Notes on engobe and glaze

Tough question: apart from the engobe made from simple liquefied clay (slip), in fact the ingredients of an unglazed coating are the same as those of a glaze:
clay or stabilizing material; silicon, as a vitrifying agent; and flux.

What then is the difference between an engobe and a glaze? When do you switch from one to the other? It is actually hard to say.

Basically, it is the relative amount of clay that determines the difference; a real boundary does not exist; there are coatings called vitreous engobes or even dry glazes. Let's say that an engobe must have at least 30, 40 or 50 percent of clay, but other factors such as permeability, glassiness/opacity ... etc. must also be considered and we must understand how materials interact with each other to get engobe or glaze, beyond the term we use.

This small and brief outline, by itself, highlights the complexity of the matter:

To better clarify, in the following table I outline the chemical composition (approximate by parts) of some dry minerals: e.g., kaolin is about 1 part alumina and 2 parts silica + water.

But in English? Are "slips" and "engobes" the same thing?

Slip = clay dissolved in water - very close to the Italian “barbottina” - with larger amounts of clay than engobes,

Engobes = often contain fluxes, various clays mixed, felspar, quartz.

Engobes change the surface of ceramic pieces; with them you can change the color of a piece, or make colorful designs.

They can be used as a completed stage or be glazed. The surface of the piece can be made smoother or, on the contrary, rougher, patterns can be made by partially removing the engobe, re-emphasizing the color of the clay of the piece, which should be contrasting.

The engobe can also serve to make the surface of the piece better or more durable.

To decide which coating is best for a certain clay, we need to know what effect we want to achieve (a certain color or surface texture), but we also needs to know how the engobe is to be applied and its compatibility with the underlying clay (and as well as with the glaze, if there is any) both during processing and firing.

We must then focus on what the shrinkage is, to know if there is compatibility between the coating and the clay, because otherwise there can be detachments or cracks: therefore, the engobe recipe must be adapted to the clay of the piece.

In order to make changes, we must proceed experimentally, starting with the physical properties, as we saw, firstly, there is shrinkage, but also other things such as:

• - the fluidity (drying rate, amount of water, viscosity) for which the mode of application is decisive, but by intervening on the components it can be partly modified;
• - the vitrification appropriate to the surface of the piece is governed by the flux. Non-glazing coatings usually have lower amounts of flux and result in surfaces that lie between naked clay and glaze. If the engobe is glassier than the underlying clay, it adheres better to it. To make it glassier, flux must be added by bringing it closer to a glaze and thus changing the interactions with the overlying glaze or crystalline;
• - the permeability of non-glaze coatings;
• - the coverage: engobe opacity can help cover physical imperfections or stains, but not clay or water evaporation stains made by salts;
• - thermal expansion coefficient: but what is that? Very important! To make clay/coating adhesion work after firing, clay and coating must have the same coefficient of thermal expansion. During the use of the piece, when the object undergoes thermal changes, contractions/dilations of clay and coating must be similar. An abrupt temperature change, may make the engobe flake if there is not proper compatibility between engobe and the clay. If there is also a glaze, the expansion of the engobe must have compatibility with both, the underlying clay and the glaze applied on top. Therefore, we first have to find the compatibility between glaze and clay, then to adapt the engobe to the glaze/clay compatibility. Hence, you first have to test the adhesion of the glaze to the piece then when you have the right glaze, you go on to test the engobe/piece adhesion in order to have a good bond on the fired product. Then the glaze/engobe compatibility is checked.

All of these factors can be changed, acting on the engobe composition.

But there is another factor. When should engobes be applied?

They can be applied to leather-hard, or bone-dry clay or to a bisque piece. The composition, for all the above varies.

To give an example, broadly speaking,
R.J. Wilson, in his "Inside Japanese Ceramics" (Ed. Weatherhill )
indicates three basic formulas for obtaining the blank engobes of traditional Japanese ceramics.

To apply to leather-hard pieces: 70% clay (kaolin and/or white plastic clay) 20% siliceous rock or quartz 10% flux (feldspar or limestone) To bone dry pieces: 40% clay (kaolin and/or white plastic clay) of which one part calcined 40% siliceous rock or quartz 20% flux (feldspar or limestone) To bisqued pieces: 40% clay (kaolin and/or white plastic clay) all calcined 40% siliceous rock or quartz 20% flux (feldspar or limestone)

The following table is an example of some other coatings reported by well-known ceramists.

Note: Feldspar: both Na and K

Ca carbonate: sometimes replaces all or part of Feldspar

Fe ox is optional

Zirconium silicate is used in ceramics as a glaze opacifier (cheaper than tin oxide, another opacifier/whitener). In a glaze-at 10% dry weight over total, it gives an opaque white.

The theme, I speak about here, has been only hinted, because it is so vast and can be developed in various aspects, as it is proper to the history of ceramics and Humanity, as well as to planet Earth’s geological history. There are endless ways to decorate ceramic surfaces, with varying results using different clays depending on the coatings, the application, temperature and firing methods. 

Bibliography

Emanuele Grill - Industrial minerals and rock minerals - Ed. U. Hoepli. 

Nino Caruso - Living Ceramics - Ed. U. Hoepli. 

Nino Caruso - Ceramic Decoration - Ed. U. Hoepli. 

Susan Peterson - Making pottery - Zanichelli ed. 

P. Rada, M. Hucek - The techniques of ceramics - Ed. Melita. 

Piero Cademartori - Complete Course in Ceramics - Ed. De Vecchi