An engobe is a white or colored coating for clay products. This substance is ideal for emphasizing the natural color of clay and for adding decorative accents. It is applied to wet or dry clay, and then burned. If necessary, it can be coated with glaze. The use of engobes can be traced back to 3000 BC. e. Samples of ceramics treated with such a substance were discovered during archaeological excavations.
Engobes for ceramics - what is it
They are a liquid ceramic mass, a mixture of clay, water and, as a rule, dye. Flux or silica (silicon dioxide) can also be used as ingredients. In their manufacture, frit material is used (frit, silica-rich glass composition, which is burned over a small fire before sintering). This reduces shrinkage on the finished product.
The properties of engobe and glaze are somewhat similar. Glaze is a thin glassy coating used for clay products. It is a powder mixture of oxides and pigments diluted with water. It is applied by dipping, spraying, watering or using a brush. These two types of coating differ on the finished product: the glaze has a glossy finish.
How is used
Engobe is a coating that is considered universal. It is used primarily to give the work a depth of color and to diversify it.
Colored engobes are used as underglaze paints. They are characterized by great juiciness of color, with their help a rich color palette with very many shades and subtle transitions is easily created.
Engobe is one of the most effective ways of painting ceramic products in the case of complex, detailed drawings, especially when using several different colors.
It can be used as a continuous or partial coating. It helps form a smoother surface. Also, using engobe painting, you can hide unwanted coloring, embossed patterns and more. They can be used as a coating for which they do not apply additional processing: thus, the product acquires a finished texture and color. It is also used as an intermediate coating between a layer of ceramic and glaze.
Using engobe, you can apply color spots, stripes and complex patterns. You can use a pencil to apply a precise drawing to mark up first. You can also use a stencil.
In the largest volume, engobe is used in the manufacture of tiles as a layer between the base and glaze. In this case, automated methods of filling / dipping are used. White engobe creates a surface on which glazes can have the same bright colors as on porcelain.
Engobes are applied in the same way as glaze by watering, dipping, spraying with a brush. In this case, the product may be moist, slightly withered, dry or preliminarily fired. After engobe has been applied, the product can be immediately coated with glaze and sent for firing. However, the greatest effect is achieved if the glaze is applied after the product coated with engobe has been pre-burned.
The main conditions for high-quality coating of products with engobes: a perfectly clean surface of the product, the correspondence of air and fire shrinkage of the engobe and the engobed material, the rough surface of the product to ensure sintering of the engobe with the main material. The thickness of the applied layer of engobe should not exceed 0.2 mm, since a thicker coating can peel off during drying and firing.
There is a certain technology for preparing engobes. Solid materials (pegmatite, chalk, glass break) are first washed, sorted and ground. Then they are dosed in accordance with the composition, placed in a ball mill, where 40% of water is added to them, and also, if necessary, coloring pigments. The grinding and mixing process takes from 20 to 25 hours, after which the resulting mixture is filtered and poured into containers.
For the first time, this technique appeared in England by accident, when a master spit chewing tobacco on a decorated product, as a result of which drawings resembling branches or corals began to creep.
The black engobe is bred to a consistency a little more liquid than usual. To this is added a squeeze of tobacco. You can also use engobe of any color with the addition of pigment or oxide. In this case, the process occurs when acidic and alkaline media are mixed. The first of them can be created using vinegar, citric acid, terpentine, fairies and even beer.
In order to apply a drawing, it is necessary to drip the resulting mixture without touching the product with a brush. After that, the product is dried, fired, covered with a transparent or translucent glaze and fired again.
Features of the creation process
Improving the rheology (fluidity of the substance), taking into account the specific gravity, viscosity and thixotropy (the ability to dilute) of a suspension, is a complex, delicate process. Great importance is attached to the equipment for mixing, since it is necessary to ensure that during the process air bubbles are not drawn into the mass. For the quick drying of the engobe, a not too large specific gravity of the base, a sufficient amount of liquid to ensure fluidity and viscosity are required. The consistency of the engobe changes during storage, so it is necessary to thoroughly mix and adjust the degree of viscosity with each use.
When using it, it should be remembered that the thicker the layer, the more this creates problems. When applied to solid products, the engobe must be mechanically bonded to the surface during drying and shrinkage. When glaze is applied to engobe, it is important that the thermal expansions of the two materials complement each other.
In each case, the engobe shrinkage coefficient is an important parameter, it must correspond to the similar clay parameter below it, otherwise the top layer will be damaged.
Materials for creating
They are divided into several groups:
- clay with kaolin or calcined kaolin, usually used instead of ball clay to counter shrinkage;
- fluxes used in glaze;
- fillers (usually silica);
- hardeners (borax, calcium borate and various resins)
Basic engobe recipes
The following three recipes for the main engobes provide a good starting point for further experiments. Engobes can be painted in any usual way.
It may include components in the following percentage:
- Kaolin - 20, talc - 25, calcined kaolin - 10, calcium borate - 15, silicon dioxide - 15, borax - 5, zirkopaks - 10.
- Kaolin - 15, talc - 10, calcined kaolin - 20, calcium borate - 10, nepheline syenite - 10, borax - 5, silicon dioxide - 20, zirkopaks (zircon muffler for glazes) - 10.
- Kaolin - 15, talc - 5, calcined kaolin - 35, nepheline syenite - 15, borax - 5, silicon dioxide - 15, zirkopaks - 10.
Getting color engobes
In the manufacture of colored engobe, a carefully measured amount of coloring pigment is taken. First, it is ground on glass with the addition of water. Then the white engobe is thoroughly mixed with the resulting mass until the color is uniform.
To create a simple blue engobe, you can use 40% ball clay, 20% red iron oxide, 20% manganese dioxide, 20% cobalt oxide.
Various oxides, carbonates, and commercial dyes can be used to color base engobe recipes. Some colors can be obtained by adding the following dyes.
A black tint is obtained by adding 3% iron oxide and 2% cobalt oxide, nickel oxide, manganese dioxide.
A dark blue color is achieved by adding 1.5% cobalt oxide.
Green color of medium saturation is obtained by adding 3% copper oxide.
Ocher is obtained by adding 4.5% yellow ocher.
Red color of medium saturation is achieved by adding 3% iron oxide.
With the addition of 6% rutile, a creamy brown tint can be achieved.
If you add 3% iron chromate, you get a dark gray engobe. When 6% manganese dioxide is added, a violet-brown color is obtained.