All about the material

The word ceramic is derived from the Greek “keramikos” for pottery. The definition of ceramic stands for all inorganic non-metallic materials which are moulded before firing, i.e. including for all products manufactured by firing clay as a basic raw material, e.g. vases, roof and wall tiles, stove tiles, floor tiles and much more.

Raw Materials

From clay to feldspar, chamotte and kaolin

Clay is the primary component of ceramic tile compounds and evolved through weathering of rock containing feldspar (e.g. granite) owing to the effects of wind, water and seasonal differences in temperature. This weathering occurred during the tertiary period millions of years ago. It comprises fine-grained minerals, whereby these minerals (layered silicate) lend the actual clay its malleable properties. Owing to the particular requirements on the ceramic manufacturing process as well as the actual product, other raw materials such as kaolin, fireclay and feldspar, for example, also need to be added to these clay materials. Processing involves homogenous mixing of all components. The ensuing compound is further processed in the ceramic manufacturing process.

Sample composition of ceramic tile compounds: clay 50-70%, fireclay 10-20%, feldspar 15-25%, kaolin 0-10%.

MOULDING PROCESS

How does the tile get its shape?

When moulding tiles, a distinction is essentially made between two processes: dry pressing and extruding. During the moulding process, the prepared compound – in the form of granulate or in an elastic state – is pressed into or through a mould under very high pressure. The choice of moulding process depends on the requisite properties of the final product. We use both processes.

Dry pressing

During dry pressing, a round-grain ceramic granulate (spray grain) with a moisture content of 5-10% is moulded as a blank under very high pressure using an upper and lower die within a mould. During this process, the granulate is compressed to 50-60% of its original volume.

Extrusion

Extrusion involves pressing a moist elastic strand of compound (moisture content of 15-25%) through a so-called mouthpiece at high pressure. The mouthpiece outlet represents the silhouette in the cross-section of the preform in one direction. Then the preform is cut to the correct length using precise cutting blades.

GLAZES

The purpose of glazes

Glazes are applied to the ceramic semi-finished product and are aqueous, ground, glass-based suspensions which can be dyed using colour pigments. From a technical perspective, glazes serve towards attributing the ceramic surface of earthenware wall tiles in particular density, hardness and a smooth finish. Thanks to their greater density and hardness, stoneware and porcelain stoneware do not necessarily require glazing.

But it goes without saying that glazes also act as stylistic elements permitting decorative design. Glazes can be coloured, transparent or opaque, glossy, semi-matt or matt. Decorative tiles can feature several glazes applied beside and on top of each other. Glazes can be applied to both raw, unfired shards as well as pre-fired, biscuit-fired shards.

Screen printing, roller printing or digital printing are applied for upgrading the surface finish. During the subsequent firing process, the glaze is melted onto the ceramic shards. This also causes the shards to sinter to their corresponding finished properties.

EARTHENWARE, STONEWARE AND PORCELAIN STONEWARE

How do the types of material differ?

The type of composition and selection of the respective raw material components combined with the fi ring temperature are decisive for the requisite product class and its properties. Depending on the water absorption capacity of the end product, distinctions are made between earthenware, stoneware and porcelain stoneware.

Stoneware can be dry-pressed or extruded. Unlike earthenware, stoneware displays a lower water absorption capacity as it is more compactly sintered at higher firing temperatures (1,150 to 1,300 °C). Stoneware is resistant to frost as it has a water absorption capacity of 0.5 to 6% and stoneware tiles and panels are used in indoor and outdoor applications, typically as floor coverings, but also on walls and facades. Their special advantages include their mechanical strength which makes them particularly durable.

Earthenware is the traditional term for ceramic tiles featuring fine-grained, crystalline, porous shards with a high water absorption capacity. Earthenware tiles are manufactured using the dry pressing method and fired at temperatures ranging from 1,000 to 1,150 °C. As these temperatures only trigger minimum sintering, the shard remains porous and displays a water absorption capacity > 10%. These products are only used in indoor wall applications as they are not resistant to frost.

0.5% < water adsorption capacity ≤ 3% (stoneware tiles and panels)

3% < water absorption capacity ≤ 6% (stoneware panels, split panels)

Although implied by its name, porcelain stoneware does not actually involve tiles and panels made from particularly finely-ground raw materials but rather stoneware tiles with a particularly low water absorption capacity of less than 0.5%. They are pressed and fired in a tunnel kiln at up to 1,300 °C which is why they display particularly dense shards. On account of its properties, which also include resistance to frost, porcelain stoneware is suitable for practically any application indoors or outdoors.