The surface layer of mortar, or painting ground, can be prepared in various degrees of coarseness of grain to suit the artist's requirements. The more smooth and polished, however, the surface is made, the greater are the difficulties in the subsequent process of fixing, owing to the absorbent qualities of such a ground being necessarily less perfect. The ground can also be prepared in any tint or colour that may be desired, and can be applied to any suitable substance, if needed for a removable decoration. Stone, tile, slate, wire-gauze, glass, and canvas form efficient substitutes for the wall in such cases. If applied to canvas, it can in this form be fixed to wood panels, millboard, ceilings, etc, and admits of being rolled with perfect safety. The advantage of this to the artist is sufficiently obvious. If a ceiling, for instance, has to be decorated by this process, it can be painted with the same convenience as an ordinary picture in the studio. After it is fixed, it can be rolled up, taken to its destination and fastened on to the ceiling, either temporarily or permanently, at the cost of very little expenditure of time or labour.

Similarly (unless it were permanently fastened up), the ceiling would admit of being removed for the purpose of being cleaned.

As to the colours used in this process. Certain pigments only are admissible, in order to ensure permanence, and regard must be had to the purity of these, and to their absolute freedom from adulteration. All the colours found available for the stereo-chrome process can be employed; these are, for the most part, composed of natural earths or metallic oxides, since experience has proved that the most permanent colours are those derived from such sources. In their preparation, due account has been taken of the well-known law in optics, which teaches that colour does not lie in the substances themselves, but in the rays of light, which are divided, reflected, or absorbed by the substances in such a manner as to produce the effect of colour upon the eye. Substances, therefore, which readily undergo change, whether by reason of their affinity to other substances with which they are brought into contact, or by the action of the light itself, which often causes molecular change, must, whenever such change takes place, lose or modify their original colour, since under their altered conditions they absorb or reflect the ray of light in a different manner.

It is clearly then of the greatest importance that each pigment should remain chemically unaffected by the substance of the painting ground on which it is laid and by the substance of any other pigment employed, as well as by that of the material used for fixing them. To meet this end, the colours in this process are treated beforehand with alkaline solutions (of potash or ammonia), to anticipate any change of hue which might result from the use of the alkaline liquids which form the fixative. In addition to this, they are further prepared with certain other substances, such as zinc oxide, baryta carbonate, felspar, powdered glass, etc, as required by the peculiar properties of each, in order to obviate any other danger of chemical change taking place.

The colours found available present a very full scale. They are 38 in number, and there are several other colours which could be added if required. They consist, speaking in general terms, of 4 varieties of white, 6 of ochre, 2 of sienna, 10 of red, 2 of brown umber, 2 of Naples yellow, 2 of ultramarine, 5 of green, 3 of black, and cobalt blue. Cadmium will shortly be added to them. The whites are, perhaps, in unnecessary profusion. Zinc white, for its opaque qualities, and baryta white for purposes where great opacity is not desirable, would be probably found quite enough in practice.

Zinc white is especially valuable in this process, forming a silicate in combination with the fixing solution, and thus adding greatly to the hardness and durability of any colours with which it is mixed.

Baryta white is useful for giving a lighter tone to colours without greatly detracting from their transparent qualities, and is on this account useful in glazing, where zinc white would be too opaque.

The reds are chiefly oxides. The chrome is a lead sub-chromate. This colour is prepared in dry powder instead of in a moist paste, as in the case of the others. The reason for this lies in the fact that the colour depends on the size of the crystals, which would be destroyed by further grinding, with the result of the pigments assuming an orange hue. It will therefore only admit of being mixed with water by the means of the brush.

The lake is only suitable for interior decoration, and has been prepared by Keim, under protest, for artists who found themselves unable to forego its use. He does not guarantee its permanence if exposed to weather in the open air. He has proposed an ultramarine red as an efficient substitute.

The colour named mennig is a lead oxide.

The umber is an iron and manganese oxide, combined with silica.

The Naples yellow is a compound of lead oxide and antimony, or lead antimoniate.

The ultramarine is artificial, and consists of silica, alumina, and sodium sulphate.

The cobalt blue is cobalt protoxide, compounded with alumina.

The cobalt green is cobalt protoxide, in combination with zinc oxide.

The green earth consists chiefly of silicic iron protoxide. It also contains magnesia alumina, and potash.

The chrome oxide green is chromium oxydiydrate.

Over no part of his process has Keim expended more labour and thought than in the preparation of the colours. From the various nature of the properties possessed by some of the pigments, it was found that their capacity for absorbing the alkaline silicate with which they were fixed varied very greatly. There was also a marked difference in the degree of mechanical cohesive capacity which they respectively possessed. To equalize them in these respects, without which the fixing would have been a work of great difficulty and uncertainty, alumina, magnesia, and silica hydrate were added as required. The result is, that all the colours are equally acted upon by the fixing solution, and all attain an equal degree of durability after fixing, both as regards the mechanical and chemical action of this process upon them.