Field's Chromatography Part 3

Perfect white is opaque, and perfect black transparent; hence when added to black in minute proportion, white gives it solidity; and from a like small proportion of black combined with white, the latter acquires locality as a colour, and better preserves its hue in painting. Both white and black communicate these properties to other colours, in proportion to their lightness or depth; while they cool each other in mixture, and equally contrast each other when opposed. These extremes of the chromatic scale are each in its way most easily denied, as green, the mean of the scale, is the greatest defiler of all colours. Rubens regarded white as the nourishment of light, and the poison of shadow.

In a picture, white should not be merely glittering or brilliant, but tender as well as bright. The eye should seek it for rest, brilliant though it may be; and feel it as a s.p.a.ce of strange heavenly paleness in the midst of the flushing of the colours. This effect can only be reached by general depth of middle tint, by the perfect absence of any white, save where it is needed, and by keeping the white itself subdued by grey, except at a few points of chief l.u.s.tre.

White, as a pigment, is of more extensive use than any other colour in oil painting and fresco, owing to its local quality, its representing light, and its entering into composition with all colours in the formation of tints. The old masters have been supposed by some to possess whites superior to our own, but this may be questioned. The pureness of whites in some celebrated old pictures is rather to be attributed to a proper method of using, careful preservation of the work, and in many instances to the introduction of ultramarine or a permanent cold colour into the white--such as plumbago--helped also by judicious contrast.

Notwithstanding white pigments are tolerably numerous, a thoroughly unexceptionable white is still a desideratum--one combining the perfect opacity or body of white lead with the perfect permanency of zinc white.

The nearest approach to it that has yet been made, is Chinese white, which possesses in a great measure the property of the former, and, being a preparation of zinc, has wholly that of the latter.

Unfortunately Chinese white is a water-colour pigment only, not retaining its several advantages, stability excepted, when employed in oil.

1. CONSTANT WHITE,

Also called _Permanent white_, and _Barytic white_, is, when well prepared, of superior body in water, but has less opacity in oil. It works in a somewhat unsatisfactory and unpleasant manner, and is considerably lower in its tone while wet than when dry, a fault which subjects even an experienced artist to great uncertainty where he uses it in compound tints. The semi-transparency of the white, while wet, prevents his judging of the true tint until his colour has dried, when he frequently finds it harsh and chalky, and out of tone with the rest of his painting. As little gum as possible should be employed with it, gum being inimical to its body, or whiteness. The best way of preparing this pigment, as well as other terrene whites, so as to preserve their opacity, is to grind them in simple water, and to add towards the end of the grinding sufficient only of clear _cold_ jelly of gum tragacanth as will connect them into a body, and attach them to the paper in painting.

Cold starch will answer the same purpose.

Constant white is a sulphate of baryta, found native and known under the name of heavy-spar, or prepared artificially by adding sulphuric acid, or a soluble sulphate, to a solution of a barytic salt. In the first mode, if the white be not well purified from free acid, it is apt to act injuriously on some pigments. Sulphate of baryta is often used for the purpose of adulterating white lead, the native salt being ground to fine powder, and washed with dilute sulphuric acid, by which its colour is improved, and a little oxide of iron probably dissolved out. Whether native or artificial, the compound is quite unaffected by impure air, and is not poisonous.

LEAD WHITES

Comprise and are known under the names of:--_White lead_, _Flake white_ and _Body-white_, _Cremnitz_, or _Kremnitz_, _Crems_ or _Krems white_, _London_ and _Nottingham white_, _Flemish white_, _Pattison's white_, _Blanc d'argent_ or _Silver white_, _Ceruse_, _Dutch white_, _French white_, _Venetian white_, _Hamburgh white_, _Kremser white_, _Sulphate of lead_, &c.

The heaviest and whitest of these are the best, and in point of colour and body, are superior to all other whites. When pure and properly applied in oil and varnish, they are comparatively safe and durable, drying well without addition; but excess of oil discolours them, and in water-painting they are changeable even to blackness. Upon all vegetable lakes, except those of madder, they have a destructive effect; and are injurious to gamboge, as well as to those almost obsolete pigments, red and orange leads, king's and patent yellow, ma.s.sicot, and orpiment. With ultramarine, however, red and orange vermilions, yellow and orange chromes, yellow and orange and red cadmiums, aureolin, the ochres, viridian and other oxides of chromium, Indian red &c., they compound with little or no injury. Lead colours must not be employed in water-colour or crayon painting, distemper, or fresco. The whites of lead are carbonates of that metal, with two exceptions:--Flemish white or the sulphate, and Pattison's white or the oxychloride. In using all pigments of which lead is the basis, cleanliness is essential to health.

