Slip, engobe, glaze and washes are terms you often hear when you delve into ceramic decoration. These terms have always been a source of confusion for me, especially since different ceramists give them different meanings. A clear definition can help to bring order in the chaos. But even more important is gaining insight.
You can memorize definitions, but I’m happy to leave that to AI models. More important is understanding both the differences and similarities between these materials for ceramic decoration. In my opinion, it is essential to recognize that all of these decorating techniques are part of a continuum.
This continuum extends from slip, through engobe and glaze, to washes. Based on the amount of clay (the source for Al2O3) and quartz (SiO2) you can put this into perspective.
This is easy to visualize with the help of the “Stull graph”. R.T. Stull made this graphic in 1912 to classify glazes. But also very useful to gain insight into the entire ceramic decoration spectrum.
Decorative slip: a coloured clay layer
Decorative slip is a clay that has been mixed with enough water to make it liquid. You can make a roof tile with red river clay, but when you apply it to a white pot for decoration, it transforms into decorative slip.
Other names for decorative slip are clay slip and the French term barbotine. Decorative slip and clay slip are synonymous, but clay slip should not be confused with casting slip. Although the composition may be similar, casting slip is used for casting ceramics, not for decoration. I use the term ‘barbotine’ for thick-liquid clay slip, with which you apply raised decorations, for connoisseurs known as slip trailing (“ringeloren” in Dutch).
To adjust the color or shrinkage of the clay slip, you can add different raw materials. For example, iron and cobalt oxide can be added for color, or quartz and calcined kaolin to modify the dry shrinkage.
Slip versus Engobe
However, if you add more than 50% of materials that are not clay, I no call it longer slip but an engobe. It’s an arbitrary boundary that I’ve taken from Daniel Rhodes. We could also have set the limit at 40 or 60%, but I think 50% clay in the recipe is a nice compromise 🙂
Although the terms slip and engobe are often used interchangeably in ceramic decoration, there is a distinct difference in clay content. Slip, with a higher clay content, will shrink more during drying than engobe. Therefore, as a rule of thumb, slip should be applied to clay that is soft-leather-hard, while engobe can be applied to leather-hard to hard-leather-hard pots.
Engobe: neither slip nor glaze
“Engobes are clay-like coatings that cover the underlying ceramic shard and are fired together with the shard at the right temperatures. They are designed to both refine the surface of the shard and to give it more color than what would be possible with the clay mass alone…”
The above definition comes from Wolf Matthes. He has an entire book dedicated to engobes, which is definitely a recommendation for those who want to delve further into this subject.
An engobe is therefore an opaque layer on the ceramic shard. It is neither a clay slip nor a glaze, but something in between. Usually, an engobe is applied to leather-hard clay. However, if the engobe has very low drying shrinkage, it can also be applied to biscuit, which is ceramic that has been fired for the first time.
Engobe versus Glaze
In addition to clay, an engobe recipe also contains other raw materials, such as quartz, feldspar and frittes. Some engobes sinter, a preliminary stage of melting, more than others. The engobes that tend more to melt are called “sinter-engobes”. Since the clay content in the recipe is relatively low, these can often also be applied to biscuit.
So, a sintered engobe looks a lot like a glaze, but the difference is that it is not completely melted into glass. Thus, an engobe is a layer of cover that has not completely melted out.
There is no “recipe definition” for the distinction between an engobe and a glaze, as there is for sludge and engobe. To determine the difference between engobe and enamel, I look at the underlying chemistry: the UMF (“Unity Molecular Formula”) also known as the Seger formula. In general, in the case of engobe, the alumina (Al2O3) in the UMF will be higher than 0.6 to 0.7, sometimes even considerably higher.
Glaze: molten glass
Glaze forms a finishing layer on ceramics, which completely melts into an aluminum-silicate glass. It is the most widely used and best-known ceramic decoration.
Usually, the amount of alumina (Al2O3) in the UMF is less than 0.6 to 0.7. However, not all ceramic finishes with anAl2O3 content below 0.6 are actually glazes. A glaze is not recognized as such until the chemical reaction that forms it into glass is complete.
There are many types and subtypes of glazes. For a durable and safe glaze, it is crucial that it is completely melted. This is clearly observable with an SEM microscope. Without such a microscope, one can only estimate this on the basis of the UMF.
A melted glaze can be recognized by its shine, but matte glazes can also be completely melted. However, many “matte glazes” are matte because they are not completely melted; I don’t consider these as glazes, but as sinter engobes. Although there is nothing wrong with this, they are not glazes and certainly not as resistant as glazes.
This means that a glaze that has been completely melted and fired at 1280 °C, becomes a sintered engobe at 1180 °C (sintered, but not completely melted) and a (matte) engobe at 1080 °C. It’s a matter of perspective 🙂
Glaze versus Wash
From slip to glaze, there is a gradual transition from unmelted clay to increasingly sintered clay through the addition of glass and fluxes, until it becomes a completely melted glass. From a chemical point of view, the process evolves from slip, with a high clay content (and therefore a high content of Al2O3), to an increasingly low content of Al2O3. When this content is low enough to melt at the temperatures of a ceramic kiln, we speak of glaze.
But are we there? Almost, but not quite. Glaze consists of molten Al2O3 and SiO2, which forms an aluminum silicate glass. However, if there is not enough Al2O3 and SiO2 present to form a glass, we call this a “wash”.
In his experiments, the glaze researcher Stull determined that a glaze must contain at least 0.1 Al2O3 and 0.6 SiO2 in order to form a glass. So if a glaze in the UMF is below 0.1 Al2O3 , it is by definition no longer a glaze. After all, there is not enough glass former present to be able to form a glaze.
Wash: colour but not glass
A wash is a colour application that is applied to, under or without glaze on the ceramic shard. It consists mainly of oxides or carbonates (often of iron, manganese, cobalt, chromium, copper, nickel, titanium or rutile) or a commercial color pigment. This mixture is usually mixed with a small amount of frit (glass) or other melting agent and applied to the ceramic. Wiping or sponging away the mixture highlights the deeper areas.
Philippe Pirard has dedicated an entire book to washes, with more than 460 recipes. It is an impressive work that I appreciate very much, especially because of its title: “Jus d’oxydes“. The French language possesses a certain elegance that is sometimes lacking in English (and Dutch).
Ceramic decoration: it’s a spectrum
With the wash, we’ve had the whole spectrum of ceramic decoration, from slip and engobe to glaze and washes.
Unfortunately, what leads to confusion is that many ceramic decoration products are sold under an exotic name. Such as “stroke and coat”, “texturizer”, “unidecor”, majolica, and so on. Some are purely a marketing term, others have been created historically.
They all fit within the spectrum, but where exactly is a matter of the chemical composition and definition. The marketing doesn’t make it easy for ceramists.
And I like clear definitions and boundaries in perspective. I’m probably somewhere on the (ceramics) spectrum too 🙂
Literature List
- Philippe Pirard, “Jus D’oxide”, (2019) (French, also published in German)
- Daniel Rhodes, “Clay and Clazes for the Potter” (originally from 1957, new revised edition from 2022. Also published in German and French)
- Wolf Matthes, “Engoben und andere tonige Überzüge auf Keramik” (2006) (German, also published in French)
- R.T. Stull, “Influences of variable silica and alumina on porcelain glazes of constant RO”, (1912)
Want to know more about glazes?
Better understand how glazes work with the help of Glazy and the Stull chart. Learn to discover enamel mechanisms and control melting temperature. Develop glazes from the right perspective!
Follow the course “Systematic Glaze Research with Glazy”
Send me an email and I’ll keep you informed!
About Post Author
