An important precaution to be observed in the 'conduct' of a painting during its progress is based upon the two actions just referred to, namely, the oxidation of the oil during its hardening, and the escape of volatile solvents. The latter action takes place more easily than the former, and so if a picture is to be carried on rapidly to completion, the earlier and lower paintings should contain less oil than those nearer the surface, into which more oil and less resin (copal or amber), dissolved in some volatile solvent, should be introduced. If the reverse order be followed, the highly oleaginous layers below, having had no sufficient opportunity for oxidizing, drying, and hardening, will be rent by the strong and quickly-drying resinous layers above them.

The harder resins, paraffin-wax, wax, and oil, possess in varying degrees the power of 'locking-up' the pigments with which they are mingled, in such a way that these become much less liable to act upon one another, and to suffer injury from external agencies. In a measure they repel and exclude moisture and oxygen - two of the chief agents of chemical change. But the value of these 'locking-up' materials has been exaggerated: they often prove quite ineffectual in preventing the oxidation or other change suffered by non-permanent pigments and the inter-action of pigments. For instance, the oil which surrounds each particle of cadmium yellow and emerald green, in a mixture of these two oil-paints, is not capable of preventing the formation of the black sulphide of copper. And Dr. A. P. Laurie has found that when a layer of linseed-oil is interposed between these oil-colours separately spread, it is the emerald green which appears to travel towards the cadmium yellow - perhaps owing to its solubility in the medium. In consequence, the production of spots of black sulphide of copper occurs chiefly, if not entirely, on that side of the oil-layer which is in contact with the cadmium yellow.

To Dr. Laurie we are also indebted for a very ingenious method of comparing the locking-up function of various oils and resins. Dr. Laurie prepared some anhydrous sulphate of copper which is white, but acquires a blue colour when exposed to moisture. He ground this white sulphate with various media, painted glass slides with the mixtures, dried them in a desiccator, and then exposed them to moist air. A solution of amber in turpentine proved superior, in its power of resisting the access of moisture, to boiled linseed-oil, oil-copal varnish, amber dissolved in oil, resin or mastic dissolved in turpentine. Another set of trials, in which the test substance was ground in linseed-oil, allowed to harden in a desiccator and then coated with different varnishes, indicated a temporary superiority on the part of mastic in turpentine, and of oil-copal varnish over amber or copal in turpentine. The inferiority of the latter solutions may be due to the rupture in continuity of the resinous films which they leave on evaporation. (See Journal of Chemical Industry, June, 1890.) But it must not be forgotten that many an old oil-picture furnishes distinct evidence of the value of resinous matters (such as Strasburg and Venice turpentine) in locking up such changeable and destructive pigments as verdigris and orpiment.

The slow and laborious execution of such paintings constituted an important element in the success achieved, for each layer dried and hardened before the next was applied.

It should be noted that different oil-paints contain very different percentages of oil. This fact should be taken into account, so far as possible, in adjusting the amount of resinous matter to be introduced during the course of work upon an oil-picture. A table giving approximately the quantities of oil required in grinding 100 parts of various dry pigments as oil-paints will be found on page 66. Further information concerning such pigments is given in Chapters XIII. to XIX.

In completing an oil-picture, the three operations of 'glazing,' 'oiling out,' and 'varnishing' remain to be considered. As to glazing and oiling out, it should be stated that drying oil, with a little copal or amber varnish, should alone be employed - mastic varnish should never be added to the oil. Of course oil-paints are used in admixture with oil and copal for glazing purposes. If mastic be introduced, a risk is incurred of its partial removal during any cleaning operation to which the picture may be afterwards subjected. The question of the kind of varnish to be finally applied to an oil-picture has been much discussed. Our choice lies between a strong irremovable varnish, and a weak one capable of being abraded by friction, or of being dissolved by the application of a suitable solvent, which will not touch the true painting beneath. Mastic dissolved in turpentine fulfils the latter conditions; copal or amber dissolved in oil and thinned with turpentine, and mixed with a little oil, constitutes a strong, hard, irremovable protection to the surface, and becomes a part of the picture itself.

Under no circumstances should any varnish be applied to the painting until the latter has become thoroughly hard and dry; the danger of tearing the layers of paint by such application will then have been reduced to a minimum. A further advantage of delay in varnishing a picture accrues through the increasing insolubility with age of the oxidized oil present therein, the pigments associated therewith becoming less liable to removal by any treatment to which the work may afterwards be submitted.

The chemistry of Gambier-Parry's spirit-fresco method, and of the process in which paraffin-wax and copal varnish are employed as the vehicle, is essentially the same as that of oil painting. The wax or paraffin-wax is introduced merely to secure a matt surface. Pictures executed in these methods are, of course, never varnished. The method of spirit-fresco was devised by the late Mr. Gambier-Parry with the object of obtaining such effects in mural paintings as are realized in true fresco, but with greater ease in working, and greater permanence under adverse atmospheric conditions. He desired to exclude linseed or other fixed drying oils completely from the medium and other materials employed. With this end in view, he directed that the pigments used should be ground up, not with oil, but with the medium itself. He was apparently unaware that the copal varnish, which enters largely into the composition of his vehicle, contains a greater proportion of oil than of any other ingredient (see Chapter XII (Varnishes And Vehicles).). So, after all, the medium used in spirit-fresco differs from that generally employed in oil-painting rather in the proportions than in the nature of its ingredients.

Thus in working with it we shall find that its binding character is obtained as a result of the same two changes which cause the fixing and solidification of an oil painting, namely, the oxidation of the oil, and the desiccation of the resin. The wax present suffers no chemical alteration at first, but merely solidifies, although after the lapse of years it is liable to produce a kind of exudation or bloom; indeed, in the course of years the wax may wholly disappear. It should be added that the painting-ground for this method of working is first prepared with the medium diluted with oil of turpentine (see Chapter II (Plaster, Gesso, Stone, Slate, Etc).).

The method of painting with the paraffin-copal medium involves the same chemical and physical changes as those which occur in the use of the spirit-fresco vehicle, and is carried out in the same manner. Colours stiffly ground in oil may be used or in a mixture of the medium with oil: dry colours ground in the medium generally are to be preferred. The medium may be diluted to any desired consistency with spirit of turpentine or with oil of spike, but no dilution further than that required to secure perfect freedom in the manipulation and use of the paints is desirable, while it is important to remember that the use of abundance of medium is necessary to bind the particles of pigments firmly together. Artists have sometimes found that a picture painted in spirit-fresco will cede colour to a cloth used in rubbing its surface. This result is due either to excessive use of a diluent in working with this medium, or to a deficiency of oil in the copal-varnish used. I have never known a friable surface to be formed where the colours employed had been ground in oil instead of in the medium, or where a little extra oil had been added to the latter.