Glazes, engobes and crystalline glazes: let's clarify!

The processing of ceramics is one of the most loved and widespread artistic activities. A practice that requires attention and care, passion, and a lot of creativity. Creativity that is not limited to the shapes and uses of objects, but is expressed at its best in colouring and decoration.

There are various ways to colour ceramics, and for each one, there are details and techniques to know in order to choose the most suitable path to follow. This choice will depend on unique and personal factors, such as your taste and the desired aesthetic effect, but also on the final use of the product. As we will see later, for example, some treatments make ceramics waterproof, while others do not.

In this article, we will briefly but clearly explain what you need to know about engobes, glazes, crystalline glazes, and under-glazes, analysing their different characteristics and methods of use. Then we will delve into the practice of glazing, with its basic techniques and precautions to follow for an optimal result. Finally, we will provide an overview of firing, aimed at fixing the glaze and completing your ceramic project.

Engobe

Let’s start with engobe (or slip), which is a type of colouring applied to raw ceramics. It consists of coloured clay mixed with water. Along with slip, an even simpler mixture of just clay and water, it is one of the oldest methods of colouring and decorating: there are findings that testify to its use as far back as 3000 BC!

Engobe is applied to ceramics that are not yet completely dry, specifically at leather-hard stage, and has a matte finish. If you desire a shiny and glossy finish, a transparent crystalline glaze can be applied in a second firing, as we will see later.

One of the advantages of engobe is its ease of use, especially if you use a ready-to-use liquid preparation, like the ones shown in the photo.

Engobes are versatile, available in practically every colour, and adhere well to clay, remaining stable during firing. Below are some decorative ideas you can create with engobe:

• marbled effect: apply a thick layer of paint and then, with a brush or an applicator bottle, draw lines of another colour, allowing them to mix with the base colour to create streaks.
• Sponging: use one or more colours to create patterns, applying the sponge with more or less decisive strokes depending on your taste.
• Stencilling: you can create a design or decoration on a stencil and then apply it to the clay before applying the colour.

Glazes, Crystalline Glazes, and Under-glazes

Glazing is one of the most well-known and common methods for colouring ceramics. Besides its decorative purpose, glaze also serves to seal your work through the second firing and make it waterproof.

Glaze can have a glossy and shiny finish, or a matte and satin finish, and in terms of colours and decorative effects, it offers an almost infinite variety of possibilities. Since glazes are made of chemical compounds that react differently to clay, you can achieve consistently different and, above all, unique results! For this reason, it can be useful to conduct some glaze tests, applying different glazes to the clays you are working with and proceeding to firing to check the effects. You can also test different colours by mixing glazes or creating successive layers, recording all results so you can repeat and recreate the desired effects.

Other important precautions for using glaze, which we will explore in greater detail later, include:

• the thorough cleaning of the surface: clean the clay well with a damp cloth or sponge and wait for it to dry completely before starting;
• the consistency of the glaze: mix thoroughly, ensuring there are no sediments at the bottom or lumps.

One important thing to keep in mind when discussing glazes, especially for the final aesthetic effect of your work, is that there is the possibility of creating either an opaque or transparent coating.

Glazes, in fact, have the characteristic of being opaque: the glassy coating obtained from glazing covers the underlying terracotta, creating a uniform layer of another colour. The crystalline glaze, on the other hand, allows for the creation of a glassy coating similar to glaze, but with a transparency effect: this means that beneath the glaze, the original terracotta will be visible, as well as any decorations you may have previously created on it, which are called under-glazes.

The term under-glazes refers to any colour applied to the surface of the ceramics before applying a transparent glaze. Under-glazes, therefore, are decorations used to create designs and patterns that will emerge under and through the layer of transparent glaze, creating effects of greater depth and richness. Crystalline glazes can be neutral or coloured, and have a glossy or satin finish, and can be mixed together while still maintaining transparency.

Glazing: Methods and Precautions

Now that we have clarified the different types of materials and methods of decoration, let’s delve into the practice of glazing.

First, the application methods: dipping, spraying, dusting, or brush?

Dipping, as the word suggests, involves immersing your object in a container full of glaze for about three seconds. It is the easiest and fastest method, suitable for beginners, and allows for a uniform coating layer. It is important to give the glaze the right consistency so that it adheres well to the ceramics. The more fluid the glaze is, the longer the immersion time required.

The brush, finally, is particularly suitable for smaller objects and precision work. Brush glazes have a much thicker consistency, and it is important to read the instructions to know how many layers need to be applied to achieve the desired colour. One of the “risks” with this type of technique is leaving brush marks. But if this is an aesthetic effect you wish to achieve, we recommend varying the thickness of the glaze layers as you apply them.

Other methods for applying glaze include sponging, perhaps using sponges of different textures for different colours and applications, and spraying, which will allow for an even distribution on all surfaces.

Now let’s talk about firing. As we have already mentioned, while engobe can be applied to fresh ceramics, glaze must instead be applied to bisque-fired ceramics, followed by a second firing. The kiln is slowly brought to the necessary temperature for the melting of the silica in the glaze, and then slowly cooled. This makes the ceramics strong, solid, waterproof, and resistant to the elements.

All glazes are classified by firing levels, ranging from medium-low temperatures to extremely high temperatures: it is therefore important to respect the indicated ranges to avoid defects and breakage. For this reason, we refer you to the last paragraph of this guide, where we have gathered the basic indications for each temperature range.

The firing phase is also when some of the most common problems related to glaze application errors or firing can occur. Let’s look at the main ones!

Under-firing or over-firing: in the case of incomplete firing, the result will be a rough and dull glaze. The good news is that this problem can be resolved by subjecting the clay to a new firing at higher temperatures. The case of excessive firing is different, from which there is unfortunately no return. In this case, the main problem is the excessive melting of the glaze, which will drip off the clay, becoming thinner at the top and too thick at the base, or will drip in the kiln damaging any other products.

Lack of adhesion between ceramics and glaze: during firing, due to the high temperatures, both the clay and the glaze will have greater fluidity and will adhere perfectly. However, since they are materials with different physical behaviours, they can lose this adhesion when cooling begins. If the glaze layer contracts too much, it will not cover the entire surface of the clay, creating cracks. Conversely, if the clay contracts too much, spaces will form between the two layers.

Incorrect application: a layer that is too thin can cause imperfect and irregular glazing; a layer that is too thick can cause the glaze to drip off or create bubbles.

Lack of adhesion: we have already emphasised the importance of having a surface as clean and dry as possible before applying glaze, as impurities, dust, and moisture can affect the adhesion of the glaze to the clay. If you intend to apply a second layer of glaze, you should not let the first layer dry too much; otherwise, the second glazing will not adhere.

Creation of bubbles: this is a defect that concerns the clay more than the glaze, but it can negatively affect the glaze as well, creating bubbles or holes. For this reason, you must ensure that there are no air bubbles in your clay.

Other types of unforeseen events, more than errors, involve problems that can occur with the kiln: from power outages to one object sliding against another, resulting in “sticking,” or the glaze dripping onto another object, or the “migration” of a colour – especially very volatile ones – from one object to another, staining it.

To conclude this guide to colouring ceramics, we have prepared a summary that presents all the temperature ranges that are more or less commonly used for glaze firing. As we have seen, indeed, the wrong temperature can create many problems. For each temperature range, you will find some usage indications, along with the main pros and cons.