
Colours, again, may be considered as either simple or compound. The simple ones are such as require no compound thing to be superadded to them, in order to make a full strong colour, without regarding whether they are formed of many or few ingredients; and in this view, white-lead, red-lead, vermilion, oxides of iron, &c. are simple colours. The compound ones are formed by the union of two or more colouring substances; as blue and yellow united together to form a green, red and yellow to form an orange, a white earth or oxide with the red colour of cochineal or brazil to form a lake, &c.; and thus carmine, lake, rose-pink, Dutch-pink, English-pink, &c. are compound colours.
The last and most important division of colours is true and into true and false. By the former are meant those false colours which retain their colour under every possible variety of circumstances, without fading in the least: the others are such as do not; but either lose their colour altogether, or change to some other. What is chiefly apt to affect colours, is their being exposed to the sun in summer, and to the cold air in winter: but to this there is one exception, viz. white-lead; which, when ground with oil, retains its whiteness if exposed to the weather, but degenerates into a brownish or yellowish colour if close kept. In water this substance is very apt to lose its colour, whether exposed to the air or not. not. The great desideratum in colour-making is to produce the first kind of colours, viz. such as will not fade by exposure to the weather; and indeed it is to be regretted, that the most beautiful are in general the least permanent. It may, for the most part, however, be expected, that the more simple any colour is, the less liable will it be to change upon exposure to the air.
The great difficulty of knowing *a priori* whether a colour will fade or not, is owing to our ignorance concerning the nature of colouring substances. With all our disadvantages, however, we may observe, that whatever change of colour is produced in any substance by exposure to the sun and air, that colour to which it changes will bid fair for being permanent, and therefore ought to be employed where it can be done. Of these changes the instances are but very rare. One is the purple of the ancients, which assumed its colour by exposure to the sun, and consequently was exceedingly permanent. Another is in the solution of silver; which, being mixed with chalk, the precipitate turns to purple black where it is exposed to the sun. A third is in solutions of indigo by alkaline substances, which constantly appear green till exposed to the air by spreading them very thin, upon which they become almost instantaneously blue, and continue so ever after. Sometimes, though still more rarely, a very remarkable change of colour happens, upon mixing two vegetable juices together. Almost the only instance of this we have on the authority of Mr George Forster, who informs us, that the inhabitants of Otaheite dye their cloth of a crimson colour, by mixing together the yellow juice of a small species of fig with the greenish juice of a kind of fern. But the most remarkable alterations of colour are effected by different metallic and saline solutions mixed with certain animal or vegetable substances; and with these the colour-maker will be principally conversant.
It is a common observation in chemistry, that acids mixed with blue vegetable juices turn them red, and alkalis green. It is equally certain, though not so generally known, that acids of all kinds generally tend to heighten red colours, so as to make them approach to the scarlet or true crimson; and alkalis to darken, or make them approach to blue or purple. Mixed with yellow colours, acids also universally tend to brighten the yellow; and alkalis to turn it to an orange, and make it become more dull. But though this is very generally the case, we are not to expect that all acids are equally powerful in this respect. The nitric acid is found to heighten the most of any, and the muriatic acid the least of the mineral ones. The vegetable, as might be expected, are less powerful than the mineral acids. Thus, if with a tincture of cochineal, either in water or alcohol, is mixed the pure nitrous acid, it will change the colour to an exceeding high orange or flame colour, which it will impart to cloth. If sulphuric acid is used, a full scarlet, inclining to crimson rather than orange, is produced. With muriatic acid, a true crimson colour, bordering on purple, is the consequence. Alkalis, both fixed and volatile, change the colour to a purple, which is brighter with the volatile than the fixed alkalis.
Here it is obvious, that whatever colours are produced by the mixture of different substances together; the permanency of these colours can only be in proportion to the ability of such mixtures to resist the weather. Thus, suppose a high scarlet or orange colour is produced by means of spirit of nitre, it is plain that, was such a colour exposed to the air, it could not last, by main no longer than the spirit of nitre which produced what determined. In proportion, therefore, as the spirit of nitre was exhaled into the air, or otherwise destroyed, the colour behaved to fade, and at last to be totally destroyed; and thus, in proportion to the destructibility of the substances by which colours are produced, will be the disposition of such colours to fade, or the contrary. In this respect alkalis are much more destructible than acids, and consequently less proper for the preparation of colours. With regard to acids, the nitric seems most destructible, the sulphuric less so, and the muriatic the least of all. From the extreme fixity of the phosphoric acid and sedative salt, perhaps they might be of service in preserving colours.
As all colours, whether derived from the animal or vegetable kingdom, must be extracted either by pure water or some other liquid menstruum, they cannot be used for the purposes of painting till the colouring substance is united with some earthy or solid matter capable of giving it a body, as the workmen call it; opaque and, according to the nature of this substance, the colour will be transparent or otherwise. This basis ought to be of the most fixed and durable nature; and ought also to be of a pure white colour, and easily reducible into an impalpable powder. For this reason all earthy substances should be avoided as being acted upon by acids; and therefore, if any of these were added to heighten the colour, they would not fail to be destroyed, and their effect totally lost. Precipitates of lead, bismuth, &c., though exceedingly fine and white, ought also to be avoided, as being apt to turn black by exposure. The only substance to be chosen in preference to all others, is oxide of tin, prepared tin, either by fire or the nitric acid. This is so exceedingly refractory as not only to be unalterable by all for fine colours, acids, or the sun and weather, but even by lous, the focus of a very large burning mirror. It is besides white as snow, and capable of being reduced to an extreme degree of fineness, insomuch that it is made use of for polishing metallic speculums. For these reasons, it is the most proper basis for all fine colours. For coarse ones, the white precipitate of lead, mentioned under the article Chemistry, will answer very well. It hath a very strong body, i.e., is very opaque, and will cover well; may be easily ground fine, and is much less apt to turn black than white lead; it is besides very cheap, and may be prepared at the small expense of 3d. per pound.
