The colours, finely ground, are mixed with diluted gum or glycerine to render them more adhesive. They must resist the temperature of vitrification of the glaze, hence their composition depends upon the fusing point of the latter. They are often mixed with fluxes. They are applied to the warmed pottery by immersion for uniform tones, or with the brush when there is a design, or by printing when the same decoration has to be reproduced many times.
The brush-work is done with the colours mixed with gum or essence of turpentine; they are applied as in oil painting. When once dry, the pieces are heated before putting on the glaze in order to remove the essence.
Printing is done with copper plates having the design engraved on them by points; these are coated by means of a pad with the colour which has been prepared with boiled linseed oil and a little resin. The plates are heated to 30° or 40° C, to facilitate the removal of the paper, to which the colour remains adherent. This paper is then applied to the pottery to be decorated and rubbed over with a pad, and is plunged into water so that the paper may be detached with a sponge. The pieces are dried and afterwards heated to 2000 to 3000 C, in order to remove the oil; they are allowed to cool, then covered with the glaze and fired.
The design obtained is monochrome, but other colours can be added with a brush, or more economically, if a large number of pieces are being treated, by printing.
For this purpose the design is divided among as many plates as there are colours, and the different colours are printed off separately upon the same paper, great care being taken to fit them together correctly. The design is afterwards transferred to the piece; this process is used in the decoration of facing quarries.
Chromolithographic printings which is much employed in over-glaze decoration, presents certain technical difficulties when used under glaze. M. Boulenger, the eminent ceramist of Choisy-le-Roi, was the first in France to overcome them. Sarreguemines (Alsace) and Mettlach (Prussia), Minton and Hollins (England), also use sub-enamel decoration.
Machine-printing renders great services to the industry, but does not give such artistic effects as painting with a brush, which alone is capable of satisfying refined tastes.
This depends upon the nature of the paste to be decorated, both with respect to the colouring oxides used, and in the choice of the fluxes which are frequently added to them. As an example we shall mention the colours from which Deck has obtained such beautiful effects in his faiences of silicious paste, recalling, in their rich colouring, the celebrated Oriental faiences.
The colours having been finely crushed, are diluted by the muller, and applied with the brush.