Engobe

Engobes are high-clay slurries that are applied to leather hard or dry ceramics. They fire opaque and are used for functional or decorative purposes. They are formulated to match the firing shrinkage and thermal expansion of the body.

Key phrases linking here: engobed, engobes, engobe - Learn more

Details

The power of a black engobe
This slab mug is made from a buff stoneware, M340. A black engobe was applied by pouring the inside and dipping the outside two-thirds of the way down. The mug was then dipped into a honey transparent, GA6-B.

A white or colored opaque-firing slip applied to clay as a coating. The term "slip" is often used interchangeably with "engobe" but they are not actually the same. Generally, potters talk about slips in the context of decorating, even using the term as a verb. But in industry engobes are employed to hide or cover an underlying body - opacity is thus vital. The tile industry uses the largest volumes of engobe by far, employing them as barriers between less-than-white bodies and the overlying glaze. Even locally available brown or red burning bodies can be made white using engobes, they almost always vitrify at much lower temperatures producing a stronger product. Another area of cost-saving is that glazes need less zircon opacifier (assuming that no zircon is needed in the engobe) over a white surface than a dark one.

The use of engobes adds steps (and time) to the production procedure. That being said, the benefits can also accelerate other parts of the process. In terracotta and stoneware processes, engobes are ideally applied to leather-hard ware (for best bonding). But in practice they are often applied to bisque ware, especially with terracotta tiles.

Engobes need to be fluid enough to pour and drain well but have a high enough thixotropy to gel and stay in place without drips after application. Perfecting the rheology (specific gravity, viscosity and thixotropy) of a slurry can be difficult and delicate, when starting out it is best to be conservative, tolerating a lower specific gravity. A valuable asset is a good wall-mounted variable-speed propeller mixer that can mix the slurry well but not so much that it pulls in air bubbles. Ideally, engobes need a specific gravity higher than glaze (often much higher) to dry as fast as possible. That means some deflocculant is needed. A specific gravity of 1.5 is a good starting point. Use enough deflocculant to make it too fluid and then bring it back to thixotropic using an addition of flocculant (e.g. Epsom salts). The system is very sensitive and it is easy to add too much - be patient adding only a little and waiting as it mixes. When right the slurry is fluid enough to apply evenly and drain well but then gels after a few moments (so you do not have to wait while it drips-drips-drips). In a gallon-size batch, this can mean additions of only drop(s) of flocculant or deflocculant to move it as needed. Trial application on pieces is the best way to determine if the rheology is right. Once learned, the whole process really is magic. One warning: If you overdo a flocculant addition when adjusting the slurry can become lumpy. Do not panic, the addition of a little deflocculant and propeller mixing will bring it back. But don't go back and forth too many times.

For tiles, engobes are applied as a curtain over a continuously moving production line or as a powder layer during the pressing of the tile itself. Potters would not generally have the equipment and expertise to apply engobes to dry ware (unless being painted on in a very layer). While it may appear possible to apply engobes to bisque ware (by adding CMC gum to harden and adhere them and VeeGum to gel the slurry somewhat), this is not advisable. The bond will be compromised as the engobe fire-shrinks on a body that has already done so. While thin layers may appear to work, they will likely fail with time because of internal stresses.

Engobe consistency will change on storage so mix thoroughly and readjust the degree of gel on each use. Once it has stabilized to the ideal consistency the slurry will gel on storage and resist thinning without propeller mixing. It will also power mix to what appears to be too thin of a consistency, but will gel up quickly to working behaviour. It is common to mix or stir the slurry before the dipping of each piece.

“Engobe” is a more specific term than “slip”. Potters use slips for decorative purposes, they are normally made from a clay of contrasting color (with added feldspar, bentonite, etc). Potters often 'traffic' slip recipes. By that term, we mean that people often just try a bunch of recipes until they find one that appears to work (without considering any technical considerations). Industrial engobes, on the other hand, are designed for a specific body or body type, technicians creating and maintaining them are faced with a balancing act and constant production issues.

Bonding is a key consideration when using engobes. The thicker the layer, or the more layers, the more issues there will be. The challenge starts at the initial application and drying stage. When applied to leather-hard ware the engobe must bond mechanically to the surface during drying and shrink with the body. To do this it needs enough clays and binders to hang on but not so much that it shrinks excessively and cracks during drying. Strangely, engobes having a plasticity (and stickiness) considerably higher than the body seem to bond best during drying. For this reason, it is common to see very high ball clay percentages and additions of as much as 10% bentonite (which slows down drying considerably). That being said, as an engobe's thickness increases its ability to impose its own shrinkage also increases, and, as it does so, it will crack to relieve the stress if applied too thickly. A note about ball clay: If you don't have the one shown in a recipe just try the ones you do have, most ball clays have very similar properties.

Engobes need to be thermal expansion compatible with the body also (but only after their fired shrinkages are matched). This is so that stresses coming with temperature change during service do not lead to flaking off in use. Of course, tableware will experience much more thermal stress than will floor tile, for example. When glaze is applied over an engobe, it is important that the thermal expansions complement each other. In tile, for example, the thermal expansion of the engobe needs to be between that of the body and the glaze.

Engobe recipes found online often look like glazes, but they should not. Engobes are like bodies, there should be little or no frit or boron source in the recipe (except possibly at low temperatures). Excessive flux will compromise opacity. More important, excessive flux will give the engobe a greater fired shrinkage than the body, introducing stresses that can severely test their bond and weaken the fired marriage. A vitreous engobe might have a fired shrinkage of 15%. Many stoneware bodies can be as little as 5-8%, vitreous bodies up to 10%. Earthenwares can exhibit a wide range of shrinkages depending on the firing temperature. Test bars of body and engobe can be fired at a range of temperatures to measure the fired shrinkages. Or, bi-clay strips can be made, these sandwich the body and engobe in a thin bar that curls during drying and firing to indicate fit or misfit. The lesson: Vitreous engobes on vitreous bodies, non-vitreous ones on non-vitreous bodies.

Thermal expansion compatibility is especially important in commercial products. Engobes cannot stand tensile stresses any better than glazes, so it is normal that an engobe should be under at least some thermal compression (not too much of course, especially if it is applied thickly). If ware is glazed (most common), fit is even more important. There are two interfaces to consider, body-to-engobe and engobe-to-glaze. Thus body/engobe compatibility should be tested separately (without the glaze) and the engobe/glaze compatibility tested (without the body, e.g. using the EBCT test). It is very important to avoid situations where the engobe is under tension from the body and the glaze under compression on the engobe (and vice versa). Engobes do not bond to the body as well as a glaze, so poor fit combinations are more likely to produce peeling and chipping at that interface. Because of the two interfaces and three expansions, it is easy to misdiagnose the cause of failure. The ideal situation is where the glaze has a lower thermal expansion than the engobe (thus being compressed by it) and the engobe lower than the body (and compressed by it). Example COE's reported to us by one tile manufacturer: Body- 200.7, engobe - 196 to 201, glossy glaze - 179 to 181.

Potters have options that are prohibitive in industry (because of cost and difficulty). For example, high percentages of zircon opacifier can be added for whiteness and opacity. Highly plastic and stickier mixes, lower specific gravities can be tolerated (they dry much slower). There is merit to creating a brushing version of your engobe, this enables close control of thickness and coverage. Use only CMC gum (not Veegum CER) to enable keeping the specific gravity as high as possible.