If what we have just now observed is attended to, General the general method of extracting colours from any vegetable or animal substance, and fixing them on preparing proper basis, must be very easily understood. For this purpose, a quantity of oxide of tin is to be procured in proportion to the quantity of colour desired. This must be well rubbed in a glass mortar, with a little of the substance designed for brightening the colour, as alum, cream of tartar, spirit of nitre, &c., after which it must be dried, and left for some time, that the union between the two substances may be as perfect as possible. If the colour is to be a very fine one, suppose from cochineal, the colouring matter must be extracted with alcohol without heat. When the spirit is sufficiently impregnated, it is to be poured by little and little upon the oxide, rubbing it constantly, in order to distribute the colour equally through all parts of the oxide. The spirit soon evaporates, and leaves the oxide coloured with the cochineal. More of the tincture is then to be poured on, rubbing the mixture constantly as before; and thus, with proper management, may very beautiful colours, not inferior to the best carmine, be prepared at a moderate expense. If, instead of cochineal, we substitute brazil-wood, turmeric, logwood, &c., different kinds of red, yellow, and purple, will be produced. For the coarser colours, aqueous decoctions are to be used in a similar manner; only, as these are much longer in evaporating than the alcohol, very little must be poured on at a time, and the colours ought to be made in large quantity, on account of the tediousness of the process.
Hitherto we have considered only the effects of the pure and simple salts, viz., acids and alkalies, on different colours; but by combining the acids with alkalies, earths, or metals, these effects may be varied almost in infinitum; neither is there any rule yet laid down by which we can judge a priori of the changes of colour that will happen on the admixture of this or that particular salt with any colouring substance. In general, the perfect neutrals act weakly; the imperfect ones, especially those formed from metals, much more powerfully. Alum and sal ammoniac considerably heighten the colour of cochineal, brazil, turmeric, fustic, madder, logwood, &c. The same thing is done, though in a less degree, by common salt, Glauber's salt, nitre, and many other neutrals. Solutions of iron in all the acids strike a black with every one of the above-mentioned substances; and likewise with sumach, galls, and other astringents. Solutions of lead, or saccharum saturni, universally debauch red colours to a dull purple. Solution of copper changes the purple colour of logwood to a pretty good blue; and, in general, solutions of this metal are friendly to blue colours. The effects of solutions of gold, silver, and mercury, are not so well known; they seem to produce dark colours of no great beauty. The most powerful solution, however, with regard to tin the most a great number of colours, is that of tin, made in nitro-muriatic acid. Hence we may see the fallacy of Mr Delaval's hypothesis concerning colours*, that the least refrangible ones are produced by the most dense metals: for tin, which hath the least density of any metal, hath yet, in a state of solution, the most extraordinary effects upon the least refrangible colours as well as those that are most so. The colour of cochineal is changed by it into the most beautiful scarlet; a similar change is made upon the colouring matter of gum-lac. Brazil-wood is made to yield a fine purplish crimson; logwood, a beautiful dark purple; turmeric, fustic, weld, and all yellow-colouring woods and flowers, are made to communicate colours far more beautiful than can be got from them by any other method. The blue colour of the flowers of violets, eye-bright, iris, &c., are heightened so as to equal, if not excel, the blue produced by a solution of copper in volatile alkali. In short, this solution seems to be of much more extensive use in colour-making, when properly applied, than any thing hitherto thought of. It is not, however, universally serviceable. The colour of madder it totally destroys, and likewise that of safflower, changing them both to a dull orange. It likewise spoils the colour of archil; and what is very remarkable, the fine red colour of tincture of roses made with sulphuric acid is by solution of tin changed to a dirty green.
The most important consideration in colour-making Directions is to make choice of such materials as produce the most durable colours; and if these can be produced, colouring an ordinary colour from them is to be preferred to materials a bright one from those which fade sooner. In what the difference consists between the colours that fade and those which do not, is not known with any degree of certainty. From some appearances it would seem, that those substances which are most remarkable for keeping their colour, contain a viscid glutinous matter, to combined with a resinous one as to be soluble both in water and alcohol. The most durable red colour is prepared from gum-lac. This is very strongly resinous, though at the same time so far glutinous, that the colouring matter can be extracted from it by water. Next to gum-lac are madder-roots and cochineal. The madder is an exceedingly penetrating substance, inasmuch that, when given to animals along with their food, it tinges their bones of a deep red colour. Its colouring matter is soluble both in water and alcohol. Along with the pure red, however, there is in madder a kind of viscid astringent substance, of a dark brown colour, which seems to give the durability to the whole. The colouring matter of cochineal, though soluble both in water and alcohol, is very tenacious and mucilaginous, in which it bears some resemblance to the purpura of the ancients, which kept its colour exceedingly well. Where the colours are fugitive, the tinging substance seems to be too resinous or too mucilaginous. Thus the colours of brazil, turmeric, &c., are very resinous, especially the latter, inasmuch that the colouring matter of turmeric can scarcely be extracted by water. Both these are perishable, though beautiful colours; and much more are the red, purple, and blue flowers, commonly to be met with. These seem to be entirely mucilaginous, without the least quantity of resinous matter. The yellow flowers are different, and in general keep their colour pretty well. Whether it would be possible, by adding occasionally a proper quantity of gum or resin, to make the fugitive colours more durable, hath not yet been tried, but seems to have some probability. What tends a little to confirm this, is a process given by M. Hellot for imparting durability to method of the colour of brazil. It consists only in letting decoctions of the wood stand for some time in wooden casks the duration of which till they grow stale and ropy. Pieces of woollen cloth now dyed in the liquor acquired a colour so durable, that they were not in the least altered by exposure to the air during four months in the winter season. Whether this change in the durability of the colour was effected by the ropelings following the fermentation, tion, or by some other cause, or whether the experiment can be at all depended upon, must be referred to future observation.
Having thus collected all that can as yet be depended upon for establishing a general theory of colour-making, we shall now proceed to give an account of the different pigments generally to be met with in the colour-shops.
1. Black. These are lamp-black, ivory-black, blue-black, and Indian-ink. The first is the finest of what are called the foot-blacks, and is more used than any other. Its preparation is described in the Swedish Transactions for the year 1754, as a process dependent on the making of common resin: the impure resinous juice collected from incisions made in pine and fir trees, is boiled down with a little water, and strained whilst hot through a bag; the dregs and pieces of bark left in the strainer are burnt in a low oven, from which the smoke is conveyed through a long passage into a square chamber, having an opening on the top on which is a large sack made of thin woollen stuff: the foot, or lamp-black, concretes partly in the chamber, from whence it is swept out once in two or three days, and partly in the sack, which is now and then gently struck upon, both for shaking down the foot, and for clearing the interstices betwixt the threads, so as to procure a sufficient draught of air through it. In this manner lamp-black is prepared at the turpentine houses in England, from the dregs and refuse of the resinous matters which are there manufactured.
