Engobes are either white or coloured. When white, they serve to modify the colour of the pottery to which they are applied. They are made of fine white earths, to which are added a thinning substance and different salts. Their composition should be such that, when applied to the paste, they shall bear the same firing, undergo the same dilatation, behave in the same manner under the influence of the atmosphere of the kiln, and finally adhere satisfactorily to the paste. The engobe is laid on by diluting to a state of paste the mixture of the proper ingredients, and dipping the object which has to be coated, in this paste, or else, by pouring the liquid engobe over the paste, or finally, by applying it with a brush.
Coloured engobes are used for the same object as white ones, or, as is more frequent, for obtaining decorative colouring effects. They are produced from earths which have been naturally or artificially coloured by the addition of metallic oxides to white or only slightly coloured earths.
In the first case, the coloured engobes are applied like the white one; it is an economical method of giving to a paste a colouring, or properties which it would be too costly to communicate to it in any other way.
In the second case, the engobe is used in two different ways: diluted with water like a water-colour, it is applied with a brush to the piece when unbaked or slightly warmed; after drying, it is fired as it is if the colour is to remain dull, or it is covered with a glaze; this mode of decoration, which is called peinture a la barbotine, always gives dull tones and not very varied effects.
The other method of applying coloured engobes consists of making hollows in the object to be covered, which are filled with a thick paste of the engobe. Several engobes of different colours can be applied to the same piece. It is allowed to dry, and the surface is then cleaned in order to bring out the designs; this is the process employed in making paving quarries, in imitation of those of the Middle Ages. If the engobe reaches a certain degree of thickness, it is called incrustation.
The Greeks and Romans used extensively engobes made with white, red, and yellow clays, to decorate their vases. By these simple and primitive methods they have produced masterpieces which we now admire in our museums. These coatings are dull, except the black one, which shines in consequence of the iron contained in it.
To-day engobes are seldom used alone, and the peinture d la barbotine is scarcely used at all except for architectural pottery. In most cases they are covered with a glaze; their composition must therefore be allied with that of the glaze. They are applied to the pieces when unbaked or slightly warmed.
These are complex metallic silicates of variable composition; they are usually divided into:
Soft glazes, colourless and transparent vitreous substances, fusible at a fairly low temperature and generally containing lead;
Hard glazes, also colourless and transparent, but melting at a high temperature and containing always an earthy or alkaline-earthy substance;
Enamels, coloured vitreous substances; these are divided into transparent enamels, which are plumbiferous or alkaline, and opaque enamels, which are generally stanniferous and are used especially in the manufacture of faiences;
Colours, metallic oxides generally mixed with verifiable substances; they are either laid direct upon the pottery and afterwards covered with a colourless glaze - this is the under-glaze decoration - or upon the glaze itself, which is the over-glaze method.
These subdivisions are clear enough; nevertheless more precision may be given to the terms employed by adding to the name of the glaze those of the principal substances which enter into its composition; we say, for instance, plumbiferous glaze, calcareous glaze, stanniferous enamel, etc.
The foundation of glazes is silica, which forms, in combination with other basic substances, fusible and transparent multiple silicates which are really glasses.
Certain bodies which enter into the composition of these glasses are called fluxes because they have the property, when mixed with other substances, of rendering them fusible if they were infusible, or of increasing their fusibility if they were already fusible. Fluxes are not necessarily fusible themselves. Lime and alumina are infusible, but the result of their mixture in certain proportions with silica, itself infusible, is the formation of fusible compounds. It is not always easy to determine which body it is which acts the part of flux to the other. Moreover, it has been shown that multiple silicates are more fusible than simple ones.
In coloured glazes, the verifiable substances are mixed with colouring metallic oxides; hence we have two series of compounds to study.