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To Prepare Cobalt Blue

Bleu de Thenard.

Having reduced the ore to powder, calcine it in a reverberatory furnace, stirring it frequently.

The chimney of the furnace should have a strong draught, in order that the calcination may be perfect, and the arsenical and sulphurous acid vapors may be carried off. The calcination is to be continued until these vapors cease to be disengaged, which is easily ascertained by collecting in a ladle a little of the gas in the furnace; the presence or absence of the garlic odor determines the fact. When calcined, boil the result slightly in an excess of weak nitric acid, in a glass matrass, decant the supernatant liquor, and evaporate the solution thus obtained, nearly to dryness, in a capsule of platina or porcelain. This residuum is to be thrown into boiling water and filtered, and a solution of the subphosphate of soda to be poured into the clear liquor, which precipitates an insoluble phosphate of cobalt. After washing it well on a filter, collect it while yet in a gelatinous form, and mix it intimately, with eight times its weight of alumina, in the same state - if properly done, the paste will have a uniform tint, through its whole mass. This mixture is now to be spread on smooth plates and put into a stove; when dry and brittle, pound it in a mortar, enclosed in a covered half an hour. On opening the crucible, if the operation has been carefully conducted, the beauearthen crucible, and heat it to a cherry-red for tiful and desired product will be found. Care should be taken that the alumina in the gelatinous form be precipitated from the alum by a sufficient excess of ammonia, and that it is completely purified by washing with water filtered through charcoal. To make Artificial Saxon Blue.

Saxon blue may be successfully imitated by mixing with a divided earth prussiate of iron at the moment of its formation and precipitation.

Into a solution of 144 grs. of sulphate of iron pour a solution of yellow prussiate of potash.

At the time of the formation of iron add, in the same vessel, a solution of 2 oz. of alum, and pour in with it the solution of potash, just sufficient to decompose the sulphate of alumina, for a dose of alkali superabundant to the decomposition of that salt might alter the prussiate of iron. It will, therefore, be much better to leave a little alum, which may afterwards be carried off by washing.

As soon as the alkaline liquor is added, the alumina precipitated becomes exactly mixed with the prussiate of iron, the intensity of which it lessens by bringing it to the tone of common Saxon blue.

The matter is then thrown on a filter, and, after being washed in clean water, is dried. This substance is a kind of blue verditer, the intensity of which may vary according to the greater or less | with a new quantity of the solution, and if this is quantity of the sulphate of alumina decomposed.

It may be used for painting in distemper.

To make Blue Verditer.

Dissolve the copper, cold, in nitric acid (aquafortis), and produce a precipitation of it by means of quicklime, employed in such doses that it will be absorbed by the acid, in order that the precipitate may be pure oxide of copper, that is, without any mixture. When the liquor has been decanted, wash the precipitate and spread it out on a piece of linen cloth to drain. If a portion of this precipitate, which is green, be placed on a grindingstone, and if a little quicklime, in powder, be added, the green color will be immediately changed into a beautiful blue. The proportion of the lime added is from 7 to 10 parts in 100. When the whole matter acquires the consistence of paste, desiccation soon takes place.

Blue verditer is proper for distemper, and for varnish, but it is not for oil painting, as the oil renders it very dark. If used it ought to be brightened with a great deal of white.

Chrome Yellow.

To a solution of bichromate of potassa add a solution of nitrate of lead as long as a precipitate falls. Wash and dry it.

Cadmium Yellow Is a compound of cadmium and sulphur. It is obtained by precipitation from a salt of cadmium by a current of sulphuretted hydrogen gas, or by an alkaline sulphide.

Lemon Yellow (Steinbuhl Yellow) Is a chromate of baryta, made by mixing hot saturated solutions of bichromate of potassa and nitrate of baryta. Wash and dry the precipitates.

It is considered superior to chrome yellow.

To make Naples Yellow.

Twelve oz. of ceruse, 2 oz. of the sulphuret of antimony, oz. of calcined alum, 1 oz. of sal ammoniac. Pulverize these ingredients, and having mixed them thoroughly, put them into a capsule or crucible of earth, and place over it a covering of the same substance. Expose it at first to a gentle heat, which must be gradually increased till the capsule is moderately red. The oxidation arising from this process requires, at least, 3 hours' exposure to heat before it is completed. The result of this calcination is Naples yellow, which is ground in water on a porphyry slab, with an ivory spatula, as iron alters the color. The paste is then dried and preserved for use. It is a yellow oxide of lead and antimony.

There is no necessity of adhering so strictly to the doses as to prevent their being varied. If a golden color be required in the yellow, the proportions of the sulphuret of antimony and muriate of ammonia must be increased. In like manner, if you wish it to be more fusible, increase the quantities of sulphuret of antimony and calcined sulphate of alumina. To make Montpellier Yellow.

Take 4 lbs. of litharge, well sifted, divide it into 4 equal portions, and put it into as many glazed earthen vessels. Dissolve also 1 lb. of sea-salt in about 4 lbs. of water. Pour a fourth part of this solution into each of the 4 earthen vessels, to form a light paste; let the whole rest for some hours, and when the surface begins to grow white stir the mass with a strong wooden spatula. Without this motion it would acquire too great hardness, and a part of the salt would escape decomposition.