On this subject Dr Lewis has some curious observations. "The foot (says he) arising in common chimneys, from the more oily or resinous woods, as the fir and pine, is observed to contain more dissoluble matter than that from the other woods; and this dissoluble matter appears, in the former, to be more of an oily or resinous nature than in the latter, alcohol extracting it most powerfully from the one, and water from the other. The oiliness and solubility of the foot seeming therefore to depend on those of the subject it is made from, it has been thought that lamp-black must possess these qualities in a greater degree than any kind of common foot. Nevertheless, on examining several parcels of lamp-black, procured from different shops, I could not find that it gave any tincture at all, either to alcohol or to water.
Suspecting some mistake or sophistication, or that the lamp-black had been burnt or charred, as it is to fit it for some particular uses, I prepared myself some foot from linseed-oil, by hanging a large copper pan over the flame of a lamp to receive its smoke. In this manner the more curious artists prepare lamp-black for the nicer purposes; and from this collection of it from the flame of a lamp, the pigment probably received its name. The foot so prepared gave no tincture either to water or to alcohol, any more than the common lamp-black of the shops. I tried different kinds of oily and resinous bodies with the same result; even the foot obtained from fish oils and tallow did not appear to differ from those of the vegetable oils and resins. They were all of a finer colour than the lamp-black commonly sold.
"Some foot was collected in like manner from fir and other woods, by burning small pieces of them slowly under a copper-pan. All the foots were of a deeper black colour than those obtained from the same kinds of wood in a common chimney; and very little, if at all, inferior to those of the oils: they gave only a just discernible tincture to water and alcohol, while the foots of the chimney imparted a strong deep one to both. The foot of mineral bitumens, in this close way of burning, appears to be of the same qualities with those of woods, oil, and resins: in some parts of Germany, great quantities of good lamp-black are prepared from a kind of pit-coal.
"It appears, therefore, that the differences of foots do not depend altogether on the qualities of the subjects, but in a great measure on the manner in which the subject is burnt, or the foot caught. The foots produced in common chimneys, from different kinds of wood, resinous and not resinous, dry and green, do not differ near so much from one another, as those which are produced from one kind of wood in a common chimney, and in the confined way of burning above mentioned.
Ivory-black is prepared from ivory or bones burnt in a close vessel. This, when finely ground, forms a black, more beautiful and deeper colour than lamp-black; but in the common methods of manufacturing, it is so much adulterated with charcoal-dust, and so grossly levigated, as to be unfit for use. An opaque deep black for water-colours, is made by grinding ivory-black with gum water, or with the liquor which settles from the whites of eggs after they have been suffered to stand a little. Some use gum water and the whites of eggs together, and report, that a small addition of the latter makes the mixture flow more freely from the pencil, and improves its glossiness. It may be observed, however, that though ivory-black makes the deepest colour in water as well as in oil painting, yet it is not on this account always to be preferred to other black pigments. A deep jet-black colour is seldom wanted in painting; and in the lighter shades, whether obtained by diluting the black with white bodies, or by applying it thin on a white ground, the particular beauty of the ivory-black is in a great measure lost.
Blue black is said to be prepared from the burnt stalks and tendrils of the vine. These, however, the colour-makers seldom give themselves the trouble of procuring, but substitute in its place a mixture of ivory-black, and the common blue used for clothes.
Indian-ink is an excellent black for water-colours; Indian-ink. It hath been discovered by Dr Lewis to consist of a mixture of lamp-black and common glue. Ivory black, or charcoal, he found to answer equally well, provided they were levigated to a sufficient degree of fineness, which indeed requires no small trouble.
2. White. The white colours commonly to be met with are, white-flake, white-lead, calcined hartshorn, colours, pearl-white, Spanish-white, egg-shell-white, and nitrate of bismuth. The flake-white and white-lead are properly the same. The preparation of the former is kept a secret; the method of preparing the latter is described under Chemistry, No 1856. These are the only whites that can be used in oil, all the rest being transparent unless they are laid on with water. Calcined hartshorn is the most useful of the earthy whites, as being the least alkaline. Spanish white is only finely finely prepared chalk. Pearl-white is made from oyster-shells; and egg-shell white from the shells of eggs. All these, by their attraction for acids, must necessarily destroy such colours as have any acid or metallic salt in their composition. The nitrate of bismuth is apt to turn black, as are also flake-white and white-lead, when used in water. The white precipitate of lead recommended under Chemistry, No 1856, is greatly superior as a water-colour to all these, being perfectly free of any alkaline quality, and not at all apt to lose its own colour, or to injure that of other substances. It is a carbonate of lead.
3. Red. The red colours used in painting are of two sorts, viz., those which incline to the purple, and such as are of a full scarlet, and tend rather to the orange. The first are carmine, lake, rose-pink, red-ochre, and Venetian-red. The second are vermilion, red-lead, scarlet-ochre, common Indian red, Spanish-brown, and terra di Sienna, burnt.
We have already laid down some general rules for the preparation of carmine and lake. Particular receipts have been delivered with the greatest confidence for making these fine colours; but all of them must necessarily prove ineffectual, because an earthy basis is recommended for striking the colour upon. From the principles of chemistry, however, we are certain, that if nitric acid, or solution of tin, is made use of for brightening a colour made with any earthy basis, it must infallibly be destroyed by that basis, by reason of its alkaline quality. Carmine is the brightest and most beautiful red colour known at present; the best comes from France. Lake differs from it in being capable of mixture with oil, which carmine is not, unless with great difficulty. The former is also much more inclined to purple than carmine. This last quality, however, is reckoned a defect; and accordingly, the more that lake approaches to the scarlet or true crimson, the more it is valued. On dropping solution of tin into an aqueous tincture of brazil wood, a beautiful precipitate falls, of a purplish crimson colour. This may be very well substituted in place of the dearer lakes on many occasions.
Rose-pink is a very beautiful colour, inclining more to the purple than scarlet. It seems to be made of chalk, coloured with a decoction of brazil-wood, heightened by an alkaline salt; for which reason it is exceedingly perishable, and but little esteemed. The colour might be made much more durable, as well as better, by employing for a basis the white precipitate of lead above-mentioned, and brightening it with solution of tin.
