A glaze is a thin transparent film of colour, laid upon another colour to modify the tone, or to aid the effect of the latter; the work thereby appearing distinctly through the superimposed layer of glaze, from which it receives a characteristic hue. Glazing is effected by diluting proper transparent colours with megilp or other suitable vehicle. Thus diluted, these colours are laid upon portions of the work, either in broad flat tints, or in touches partially and judiciously distributed. The object of this process is to strengthen shadows, and to give warmth or coldness to their hue: to subdue lights that are unduly obtrusive, or to give additional colour and tone to those that are deficient in force and richness. Should it be necessary to lighten the tone of any part of the picture, this cannot be done by merely glazing; the first tints must first be concealed with brighter colours, of sufficient body for that purpose, and the glaze may then be applied. The glaze should usually be darker than the ground colour upon which it is to be laid; and as a rule, it may be observed that the first painting of the picture should be brighter than the subject may require, in order that the subsequent glazings may lower and obscure it to a proper and effective degree of tone.
Glazing is generally effected by the application, of diluted transparent colours; but occasionally semi-transparent colours are used for this purpose, provided they are rendered sufficiently transparent by the admixture of a large proportion of vehicle. These latter glazings are capable of being applied with excellent effect, where it may be necessary to modify the tones of those parts of the picture which do not appear satisfactory, or to produce particular effects, such as representations of smoke, dust, mists, and the like. Caution is however, necessary in glazing with opaque colours; because, if used in excess, they will deteriorate the picture, by destroying its transparency. Should a glazing produce a result different from what was intended, the glaze may easily be removed by a rag, or, if the spot be small, by the finger, provided the removal be effected immediately, that is, before the glaze has had time to fasten itself upon, or to soften, the colour on which it is laid, and in no case must glazing be attempted before the colours over which it is laid have become perfectly dry and firm.