As the consistence increases dilute the matter not sufficient recourse must be had to simple water to maintain the same consistence. The paste will then be very white, and in the course of 24 hours becomes uniform and free from lumps; let it remain for the same space of time, but stir it at intervals to complete the decomposition of the salt. The paste is then well washed, to carry off the caustic soda (soda deprived of carbonic acid) which adheres to it; the mass is put into strong linen cloth and subjected to a press. The remaining paste is distributed in flat vessels, and these vessels are exposed to heat, in order to effect a proper oxidation (calcination), which converts it into a solid, yellow, brilliant matter, sometimes crystallized in transverse striæ.

This is Montpellier yellow, which may be applied to the same purposes as Naples yellow. To prepare Carmine.

This kind of fecula, so fertile in gradation of tone by the effect of mixtures, and so grateful to the eye in all its shades, so useful to the painter, and so agreeable to the delicate beauty, is only the coloring part of a kind of dried insect known under the name of cochineal.

A mixture of 36 grs. of chosen seed, 18 grs. of autour bark, and as much alum thrown into a decoction of 5 grs. of pulverized cochineal, and 5 lbs. of water, gives, at the end of from 5 to 10 days, a red fecula, which, when dried, weighs from 40 to 48 grs. This fecula is carmine. The remaining decoction, which is still highly colored, is reserved for the preparation of carminated lakes.

Superfine Carmine of Amsterdam.

Heat 6 buckets of rain-water, and when it commences to boil throw in 2 lbs. of finely-powdered cochineal; continue boiling 2 hours, and then add 3 oz. of pure water, and immediately afterwards 4 oz. of binoxalate of potash. Boil again 1 minute, then remove the vessel from the fire, and let the decoction stand 4 hours. Draw off the supernatant liquid with a syphon into numerous basins, and put them aside upon a shelf for about 3 weeks, at the end of which time a mouldy pellicle will be formed, which is to be carefully removed with a whalebone, or by means of a small sponge attached to the end of a stick. The water is then run off through a syphon, which must reach to the bottom of the pans, the carmine being so compact that it adheres. This carmine is dried in the shade, and is of an intensely brilliant hue.

To prepare Dutch Pink from Woad.

Boil the stems of woad in alum-water, and then mix the liquor with clay, marl or chalk, which will become charged with the color of the decoction. When the earthy matter has acquired consistence, form it into small cakes and expose them to dry. It is under this form that the Dutch pinks are sold in the color shops.

Dutch Pink from Yellow Berries.

The small blackthorn produces a fruit which, when collected green, is called yellow berries.

These seeds, when boiled in alum-water, form a Dutch pink superior to the former. A certain quantity of clay, or marl, is mixed with the decoction, by which means the coloring part of the berries unites with the earthy matter and communicates to it a beautiful yellow color. Brownish Yellow Dutch Pink.

Boil for an hour in 12 lbs. of water 1 lb. of yellow beries, lb. of the shavings of the wood of the Barberry shrub, and 1 lb. of wood-ashes.

The decoction is strained through a piece of linen cloth. Pour into this mixture, warm, and at different times, a solution of 2 lbs. of the sulphate of alumina in 5 lbs. of water; a slight effervescence will take place, and the sulphate being decomposed, the alumina which is precipitated will seize on the coloring part. The liquor must then be filtered through a piece of close linen, and the paste which remains on the cloth, when divided into square pieces, is exposed on boards to dry.

This is brown Dutch pink, because the clay in it is pure. The intensity of the color shows the quality of the pink, which is superior to that of the other compositions.

Dutch Pink for Oil Painting.

By substituting for clay a substance which presents a mixture of that earth and metallic oxide, the result will be Dutch pink of a very superior kind.

Boil separately 1 lb. of yellow-berries and 3 oz. of the sulphate of alumina in 12 lbs. of water, which must be reduced to 4 lbs. Strain the decoction through a piece of linen, and squeeze it strongly. Then mix up with it 2 lbs. of ceruse, finely ground on porphyry, and 1 lb. of pulverized Spanish white. Evaporate the mixture till the mass acquires the consistence of a paste; and, having formed it into small cakes, dry them in the shade.

When these cakes are dry, reduce them to powder, and mix them with a new decoction of yellow-berries. By repeating this process a third time a brown Dutch pink will be obtained.

In general the decoctions must be warm when mixed with the earth. They ought not to be long kept, as their color is speedily altered by the fermentation. Care must be taken also to use a wooden spatula for stirring the mixture.

When only one decoction of wood or yellowberries is employed to color a given quantity of earth, the Dutch pink resulting from it is of a bright-yellow color, and is easily mixed for use.

When the coloring part of several decoctions is absorbed the composition becomes brown, and is mixed with more difficulty, especially if the paste be argillaceous; for it is the property of this earth to unite with oily and resinous parts, adhere strongly to them, and incorporate with them. In the latter case the artist must not be satisfied with mixing the color; it ought to be ground, an operation equally proper for every kind of Dutch pink, and even the softest, when destined for oil painting.

To make Lake from Brazil-wood.

Boil 4 oz. of the raspings of Brazil-wood in 15 pts. of pure water till the liquor is reduced to 2 pts. It will be of a dark-red color, inclining to violet; but the addition of 4 or 5 oz. of alum will give it a hue inclining to rose-color. When the liquor has been strained through a piece of linen cloth, if 4 oz. of the carbonate of soda be added with caution, on account of the effervescence which takes place. the color, which by this addition is deprived of its mordant, will resume its former tint, and deposit a lake, which, when washed and properly dried, has an exceedingly rich and mellow violet-red color.

Another. If only one-half of the dose of mineral alkali be employed for this precipitation, the tint of the lake becomes clearer, because the bath still retains the undecomposed aluminous mordant.