Red ochre and Venetian red differ in nothing from the colcothar of vitriol well calcined. The oxides of iron may be made to appear either purplish, or inclining to the scarlet, according to the manner in which the calcination is performed. If the matter is perfectly deprived of its phlogiston, and subjected to an intense fire, it always turns out red; but the mixture of a small quantity of inflammable matter gives it a purplish cast. Hence various paints are kept in the shops under different names, which yet differ from each other only in the flight circumstances above mentioned; and such are the scarlet-ochre, Spanish brown, and terra di Sienna burnt. It is remarkable, that the oxides of iron never show their colour till they become cold.
Colcothar of vitriol, while hot, always appears of a very dark dusky purple.
Of the preparation of vermilion and red-lead, an account is given under the article Chemistry, No 1701. 1832. These are very durable colours: the first is the best red used in oil painting, but does not answer well in water; the other is rather an orange; and, like other preparations of lead, is in some cases apt to turn black.
4. Orange. The only true orange-coloured paints are red orpiment and orange lake. The frit is a sub-colours, lime formed of arsenic and sulphur; the other may be prepared from turmeric infused in alcohol having its colour struck upon oxide of tin, and brightened by a solution of that metal. All the shades of orange, however, may be extemporaneously prepared by mixing red and yellow colours together, in due proportions.
5. Yellow. The yellow paints most commonly in use are, king's-yellow, Naples-yellow, Dutch-pink, English-pink, masticot, common orpiment, yellow-ochre, terra di Sienna unburnt, and turpith mineral.
King's-yellow is evidently an arsenical preparation. Its colour is exceedingly beautiful, but apt to fade; on which account, and its great price, it is seldom used.
Naples-yellow was for a long time thought to be a preparation of arsenic, but is now discovered to have lead for its basis. It is therefore apt to turn black and lose its colour, which makes it less valuable. It is nevertheless used in preference to king's-yellow, on account of its inferiority in price. This colour is particularly liable to be spoiled by iron when moist, and therefore should never be touched by that metal unless previously ground in oil.
Dutch-pink is said to be prepared by striking the colour of yellow berries upon finely levigated chalk. But of this there is great reason to doubt; the basis of Dutch-pink seems much more hard and gritty than chalk, and its colour more durable than those struck upon that earth usually are. Very good yellows may be prepared with the white precipitate of lead, formerly mentioned, by using either yellow berries, fusio, or any other substance capable of yielding that colour. English pink is paler than the Dutch, and keeps its colour greatly worse.
Masticot is prepared by calcining white-lead till it affumes a yellowish colour. It is not apt to change, but the colour is so dull that it is seldom used either in oil or water.
Common orpiment is a pretty bright greenish-yellow, prepared by subliming arsenic with sulphur. Its nauseous smell, which is greatly increased by grinding in oil, makes it very disagreeable; nor does it keep its colour for any length of time. That kind of orpiment least inclined to green is to be preferred for the purposes of painting.
Yellow-ochre and terra di Sienna are ferruginous earths, capable of becoming red by calcination. Green vitriol precipitated by lime may be advantageously substituted for either of them. See Chemistry.
Turpith mineral is but little used in painting, though its fine yellow colour seems greatly to recommend it. This preparation is in all probability very durable; and and should seem therefore worthy of a preference either to king's or Naples yellow. See Chemistry Index.
Gamboge is a paint that can only be used in water, and is the most common yellow made use of for colouring maps, &c.; but for this it is not very proper, being neither quite transparent, nor very durable.
6. Green. The only simple green colour that hath a tolerable degree of brightness is verdigris, or preparations of it. This, however, though a very beautiful colour, is far from being durable. It is improved in colour, though not in durability, by dilution and crystallization in distilled vinegar, in which state it is called distilled verdigris. A more durable water colour is made by dissolving the verdigris in cream of tartar, or rather the pure tartaric acid; but in oil this is found to be equally fugitive with the verdigris itself. See Chemistry Index.
Compound greens are either made of Prussian or some other blue, mixed with yellow; but in whatever way these colours can be compounded, the beauty of the green produced is greatly inferior to distilled, or even common verdigris. The tartaric solution of verdigris, mixed with a little gamboge, is the best transparent green water-colour we have had an opportunity of trying; and a mixture of Prussian blue and tartrate-mineral is probably the best opaque one.
Sap green is a simple colour, but exceedingly inferior to distilled verdigris, or even to the tartaric solution of verdigris with gamboge. It is prepared from the juice of unripe buckthorn berries evaporated to the consistence of a gum. Its green colour is greatly inclined to yellow. A kind of compound green has been sometimes used, called Prussian green, which consists only of Prussian blue and yellow ochre. It has no beauty, nor is it durable. It is prepared as Prussian blue, only not pouring on any muriatic acid to dissolve the ochreous sediment which falls at the same time.
Another green sometimes used is called terre verte. This is a native earth, probably impregnated with copper. It is of a bluish-green colour, much of that tint called sea-green. It is gritty, and therefore must be well levigated before it is used. Its colour is durable, but not very bright.
7. Blue. The blue colours are ultramarine, Prussian blue, verditer, smalt, bice, and indigo. Of these the ultramarine is the finest, but its great price hinders it being much used. It is a preparation from lapis lazuli; is an exceeding bright colour, and never fades with whatever substance it is mixed. It is now, however, in a great measure superseded by Prussian blue, to the disadvantage of painting in general; as Prussian blue, though very beautiful, is far from being durable. For an account of its preparations see the article Ultramarine.
The process for making Prussian blue is described, and its nature fully considered, under Chemistry, No. 774; so that it is sufficient here to observe, that Prussian blue is to be accounted of the best quality when it is deep, bright, and not inclined to purple. It ought to be tried by mixture with white lead, as the brightness of the colour will appear much more when diluted than when concentrated in the lumps of the blue itself.
