Glazes and Raku glazes

In ceramics, glazes are used to seal the surface and to seal earthenware, while still being decorative. Glazed vessels are more resistant to scratches from cutlery, for example. Due to their very smooth surface,áconsumer ceramics are easier to clean and therefore much more hygienic.

Ceramic glazes have been known to man for a long time. The first glazes were developed in ancient Egypt and the Mediterranean region, in each case in the 6th to 4th millennium BC, and then became an everyday object in the 3rd/4th millennium BC. In Germany, mainly thanks to the Romans, pottery and the associated glazing developed in the Rhine region on a large scale around the turn of the century. In the "Scheduladiversarumartium" (around 1120), different coloured glazes for the ornamental decoration of ceramic vessels are described.

Then as now, glaze consists of silicon oxide. This frequently occurring raw material is usually added to glazes in the form of the minerals quartz, flint, feldspar or kaolin. However, the melting point of silicon oxide is 1700░C! In order for the glaze to melt out at lower temperatures, a flux (e.g. lead, potassium, sodium, barium and/or lithium compounds) is used, which lowers the melting point of silicon oxide. Of course, with the Ceramic Cartel, we only use glazes that are free of lead for all our beautiful handmade ceramics. Furthermore, we use glazes that are free of markings for all our ingenious and functional utility ceramics. The beautiful different colours are created by pulverised oxide compounds, which are contained and distributed as pigments in the glaze.

Cleaning prior to glazing

Before glazing, it is important to wipe the ceramics with a clean, damp sponge on the surfaces to be glazed to remove any dust. This would get into the glaze during application and the glaze would become blind over time due to the dust. Glazes can be applied to the bisqued ceramics either in powder form or wet in Formeiner viscous emulsion. When using powder glazes, the work risk is extremely high, as the glaze dust can cause pneumoconiosis and, as a result, lung cancer. It is therefore better for life and limb to use glaze in a slurry form, i.e. "in liquid". It is also a lot easier, as the clean, grease-free ceramics can then be easily doused, dipped or glazed with a spray gun. The water contained in the glaze penetrates into the porous ceramic during glazing and leaves a layer of powder when drying. This layer is touch-sensitive and should be applied as evenly as possible.

rohe glasierte Keramiken auf dem Tisch - Detailaufnahme
Keramik-Kartell - Glasieren

The glaze firing

During the subsequent firing between 700 and 1400 ░C, this melts into a homogeneous and above all dense glass. As you can see, the range of melting out is quite wide. The respective melting point depends on the offset - i.e. the respective composition of the glaze. The composition also influences the viscosity of a glaze, which in turn is an important criterion for firing: viscous glazes run little, but prevent gases from escaping from the ceramic and therefore tend to form bubbles. Low-viscosity glazes, on the other hand, like to run but allow the gases to escape freely from the ceramic. It is always a tightrope walk and requires time, sensitivity, willingness to experiment and above all experience. This is exactly what the potters had centuries ago, because how do you know what temperature the kiln is at without using a thermometer?
Indicators are of course the colour of the fire in the firing chamber and the sparkle of the glaze. Especially for theáRaku technique andáRaku firing these are important indicators, as the small portable thermometers are sometimes quite inaccurate. A better indicator is the shrinkage of the standardized clay strips. These are added during firing, observed and when the specified shrinkage is reached the firing is finished. It became simpler in 1890 with the introduction and use of Seger cones. Hermann August Seger had experimented with shrinkage and thus invented the Seger cones that are still used today. These change their shape (tilt) at a certain temperature and are available for different temperature ranges.

Insights into the classification of glazes

Since there is a myriad of glazes and techniques available worldwide, they are divided into various categories. For hobby use, the most common is according to colour and temperature. In the specialist field, a distinction is made between main fluxes (such as lead glazes or lead boron glazes), the main raw material(s) (e.g. feldspathic or ash glaze), the colour (coloured glazes), the way they are created (ash glazes and salt glazes) and their use (earthenware or stoneware glazes). In principle, the higher the temperature, the more limited the colour palette.

Glazes used for stoneware contain frits mixed with quartz and feldspar as sources of silicon and with colouring components such as oxides. Ceramic frits are pre-melted, silicate glasses. This process allows the melting point of glazes to be lowered and it is also possible to melt out silicate at lower temperatures such as 980 ░C - 1100 ░C. Another advantage of using frits is that water-soluble substances are converted into relatively insoluble products and are bound within the glaze.

Bunzlauer Keramik in der Küche

Earthenware and stoneware?

In this way we get from the Keramik-Kartell also the bright colors on our beautiful handmadeádecorative ceramics and since stoneware is fired at only around 1000 ░C, a wide range of colors is possible. Unfortunately, however, this ceramic is not waterproof, as the silicate contained in the clay itself cannot melt out at such low temperatures. As a result, the water penetrates the porous ceramic through the cracks created during the cooling process and blows off the glaze. Because of this, most of the ceramics used in the garden (e.g. terracotta) must be brought in in winter, as the ice that forms in the shards breaks them.

Stoneware glazes are fired at higher temperatures, around 1200 ░C. Feldspar is used as a source of silicon here, which is mixed with highly fluid clay, loam, ash or salt to form a glaze. During firing, the respective components react differently, depending on the oxygen supply. In oxidising or reducing kiln gases, correspondingly different colours are produced, for example, brownish or grey in Westerwalder Steinzeug. However, the colour range is very limited and earthy colours such as brown, beige and ochre are produced to a large extent. By adding metal oxides, a larger colour palette was achieved. Thus, the iron oxides contained in a clay glaze form enchanting celadon glazes with colour nuances from green to olive yellow in the reduction firing. In the oxidation firing (i.e. with oxygen supply), on the other hand, ceramics such as the world-famous blue tableware from Bunzlau (Bolesławiec) are produced.

The wood firing and the ash landing glaze

The ash landing glaze takes place during wood firing and is a special form of stone draught glaze. With this type of glaze, the wafting gases and the flame itself are conducted into the combustion chamber and remain there for a certain time. The organic compounds contained in wood or even bones form a kind of flux on the ceramics and thus coat them with a glassy layer. This type of firing is very time, energy and above all resource-intensive, as the kiln must be fired continuously with wood for 12 hours.

The best known stoneware glaze is probably the salt glaze. The kiln is opened at 1250 ░C and common salt is put in and the sodium ions contained in the common salt are deposited on the ceramic when the common salt is burned. During the burning process, however, chlorine is also produced, which damages the square wire in the kiln and of course sufficient ventilation must be provided. The resulting sodium silicate glass, however, is particularly resistant, hard, acid-proof and often has a slightly grained surface.

The special glazes

The last point to mention are the special glazes. These include the Tenmoku glaze (named after its region of origin in China), the snakeskin glaze or the tea dust glaze. Also transparent glazes, opaque glazes, matt glazes, crystal glazes or crackle glazes such as those foráRaku are to be included here. Basically, all low firing glazes are suitable for Raku firing. However, due to their low tendency to crackle, they are usually not suitable for Raku firing. Raku glazes are not acid-resistant due to their composition. For this reason, our beautiful handmade Raku ceramics, especially our Chawans, should be enjoyed in the traditional Japanese way (without lemon!). After all, it is also a Japanese technique!