White lead, by which we must be understood to mean the carbonate, always contains when commercially prepared a certain proportion of hydrated oxide. The less of the latter there is present, the better does the white cover, and the less liable is it to turn brown. The products formed by precipitation have proved to be inferior in body: otherwise, pure mono-carbonate of lead-oxide, obtained by mixing solutions of carbonate of potash and a lead-salt, might be best adapted for a pigment. However, such a carbonate has been lately produced by Mr.

Spence's process of pa.s.sing carbonic acid gas into a caustic soda or potash solution of lead, and for this white an opacity is claimed equal to that of the ordinary compound.

Great as is the opacity of white lead, it is apt to lose that property in some measure in course of time, and become more or less transparent.

If, over a series of dry oil-colour rubs of varied hues, there were brushed sufficient white lead paint to utterly obscure them, after some years those rubs would indistinctly appear, and by degrees become more and more visible, until at last their forms--if not their very colours--could be recognised. From this it would seem that white lead must slowly but surely part with some of its carbonic acid, and be at length converted into dicarbonate, a compound possessing less carbonic acid, and less coating power.

Impure air, or sulphuretted hydrogen, browns or blackens white lead, converting it partially or wholly into sulphide. It would appear from the recent investigations of Dr. D. S. Price, that white lead is less liable to be thus affected, when the pictures in which it is used are exposed to a strong light; also, that when such pictures have so suffered, a like exposure will restore them. We have ourselves noticed the rapidity with which an oil rub of white lead that has been damaged by foul gas, regains its former whiteness when submitted to air and sunshine. The action of drying oils has been likewise proved to be very powerful upon sulphide of lead, an exposure to light for a few days only being sufficient to change a surface of it, coated with a thin layer of boiled linseed oil, into a white one. Probably, these agents may have a similar effect upon other pigments injured by sulphuretted hydrogen, and many of the colours in paintings may be restored by treating them with boiled linseed oil, and submitting them to a strong light. That the result is due to oxidation, there can be no doubt. Indeed, the eminent French chemist, M. Thenard, was consulted some years back upon the means of bringing to their original whiteness the black spots which had formed upon a valuable drawing, by the changing of the white lead, and employed for that purpose oxygenated water. He had ascertained its power of converting the black sulphide of lead into the white sulphate, and, by touching the spots with a brush dipped in the fluid, soon succeeded in restoring the drawing to its primitive state. Here, again, the use of the agent might doubtless be extended to other colours, to which foul air is inimical.

In oil painting white lead is essential in the ground, in dead colouring, in the formation of tints of all colours, and in sc.u.mbling, either alone or mixed with other pigments. It is also the best local white, when neutralized with ultramarine or black; and it is the true representative of light, when warmed with Naples yellow, or orange vermilion or cadmium, or with a mixture of the yellow and either of the orange pigments, according to the light.

Ordinary white lead is often mixed with considerable quant.i.ties of heavy spar, gypsum, or chalk. These injure it in body and brightness, dispose it to dry more slowly, keep its place less firmly, and discolour the oil with which it is applied, as well as prevent it dissolving completely in boiling dilute potash-ley, a test by which pure white lead may be known.

The adulteration of pigments, which we have in some instances found practised to a large extent abroad, is comparatively unfrequent in our own country, so far at least as regards the superior cla.s.s of colours employed by artists. As a rule, such colours when manufactured in England may be fairly a.s.sumed to be genuine; and certainly the respectable colourmen of the present day are not in the habit of sophisticating them. We must bear testimony, indeed, to the zeal with which they purvey, regardless of necessary expense, the choicest and most perfect materials. This should be a matter of congratulation to the painter, who must of necessity rely on the faith and honesty of his colour-dealer; for if he were ever so good a chemist, it would be impossible for him to a.n.a.lyse each pigment before proceeding to use it.

The fault must rest with himself, therefore, if, through a mistaken economy, he do not frequent the best houses and pay the best prices. Of a surety, the colours of the artist are not among those things in which quality can, or should, be sacrificed to cheapness.

2. BLANC D'ARGENT, OR SILVER WHITE.

These are false appellations of a white lead, called also _French white_. It is brought from Paris in the form of drops, is exquisitely white, but of less body than flake white, and has all the properties of the best white leads. Being subject to the same changes, it is unfit for general use as a water-colour, though good in oil or varnish.