The preparation of blue verditer is kept a secret, and the best chemists have been puzzled to find out the method. The colour is exceedingly bright, and has a considerable tinge of green. A method of preparing a colour equally beautiful, and agreeing in all respects with what is sold in the shops, except that of effervescing with acids, we have found to be as follows: Dissolve copper in strong caustic alkali, until the liquid has assumed a very deep blue colour; and the deeper this colour is, the finer will your verditer be. When the menstruum has dissolved as much of the metal as it can take up, it is to be poured out into a broad and well glazed earthen pan, held over a very gentle fire; and from the moment it is put on, the liquor is to be continually agitated with a wooden spatula, so that the liquor may be heated as equally as possible. The whole secret consists in properly regulating the degree of heat; for if it exceeds the due proportion ever so little, the verditer will turn out of a dirty green. The proper degree is about 92° of Fahrenheit's thermometer. In this gentle heat the alkali slowly evaporates; and in proportion to its doing so the verditer falls to the bottom. After it is once formed, freed from the alkaline liquor, and dried, it can bear the effusion of boiling water without the least injury. Dr Priestley, in his sixth volume, takes notice, that a solution of copper in volatile alkali affords a blue precipitate by heat, but without taking notice of the requisites for its success. In making this preparation it is necessary to dissolve copper in its metallic state; for the solution of any oxide will not yield a blue but a green colour. This colour is durable in water, but dissolves in oil, and has then all the inconveniences of verdigris above mentioned.
Smalt is glass coloured with zaffre, a preparation from cobalt. It is commonly so grossly powdered that it cannot be used in painting, and its texture is so fine and hard that it cannot easily be levigated. Its colour is smalt, exceedingly bright and durable; so that when finely levigated it is used instead of ultramarine. The most proper materials for levigating this substance seem to be the plates of M. Reaumur's porcelain recommended by Dr Lewis. See Chemistry Index. For the preparation and qualities of bice, see the articles AMENUS Lupis and BICE.
Indigo is but little used in painting either in oil or water, on account of the dulness of the colour. It requires no other preparation than being washed over. Its goodness is known by the darkness and brightness of the colour. See INDIGO.
8. Purple. The only simple colour of this kind used Purple at present is celesthar of vitriol. A beautiful purple colour lake may be prepared from logwood by means of solution of tin; but this method of preparing colours is very little known as yet.
9. Brown. The brown colours are, bistre, brown-brown, ochre, Cologne-earth, umber, and brown-pink. Under these, the article BISTRE is given a process for making that colour, by infusing foot in water, pouring off the tincture, and then evaporating it to an extract; but Dr Lewis's opinion, with M. Landois in the French Encyclopédie, that the foot is either boiled in water, bistre, or ground with a little liquid of some kind into a smooth paste; it is then diluted with more water, and after standing for about half an hour till the groser substance of the foot has settled, the liquor is poured off into into another vessel, and set by for two or three days, that the finer parts may fall to the bottom, and this fine matter is the biftre. This is a very useful colour in water, being exceedingly fine, durable, and not apt to spoil any other colours with which it is mixed. The brown pink is said to consist of chalk tinged with the colouring matter of fustic, heightened by fixed alkaline salts. It is therefore very perishable, and is seldom used. The other browns are a kind of ochreous earths; for a description of which see their proper articles.
Having now considered most of the colouring substances usually to be met with in the shops, we shall next take notice of some attempts that have been made to produce all the different colours from vegetables, after the manner of lakes; though, though the methods hitherto tried have for the most part failed of success, may perhaps some time or other be found applicable to valuable purposes.
From infusions of astringent vegetables mixed with green vitriol, is produced a deep black liquor of very extensive use in dyeing*. The substances which produce the deepest blacks are galls and logwood. When a decoction or infusion of the galls is dropped into a solution of the vitriol largely diluted with water, the first drops produce bluish or purplish red clouds, which soon mingling with the liquor, turn it uniformly of their own colour. It seems to be on the quality of the water that this difference in the colour depends. With distilled water, or the common spring waters, the mixture is always blue. If we previously dissolve in the water the most minute quantity of any alkaline salt, too small to be discovered by any of the common means by which waters are usually tried, or if the water is in the least putrid, the colour of the mixture proves purple or reddish. Rain-water, caught as it falls from the clouds in an open field in clean glass vessels, gives a blue; but such as is collected from the tops of the houses, grows purple with the mixture of vitriol and galls: from whence it may be presumed, that this last has contracted a putrid tendency, or received an alkaline impregnation, though so slight as not to be sensible on other ways of trial.
Both the purple and blue liquors, on adding more of the astringent infusion, deepen to a black, more or less intense according to the nature of dilution: if the mixture proves of a deep opaque blackness, it again becomes bluish or purplish when further diluted. If suffered to stand in this diluted state for two or three days, the colouring matter settles to the bottom in form of a fine black mud, which by slightly shaking the vessel is diffused again through the liquor, and tinges it of its former colour. When the mixture is of a full blackness, this separation does not happen, or in a far less degree; for though a part of the black matter precipitates in standing, yet so much remains dissolved, that the liquor continues black. This suspension of the colouring substance, in the black liquid, may be attributed in part to the gummy matter of the astringent infusion increasing the consistence of the watery fluid; for the separation is retarded in the diluted mixture by a small addition of gum arabic. If the mixture either in its black or diluted state is poured into a filter, the liquor passes through coloured; only a part of the black matter remaining on the filter. The filtered liquor on standing for some time becomes turbid and full of fine black flakes; being freed from these by a second filtration, it again puts on the same appearance: and thus repeatedly till all the colouring parts are separated, and the liquor has become colourless.
Dr Lewis, from whose Philosophical Commerce of Arts this account is taken, further informs us, that this colouring matter, when separated from the liquor and dried, appeared of a deep black, which did not seem to have suffered any change from the air by exposure for upwards of four months. Made red hot, it glowed and burnt, but did not flame, and became a rusty brown powder, which was readily attracted by a magnetic bar; though in its black state the magnet had no effect upon it. Sulphuric acid, diluted with water and digested on the black powder, dissolved the greatest part of it, leaving only a very small quantity of whitish matter. Solution of pure fixed alkaline salt dissolved very little of it: the liquor received a reddish brown colour, and the powder became blackish brown. This residuum was attracted by the magnet after being red hot, though not before: the alkaline tincture, passed through a filter, and mixed with a solution of green vitriol, struck a deep brownish black colour, nearly the same with that which results from mixing with the vitriolic solution an alkaline tincture of galls.
