The word " tempera99 is used for a glutinous medium which binds colours and is soluble in water, such as size, gums, or egg; and in a more restricted sense for a vehicle in which yolk of egg is the principal ingredient. The analysis of fragments of coloured plasters from ancient buildings shows that a form of tempera was the usual manner of applying the colours to wall surfaces. In Egypt wax was found together with substances coming from animal blood and glue; at Athens, wax, odoriferous gums, and substances thought to be vegetable. In Sicily yellow organic matters were found in considerable quantities mixed with a little of a fat substance and lime. At Pompeii there were found: (1) Organic matters in less quantity than in Sicily, and containing portions of fat substances and lime, with traces of silica, aluminum, magnesia, and sometimes chlorate of sodium; (2) Animal matters, supposed to be milk, mixed with lime.
In the Middle Ages yolk of egg diluted with fig juice, which was used for painting, was also used medicinally.
For wall-painting warm size was sometimes used, the distemper colour of the present day. A - fifteenth-century MS. in the public library at Strassburg, quoted by Eastlake, gives a receipt for making size which would keep and not smell—very useful, since stale size does not dry well. It was made from parchment cuttings like the gilder's size of the present day, but vinegar was to be added after it was made and the whole well boiled again. (Mrs. Herringham, however, says that boiling size diminishes its tenacity.) When it was used equal quantities of size and water were taken, honey was added, and the whole warmed. Professor Church says that in selecting a size the special purpose for which it is to be used must be considered, and whether one which is insoluble in cold water and strongly gelatinising or one which is partially soluble and very adhesive would be best. " The former is less liable to crack when dry than the latter. The very fine gelatins used in photography will often be found suitable. As caustic lime, caustic soda, chloride of lime, sulphurous acid, and certain mineral acids are frequently employed in their manufacture, they must be tested. A hot-water solution must not redden blue litmus paper, nor bleach dahlia paper, nor embrown turmeric paper".
In using a size medium care must be taken that the ground has no grease in it: the colour when mixed should drop from the brush in a thread, the size being used warm.
Detail from Botticelli's fresco of the life of Moses, in the Sistine Chapel, Rome.
Glutinous vehicles were diluted with wine for a long time in Italy; the northern artists were content to use beer. Honey was added to retard the drying of the colours. As long ago as the latter half of the thirteenth century it was the English practice to varnish tempera pictures; and the numerous black Madonnas revered in various churches show that such varnishing was usual still earlier, the darkness of tone being usually the result of the darkening of the varnish. In dry climates egg tempera attains a firm consistency and withstands ordinary solvents. It sets rather than dries, and to be solid must be painted directly and left undisturbed. Tempera was used for retouching frescoes "a secco," adding details which would have taken too long to put in at the time that the main painting was done, and also for painting on the dry wall as well as on panel and cloth fastened to wall or panel. Sometimes the medium used was the whole egg diluted with fig juice, which acted as a solvent and also contained a small amount of india-rubber. If the yolk should become too thick to use it may be diluted by the addition of a small quantity of water, being well mixed by shaking. Cennino recommends two temperas for finishing buon fresco, the first that just mentioned and the second yolk of egg only, and " you must know that it (the latter) is of universal application—on walls, on panels, or on iron, and you cannot use too much of it" In chapters lxxii. and xc. he directs the artist to check the over-absorbency of the wall with a diluted wash of egg before commencing to paint He mentions seven to ten coats of colour, so that it is evident that it was the custom to paint with thin coats of colour, and without using impasto. It was usual to make an under-painting in green, called "verdaccio," then to glaze transparent colours over the shadows and mix white with the same tint for the lights.
The yolk of egg medium does not protect the colours from deterioration with absolute completeness, but answers well enough for most purposes; and the best fifteenth-century pictures are still in a state of pre-preservation which may almost be called perfect. Professor Church says that "yolk and white of egg, like other nitrogenous bodies, are susceptible of coagulation, and thus may become a substance which is virtually leather. This may be effected by treating them with a solution containing tannin. Egg yolk being one-third oily or fatty matter is really well on the way to be an oil medium. As it dries the oil hardens and remains intimately commingled with the albuminous substances left behind on the evaporation of the water present These albuminous substances coagulate and become insoluble in the lapse of time a change greatly accelerated by the old practice of exposing the finished tempera picture to the sun." He gives the following analysis and says that temporary preservation from putrefaction may be effected by means of a lump of camphor or a few drops of eugenol (from oil of cloves). "A saturated solution of eugenol in five per cent, acetic acid is made, and this is added, drop by drop, with constant agitation, to the yolks in a wide-mouthed bottle, the change of colour in a slip of turmeric paper giving the stopping point, when this paper j ust regains its original colour, which was turned brownish red by the yolks. Water may now be added and a lump of camphor." He lays great stress on the correction of the alkaline reaction of the yolks, returning to the subject again; this time he recommends the addition of a few drops of white vinegar. White of egg needs dilution with water, thorough shaking and filtering through muslin. Used alone it dries too hard for painting with, has very little body, and after a time has more tendency to crack than when mixed with the yolk. The media serve not only to bind the pigments to the ground, but also the coloured particles to each other. Lead and copper paints are now excluded from the tempera palette, so that the sulphur in the egg does not alter any of the colours.