Conservation Of Pictures

It is generally conceded that a finished oil painting is best kept in such a position that it is exposed to daylight of just sufficient intensity for it to be well seen, the direct beams of the sun being excluded. In darkness, or even in approximate darkness, the lead-whites may tarnish, and the oil and resins darken. Even with the most moderate illumination, however, the more fugitive pigments, such as the cochineal and quercitron lakes, will in no long time alter and fade to such a serious extent as to destroy the 'keeping' of the work. But there is really no need to introduce these evanescent pigments, for every nuance the artist can desire may be produced with paints having a sufficient if not perfect degree of permanence. The question of artificial lighting here comes in. Against oil lamps, properly constructed and managed so as to avoid the production of smoke and soot, nothing can be urged; the same opinion may be given in reference to the use of electric incandescent lamps. The introduction of electric arc lamps seems less safe, even when the illumination they afford is reduced to the necessary minimum, for the light they emit is richer in those rays which, as a general rule, are peculiarly effective in bringing about chemical changes in the less stable pigments.

But the light by which pictures are to be seen is but one of the conditions out of several which have to be considered in their conservation, though perhaps the most important. The mode of securing a picture in its frame is not an altogether trivial matter. In the majority of cases the expansion and contraction with variations of moisture and temperature, of panel and canvas, do not correspond accurately with the similar changes of the frame. In consequence, too great rigidity in the system of fixing adopted should be avoided. Duly adjusted springs or blocks of indiarubber (not vulcanized), secured in the rebate, afford convenient means of obtaining the necessary freedom of movement, while preventing the jar caused by accidental concussions. An equable temperature is another important condition; on no account should currents of hot or of cold air impinge directly upon the front or back of a painting. Moreover, this is not a mere question of temperature, for such currents of air may bring in particles of dust and other impurities, while their hygroscopic condition is sure to vary. This question of moisture is of some moment.

For if freshly-warmed air, which is pretty sure to be comparatively dry air, is allowed to come in contact with panels or canvases, it will withdraw from them some of their necessary hygroscopic moisture, and thus cause capricious and hurtful changes of size. Such changes, often repeated, cannot but result in the production of cracks and fissures in the oil paintings subjected to these varying conditions. The hygroscopic balance between picture and air can be maintained only by a due supply of moisture to the warmed air before the latter comes in contact with the painting; the warmer the air the more moisture must be added to it. The same reasoning applies to the entrance of cold air, but in this case, care must be taken that it is sufficiently dry not to deposit water upon the picture. For the purpose of regulating the hygroscopic condition of the atmosphere in a picture-gallery, the introduction of a dew-point thermometer is advisable. And there is another contrivance by means of which the presence of the right proportion of moisture in the air may be recognised. A strip of drawing-paper, another of primed canvas, and another of mahogany, all three being in a normal hygroscopic state, are to be separately balanced by means of counterpoises.

When the air gets too dry, the strips will rise, owing to their loss of water; when excess of moisture is present, they will sink. So long as the equilibrium of the beams to the ends of which the strips are attached remains practically true, the air may be regarded as in a satisfactory hygroscopic condition. Three pairs of ordinary apothecaries' scales (or spring-balances) suffice for the construction of this apparatus, which should be protected by a glass case to which the air has free access. This question of the due amount of moisture in the air - neither in excess nor in defect - has scarcely received the attention it deserves. But it will be allowed on all hands that few conditions are more trying to pictures in oil or water-colour than those caused by currents of hot, dry air rising directly below them during the day, succeeded by currents of cold, moist air descending upon them down the surfaces of the walls at night.

The covering of an oil picture with glass, whatever objections may be urged against it from an artistic point of view, certainly secures the protection of its surface from the solid and liquid and, to some extent, from the gaseous impurities in the air. But the backs of pictures, especially of those painted on canvas, are often forgotten, yet excess of moisture and deleterious vapours and gases often enter from behind, and seriously discolour the painting-ground, and even the paint itself. Mention has previously been made of methods by which this cause of injury may be prevented by means of a double canvas, or a layer of white lead in powder mixed with starch-water, applied to the back of the original canvas; American leather cloth, or even parchment-paper, affixed to the frame behind is nearly as effective.