It hath also been attempted to produce black from a combination of other colours; as green may be produced from a mixture of blue and yellow. M. le Blon, in his Harmony of Colours, gives a method of forming black, by mixing together the three colours called primitive, viz. blue, red, and yellow; and M. Caffet, in his Optique des Couleurs, published in 1740, says that this compound black has an advantage in painting, above the simple ones, of answering better for the darkening of other colours. Thus, if blue, by the addition of black, is to be darkened into the colour called blue-black, the simple blacks, according to him, if used in sufficient quantity to produce the requisite deepness, conceal the blue, while the compound blacks leave it distinguishable. Le Blon does not mention the proportion of the three colours necessary for producing black. Caffet directs 15 parts of blue, five of red, and three of yellow; but takes notice that these proportions are rather speculatively than practically just, and that the eye only can be the true judge; our colours all being very imperfect, and our pigments or other bodies of one denomination of colour being very unequal in their degree of intensity. He observes, that the pigments should all be of the deepest and darkest kind; and that instead of taking one pigment for each colour, it is better to take as many as can be got; for the greater discord there is of heterogeneous and discordant drugs, the more true and beautiful, he says, will the black be, and the more capable of uniting with all other colours, without suppressing them, and even without making them tawny.
Dr Lewis acquaints us, that by mixing different blue, red, and yellow colours, he has not been able to produce a perfect black; but has often obtained from them very dark colours, such as may be called brown-blacks, or gray-blacks; such as we commonly see in the dark dark parts of paintings, and such as the charcoal and foot blacks appear when diluted a little. The ingredients being each of a dark deep colour is a very necessary condition; for bright blues, bright reds, and bright yellows, mixed in such proportions that neither colour prevailed, produced only a gray. In effect, all compositions of this kind, physically considered, can be no other than grays, or some of the intermediate tints between whitened and darkened; and these grays will be so much the lighter or darker as the component colours of themselves are bright or dark.
With regard to the extraction of the colouring matter from the different kinds of vegetables commonly to be met with of all colours, this would certainly be a very valuable acquisition, could the colours so procured be made durable. On this subject nothing hath yet appeared more satisfactory than what is delivered by Dr Lewis in his notes on Neumann's chemistry. His observations are curious, but promise very little success to any who shall attempt to fix these vegetable colours.
"Among the infinite variety of colours (says he), which glow in the flowers of plants, there are very few which have any durability, or whose fugitive beauty can be arrested by art, so as to be applied to any valuable purposes. The only permanent ones are the yellow, the red, the blue; and all the intermediate shades of purple, crimson, violet, &c. are extremely perishable. Many of these flowers lose their colours on being barely dried; especially if they are dried slowly, as has been usually directed, in a shady, and not warm place. The colours of all of them perish on keeping even in the closest vessels. The more hastily they are dried, and the more perfectly they are secured from the air, the longer they retain their beauty. The colouring matter extracted and applied on other bodies is still more perishable: often times it is changed or destroyed in the hands of the operator.
"The colour of many blue flowers is extracted by infusion in water; but there are some from which water gains only reddish or purplish blue. Of those that have been tried there is not one which gives any blue tincture to spirituous liquors: some give no colour at all, and some a reddish one. The juice pressed out from the fresh flowers is for the most part blue. The blue juices and infusions are changed red by all acids. The muriatic acid seems to strike the most florid red. The flowers themselves, macerated in acid liquors, impart also a deep red tincture. Alkalies, both fixed and volatile, and lime-water, change them to a green. Those infusions of the juices which have nothing of the native colour of the flowers, suffer the same changes from the addition of acid and alkaline liquors: even when the flowers have been kept till their colour is lost, infusions made from them acquire still a red colour from the one, and a green from the other, though in a less degree than when the flowers were fresh. The red colour produced by acids is scarcely more durable than the original blue: applied upon other bodies and exposed to the air, it gradually degenerates into a faintish purple, and at length disappears, leaving hardly any stain behind. The green produced by alkalies changes to a yellow, which does not fade so soon. The green, by lime-water, is more permanent and more beautiful; green lakes, prepared from these flowers by lime-water, have been used as pigments by the painter. The flowers of cyanus have been greatly recommended, as affording elegant and durable blue pigments; but I have never been able to extract from them any blue colour at all. They retain their colour indeed, when hastily dried, longer than some other blue flowers; but they communicate nothing of it to any kind of menstruum. Infusions of them in watery, spirituous, and oily liquors, are all of them more or less of a reddish cast, without any tendency to blue. Alum, which is said to heighten and preserve their blue colour, changes it, like that of other blue flowers, to a purplish red; acids to a deep red; alkalies and lime-water to a green: solution of tin added to the watery infusion, turns it to a fine crimson; on standing, a beautiful red facula subsides, but it loses all its colour as soon as it is dry. The watery infusion, insipidified to the consistence of an extract, appears of a dark reddish brown: an extract made with rectified spirit is of a purplish colour. The colour of both extracts spread thin, and exposed to the air, quickly fades. The flowers employed in these experiments were those of the common blue-bottle of the corn-fields.
"Red flowers readily communicate their own red colour to watery menstrua; among those that have been tried, there is not one exception. Those of a full red colour give to rectified spirit also a deep red tincture, brighter, though somewhat paler, than the watery infusion: but the lighter red flowers, and those which have a tendency to purplish, impart very little colour to spirit, and seem to partake more of the nature of the blue flowers than of the pure red. Infusions of red flowers are supposed to be heightened by acids, and turn green by alkalies, like those of the blue; but this is far from being universal. Among those I have examined, the rose-colours and purplish reds were changed nearly in the same manner as the blues; but the full deep reds were not. The deep infusion of red poppies is changed by alkalies, not to a green, but to dusky purple.
