Raw materials

Raw materials and ingredients of ceramic glazes

The glaze of standard hard porcelain is similar in composition to glass - hence the name "glaze". According to the so-called "Ditzel formula", porcelain glaze consists of 65% glass, 25% mullite and around 10% quartz. This "glassy" coating is also the reason why porcelain is translucent and appears transparent against the light. This is one of the main differences between porcelain and other ceramic bodies, e.g. stoneware or ceramics. It is only during the firing process that the body is inseparably bonded to the glaze coating.

Porcelain glazes contain oxides, mainly of silicon, boron, aluminium, potassium, sodium, calcium, magnesium, strontium, barium, lead, zirconium, etc. The composition and thermal expansion of the glaze is adapted to the ceramic body. If it expands more than the body, hairline cracks appear; if it expands less, it flakes off. The glaze for ordinary hard porcelain, for example, is a silica-rich, alumina-containing, low-lime, lead-free glass. During glazing, after the annealing firing (porcelain), the now very porous object is dipped into a fine aqueous suspension of the pulverised glaze components.

The porous body absorbs water. A thin layer of glaze powder is deposited on the surface of the body. During cooking or smooth firing at up to 1450 °C, the coating melts into a smooth, shiny layer and bonds intimately with the base through an intermediate reaction layer. Pottery is coated with a lead glaze. The raw glaze is applied in a similar way to porcelain. In addition to crockery, wall panels, oven tiles, roof tiles and chemical-technical porcelain and stoneware are also glazed. Coloured glazes are created by adding colour oxides such as cobalt oxide (blue), chromium oxide (green), manganese oxide and iron oxide (brown); the addition of tin oxide and zirconium oxide results in white, cloudy glazes.

Other additives in ceramic glazes

Aluminium oxide
Al2O3 - also known as alumina - hardens the surface resistance (Mohs hardness) of the body when added to the glaze.

Ball Clay
Plastic refractory clay powder, dark coloured due to organic additives, light firing. Increases plasticity in masses. Reduces the settling of glaze slip. Addition 3-6%.

Barium sulphate
Flux for matt and semi-gloss glazes. In combination with ZnO and SrCO3, beautiful crystal glazes can be achieved.

Barium carbonate
Flux for embellishing the glaze, but not suitable for LFGB. Barium carbonate glazes are toxic and corrosive.

Calcium borate
Calcium borate frit - flux for glazes. Reduces stresses and cracking between glaze and body, CaO B2O3.

Strontium carbonate
Reduces boron haze, pinholes and increases the lustre and hardness of the glaze, SrCO3.

Wollastonite
Natural calcium silicate. Achieves smooth glazes without pinholes and prevents run-off in alkali glazes. Addition of 20-100%, CaO SiO2.

Spodumene
Raw material for glazes and bodies. The addition in bodies increases the resistance to temperature fluctuations, raking masses, Li2O Al2O3. 4 SiO.

Tin oxide
Opacifier for white glazes. Increases the impact and resistance of the glaze. Additives 4-10%. SnO2. Although tin oxide is good against impact, it does not withstand aggressive detergents in long-term commercial use.