3. CREMNITZ WHITE,

Known likewise as _Kremnitz_, _Crems_, or _Krems white_, is a carbonate of lead which derives its names from Kremnitz in Hungary, or Crems or Krems in Austria. It is also called _Vienna white_, being brought from Vienna in cakes of a cubical form. Cremnitz white is the brightest white that is used in oil: it possesses rather less body than flake white, because the particles are finer. When newly prepared, it gives out a strong smell of vinegar.

4. FLAKE WHITE,

Called, when levigated, _Body white_, is an English white lead in the form of scales or plates, sometimes grey on the surface. It takes its name from its figure, is occasionally equal to Crems white in colour, and generally surpa.s.ses in body all other white leads. In composition, it is a mixture of protocarbonate and hydrated oxide of lead, the latter decreasing the opacity of the product according to the greater proportion in which it is present.

5. FLEMISH WHITE, OR SULPHATE OF LEAD

Is an exceedingly white precipitate from any solution of lead by sulphuric acid, much resembling the blanc d'argent. It is inferior, however, both in body and permanence to the ordinary carbonate. Hence, white lead which has more or less been converted by sulphuretted hydrogen into sulphide, and again been converted into sulphate by oxidation, with a view to restoring its colour, becomes peculiarly liable to the influence of impure air.

6. LONDON AND NOTTINGHAM WHITES.

The best of these do not essentially differ from each other, nor from the white leads of other manufactories. The latter variety, being prepared from flake white, is usually the greyer of the two.

7. PATTISON'S WHITE, OR OXYCHLORIDE OF LEAD

Is a mixture of chloride and oxide of lead, formed by precipitating a solution of chloride of lead with soda, potash, lime, or baryta, in the caustic or hydrated state. It would appear that when the oxychloride is used as a paint, the oxide contained in it gives rise to an oleate of lead, and, in consequence of this saponaceous matter, is capable of spreading over an extended surface. The product has been described as possessing properties which are superior to those of white lead, inasmuch as it does not so readily blacken as the latter body. Dr. Ure, however, found that water removes the chloride of lead from the paint compounded of this article, and, consequently, that it is not so effectual as the carbonate. As an artist's pigment, a partially soluble compound of lead can decidedly not be eligible.

8. ROMAN WHITE

Is of the purest white colour, and differs only from blanc d'argent in the warm flesh tint of the external surface of the large square ma.s.ses in which it is commonly prepared.

Besides the foregoing, there are other white leads, generally foreign, cheaper, and adulterated. Many of these are mixed with a small quant.i.ty of charcoal, indigo, or Prussian blue, so that the dead yellowish shade which they present may be enlivened to a brighter hue. Among them may be named--

9. CERUSE.

A French variety, not necessarily, but not unfrequently, mixed with different chalky earths in various proportions; and the following Belgian kinds:

10. DUTCH WHITE,

Containing three fourths of sulphate of baryta.

11. HAMBURGH WHITE.

A mixture of two parts of heavy spar and one of the plumbous compound.

12. KREMSER WHITE,

Differing from the rest in being unadulterated.

13. VENETIAN WHITE

Composed of heavy spar and the carbonate in equal proportions.

ZINC WHITES.

14. CHINESE WHITE.

The introduction, in 1834, of this peculiar preparation of oxide of zinc has proved an incalculable boon to water-colour painters, who formerly had no white which combined perfect permanency with good body in working. Its invention obviated the necessity for using white lead, a pigment which, though it may be employed with comparative safety in oil, is quite unfitted for water. Since the period of its production, Chinese white has been generally preferred by water-colour artists, as being the most eligible in their peculiar department. Previous to that period, the complaints of whites changing were of constant occurrence; but in no instance has any picture, in which this white has been used, suffered from its employment. To the colour of oxide of zinc, sulphuretted hydrogen is altogether harmless; sulphide of zinc being itself white.

The variety under notice works and washes well, possesses no pasty or clogging properties, and is prepared beautifully white. Moreover, it has the desirable quality of dense body; so much so, that, as the painter works, his effects remain unaltered by the drying of the colour. It may likewise be safely mixed with all other pigments, the following blending very satisfactorily with the white for opaque lights--cadmium yellow, orange, and red; gamboge; aureolin; yellow ochre; vermilion; and light red. Without the artistic drawbacks of constant white or the chemical defects of white lead, and retaining the advantages of both, Chinese white cannot but be considered as a most important addition. It is a matter of regret that this pigment is not equally efficacious in oil.

15. ZINC WHITE