"The colours of yellow flowers, whether pale or deep, are in general durable. Many of them are as much so, perhaps, as any of the native colours of vegetables. The colour is extracted both by water and by spirit. The watery infusions are the deepest. Neither alkalies nor acids alter the species of the colour; though both of them vary its shade; acids rendering it paler, and alkalies deeper; alum likewise considerably heightens it, though not so much as alkalies. An infusion of the flowers, made in alkaline ley, precipitated by alum, gives a durable yellow lake. In some of the deep reddish yellow, or orange-coloured flowers, the yellow matter seems to be of the same kind with that of the pure yellow flowered, but the red to be of a different kind from the pure red ones; watery menstrua take up only the yellow, and leave the red, which may afterwards be extracted by alcohol, or by water acuated by fixed alkaline salt. Such particularly are the saffron-coloured flowers of carthamus. These, after the yellow matter has been extracted by water, are said to give a red tincture to ley; from which, on standing at rest for some time, a deep bright red facula subsides; called from one of the names..." names of the plant which produces it, safflower; and from the countries whence it is commonly brought to us, Spain/bred, and China lake. This pigment impregnates alcohol with a beautiful red tincture, but communicates no colour to water. I have endeavoured to separate, by the same treatment, the red matter of some of the other reddish yellow flowers, as those of garden marigold, but without success. Plain water extracted a yellow colour, and alkaline ley extracted afterwards only a paler yellow; though the digestions were continued till the flowers had lost their colour, the tinctures were no other than yellow, and not so deep as those obtained from the pure yellow flowers. The little yellow flocculi, which in some kinds of flowers are collected into a compact round disc, as in the daisy and corn marigold, agree, so far as they have been examined, with the expanded yellow petals. Their colour is affected in the same manner by acids, by alkalis, and by alum; and equally extracted by water and by spirit. But the yellow farina, or fine dust, lodged on the tips of the stamina of flowers, appears to be of a different kind. It gives a fine bright yellow to spirit, and a duller yellow to water; the undissolved part proving in both cases of a pale yellowish white. Both the watery and spirituous tinctures were heightened by alkaline liquors, turned red by acids, and again to a deep yellow on adding more of the alkali: I know no other vegetable yellow that is turned red by acids.
"White flowers are by no means destitute of colouring matter. Alkaline lixivia extract from some of them a green tincture, and change their colourless expressed juices to the same colour; but I have not observed that they are turned red by acids. The flowers of the common wild convolvulus or bind-weed, which in all their parts are white, give a deep yellow or orange tincture to plain water; which, like the tinctures of flowers that are naturally of that colour, is rendered paler by acids, heightened a little by alum, and more considerably by alkaline salts. The vapours of the volatile sulphuric acid, or of burning sulphur, which whiten or destroy the colour of the coloured flowers, make no change in the white.
"The red juices of fruits, as currants, mulberries, elder-berry, morello, black cherries, &c., gently inspissated to dryness, dissolve again almost totally in water, and appear nearly of the same red colour as at first. Rectified spirit extracts the tingling particles, leaving a considerable portion of mucilaginous matter undissolved; and hence the spirituous tincture proves of a brighter colour than the watery. The red solutions, and the juices themselves, are sometimes made dull, and sometimes more florid by acids, and generally turned purplish by alkalis. The colours of these juices are for the most part perishable. They resist, indeed, the power of fermentation, and continue almost unchanged, after the liquor has been converted into wine; but when the juice is spread thin upon other bodies, efficated, and exposed to the air, the colour quickly alters and decays; the bright lively red changes the soonest: the dark dull red stain from the juice of the black cherry, is of considerable durability. The fruit of the American opuntia or prickly pear, the plant upon which the cochineal insect is produced, is perhaps an exception: This bright red fruit, according to Labat, gives a beautiful red dye. Some colour experiments, however, made upon the juice of that making, fruit, as brought into England, did not promise to be of any great advantage; but the particulars I cannot now recollect.
"The ripe berries of buckthorn stain paper of a green colour. From these is prepared the substance called fip green, a pigment sufficiently durable, readily soluble in water, but not miscible with oil. The berries dried while green, and macerated in alum-water, are said to yield a yellow pigment; and when they have grown over ripe so as to fall off spontaneously, a purple one. It is said that the berry of the heliotropium tricoccum, which grows wild about Montpelier, stains paper of a green colour, and that this green turns presently to a blue: that the common blue paper receives its colour from this juice; and that the red rags called turnful, employed for colouring wines and other liquors, are tinctured by the same juice turned red by acids. According to M. Nifole of the French academy of sciences (as quoted by Savary in his Dictionnaire de Commerce), the following juice is obtained, not from the berries, but from tops of the plant gathered in August, ground in mills, and then committed to the press. The juice is exposed to the sun about an hour, the rags dipped in it, dried in the sun, moistened by the vapour which arises during the flacking of quicklime with urine, then dried again in the sun, and dipped again in the juice. The Dutch and others are said to prepare turnful rags, and turnful in the malt, from different ingredients, among which archil is a principal one.
"In some plants, peony for instance, the seeds at a certain point of maturity are covered with a fine shining red membrane. The pellicles of the seeds of a certain American tree afford the red matter brought into Europe under the names of annatto, orelean, and raucous*. Mr Pott, in the Berlin Memoirs for the year 1752, mentions a very extraordinary property of this concrete. 'With sulphuric acid it produces a blue colour, of extreme beauty; but with this capital defect, that all salts and liquors, and even common water, destroy it.' The specimen of annatto, which I examined, was not sensibly acted upon by sulphuric acid; it received no change in its own colour, and communicated none to the liquor. Nor did any visible changes ensue upon dropping the acid into tincture of annatto made in water, or in spirit.
"The green colour of the leaves of plants is extracted by rectified spirit of wine and by oils. The leaves, from spirituous tinctures are generally of a fine deep green, even when the leaves themselves are dull-coloured, or yellowish, or hoary. The colour, however, seldom continues long even in the liquor; much less when the tingling matter is separated in a solid form, and exposed with a large surface to the air. The editor of the Wirtemberg Pharmacopoeia observes, that the leaves of acanthus, frankincense, or bear's-breath, give a more durable green tincture to spirit than those of any other herb. Alkalis heighten the colour both of the tinctures and green juices; acids weaken, destroy, or change it to a brownish: lime water improves both the colour and durability: by means of lime, not elegant green lakes are procurable from the leaves of acanthus, lily of the valley, and several other plants. There... There are very few herbs which communicate any share of their green colour to water; perhaps none that give a green of any considerable depth. It is said, however, that the leaves of some plants give a green dye to woollen, without the addition of any other colouring matter; particularly those of the wild cervil, or cow-weed, the common ragwort, and devil's-bit. The leaves of many kinds of herbs and trees give a yellow dye to wool or woollen cloth that has been previously boiled with a solution of alum and tartar. Weld, in particular, affords a fine yellow, and is commonly made use of for this purpose by the dyers, and cultivated in large quantity in some parts of England. There is no colour for which we have such plenty of materials as for yellow. M. Hellot observes that all leaves, barks, and roots, which on being chewed discover a slight astringency, as the leaves of the almond, peach, and pear trees, ash bark, (especially that taken off after the first rising of the sap in the spring), the roots of wild patience, &c., yield durable yellows, more or less beautiful according to the length of time that the boiling is continued, and the proportions of alum and tartar in the preparatory liquor: that a large quantity of alum makes these yellows approach to the elegant yellow of weld: that if the tartar is made to prevail, it inclines them to an orange; that if the roots, barks, or leaves be too long boiled, the yellow proves tarnished, and acquires shades of brown." See the article DYEING.
The most capital preparations from the leaves of plants are those of indigo, and weld; which are both very much used in dyeing, though the first only in painting.* Both the indigo and woad plants, give out their colour, by proper management, to water, in form of a blue faecula or lake. M. Hellot suspects that a like blue faecula is procurable from many other vegetables. Blue and yellow blended together, compose a green. He supposes the natural greens in vegetables to be compounded in like manner of these two colours; and that the blue is oftentimes the most permanent, so as to remain entire after the putrefaction or destruction of the yellow. The theory is specious, and perhaps just: we know of no other that accounts in any degree for the production of the indigo and woad blue. Dr Lewis, however, informs us, that he never was able to produce the least appearance of either blue or yellow from any of the plants he tried by treating them in the manner used for the preparation of indigo.
There are sundry mosses, which in their natural state, like the indigo and woad plants, promise nothing of the elegant colours that can be extracted from them by art. The most remarkable of these is archil; for the preparation of which, and the colours that may be produced from it,* see the article. Linnaeus suspects that there are several other more common mosses from which valuable colours might be extracted: a quantity of sea-moss, having rotted in heaps on the shore, he observed the liquor in the heaps to be as red as blood; the sea-water, the sun, and the putrefaction, having brought out the colour. Mr Kalm, in an appendix to Linnaeus's paper, in 1745, mentions two sorts of mosses actually employed in Sweden for dyeing woollen red: one is the lichenoides coralliforme apicibus coccineis of Ray's Synopsis; the other the lichenoides tartareum, farinacum, scutellatum umbone fusco, of Dillenius. This last is a white substance like meal clotted together, found on the sides and tops of hills. It is shaved off from the rocks after rain, purified from the stony matters intermixed with it by washing with water, then dried in the sun, ground in mills, and again washed and dried: it is then put into a vessel with urine, and set by for a month: a little of this tincture added to boiling water makes the dyeing colour. In the same Transactions for the year 1754, there is an account of another moss which, prepared with urine, gives a beautiful and durable red or violet dye to wool and silk. This is the lichen foliacus umbilicatus subtus lacunensis, Linn. flor. Suec. It grows upon rocks, and is readily distinguishable from others of that class, by looking as if burnt or parched, consisting of leaves as thin as paper, convex all over on the upper side, with corresponding cavities underneath, adhering firmly to the stones by a little root under the leaves, and coming offunder, when dry, as soon as touched. It is gathered after rain, as it then holds best together, and parts easiest from the stone. In France, a crustaceous moss, growing upon rocks in Auvergne, is prepared with lime and urine, and employed by the dyers as a succedaneum for the Canary archil, to which it is said to be very little inferior. M. Hellot relates, that he has met with several other mosses, which on being prepared in the same manner, acquire the same colour. The most expeditious way, he says, of trying whether a moss will yield an archil or not, is to moisten a little of it with a mixture of equal parts of spirit of sal ammoniac and strong lime water, and add a small proportion of crude sal ammoniac. The glass is then to be tied over with a piece of bladder, and set by for three or four days. If the moss is of the proper kind, the little liquor which runs from it upon inclining the vessel, will appear of a deep crimson colour; and this afterwards evaporating, the plant itself acquires the same colour. Dr Lewis informs us, that he has tried a good number of the common mosses, many both of the crustaceous and foliaceous kind, and not a few of the fungi; as also the herbs chamomile and milfoil, which yield a blue essential oil; and thyme, whose oil becomes blue by digestion with volatile spirits; but never met with any that yielded a colour like archil. Most of them gave a yellow or reddish brown tincture. A few gave a deep red colour to the liquor; but when diluted, it showed a yellowish cast, and when applied on cloth it gave only a yellowish red.
To these observations we shall only add, that though in general the blue colours of flowers are exceedingly perishable, there seem to be at least two exceptions probably to this rule; for the blue flowers of iris, or flower-yield perpetual luce, and those of columbine, when treated with solution of tin, yielded a colour tolerably permanent. Indeed when experiments are made with a view to extract the colour from any part of a vegetable, it will always be proper to try whether it can bear a mixture with this solution. If the colour is not destroyed by it, there is a very great probability that the solution will, by proper management, preserve, and give a durability to it, which could scarcely be obtained by by any other method. It must, however, be observed, that there are several substances used in colour-making, which solution of tin cannot bear to be mixed with. These are principally sugar of lead and cream of tartar, as well as all the calcareous earths and alkaline salts. With alum it may be mixed very safely, and is in many cases the better for it. The roots of plants, however, seem to promise more durability of colour than the upper parts. We have seen a blue colour of considerable durability and brightness prepared from the roots of common radishes by expressing the juice, combining it with tobacco-pipe clay, and brightening it with a little alum. The root of the red beet is also said to yield a durable colour of a beautiful red, inclining to scarlet; but this we cannot affirm from our own experience.
With regard to liquid colours for maps, &c., we apprehend there can be very little difficulty in preparing all the possible varieties of them, if what we have above laid down is attended to. The only colour with which there can be any difficulty is blue; but the common solution of indigo in alkalies or acids may be made to answer this purpose, though, on account of their strongly saline quality, they are not very proper. A very curious method of procuring a beautiful transparent blue colour is by extracting the colouring matter from Prussian blue, by means of a caustic alkali. This, when laid upon paper, appears of a dirty brown colour; but if washed over with a weak solution of green vitriol, is instantly changed to a most beautiful blue. This seems to afford a method of procuring blue transparent colours of greater beauty than they are usually met with.—See specimens of transparent colours prepared according to the above rules, on the Chart subjoined to History.
COLOURING, among painters, the manner of applying and conducting the colour of a picture; or the mixtures of light and shade, formed by the various colours employed in painting. See PAINTING.
COLOURING of Glass. See GLASS.
COLOURING of Porcelain. See PORCELAIN.