Colour Glazes

Coloured glazes in solid colours

In keeping with the porcelain maker's professional standards, we have explained the dipping glaze in the glazes section and the coloured glaze in the decoration section, although both are basically the same thing. The only difference is that the glaze is transparent for white porcelain and coloured for coloured glaze.

In contrast to the fund spraying process, the decorative colour is not applied to the finished porcelain body after smooth firing, but is added to the mushy glaze as a colour additive. This results in the so-called coloured glaze, which has to withstand temperatures of 1,320 °C and more when firing porcelain. Coloured glazes on ceramic tableware, e.g. crockery or earthenware, are only fired at around 1,100 °C. This explains why the variety of colours and luminosity of ceramic tableware is considerably greater than that of porcelain. However, ceramic crockery is nowhere near as durable and long-lasting.

Coloured glazes, in which the inner surfaces of hollowware (bowls, cups, mugs, etc.) and the undersides of flatware (plates and platters) are fully coated with colour, are glazed using the immersion glazing process. In this process, the porcelain body is completely immersed in the glaze and thus receives its coloured coating over the entire surface.

The colours used generally consist of metal oxides, salts and silicate additives to which a glaze additive or a mixture of kaolin, quartz and feldspar is added. This additive, often also called fire kit, gives the liquid glaze the necessary adhesive strength to bond with the underlying green or raw body.

Up to this point, all factories and production techniques are pretty much the same. Whether vat or glaze line, the same type of body is wetted with the same mixture of glaze in both processes. The industrial machine merely replaces human labour, works somewhat more precisely, but requires far higher production quantities.  

We could now go on and on about the different reactions and effects of coloured glazes on biscuit and green body and even highlight distinctive country-specific characteristics. Ultimately, however, this is of no interest to any customer and remains the concern of us manufacturers.

What you need to know about coloured glazes

For the consumer, the following characteristics apply equally to all porcelain manufacturers when using coloured glazes. You may not be informed about this in such detail anywhere, but we are not talking "rubbish" about "unique and flawless quality".

Orange peels & craters

Coloured glazes generally enhance the visual effect of the small natural imperfections on the porcelain. Orange peels and small imperfections are much more noticeable on large flat pieces than on white porcelain! The rule of thumb is: The darker the coloured glaze, the more likely it is that small imperfections will be visible.

Translucent surfaces

Colour glazes on ceramic bodies never appear 100% uniform or the same. Ceramic materials react naturally and are not plastic. Beneath the glaze of the porcelain lies a bright white body, which can shine through slightly with coloured glazes. The thinner and finer the thickness of the body, the shallower the penetration depth of the liquid glaze and the more likely it is that segmental translucency will occur. Especially on snouts, handles and rims, the coloured glaze is generally always somewhat thinner and more transparent than on straight body shapes. If the glaze is thickened, which is technically not a problem, it unfortunately becomes heavier and tends to form a glaze nose. Such glaze runs look far less attractive than the subtle shimmering through of the white body. This balancing act between consistency and pigmentation leads to minor differences between the production batches of all manufacturers. The rule of thumb applies: The darker the solid colour, the more likely it is that the white body will be discreetly visible in some areas.

Colour shades

According to colour theory, the subtle shining through of the bright white body leads to slightly different colour tones. A red full-body colour, for example, appears slightly orange, a grey full-body colour shimmers slightly greenish. Such colour shades are therefore not the result of a fluctuating colour, but are due to the degree of translucency.

Horizontal uneven terminations

Depending on the production method of the solid colour, whether as a dip glaze or as a spray decoration, slightly irregular finishes of base rings and mouth rims may occur. If the inside of a cup or bowl is left white, the item must be covered or sprayed upside down. The edges of the covers can make it difficult to spray the solid colour evenly, so that irregular, horizontal finishes can easily form. This usually only occurs with hollow parts. If the solid glaze colour is only applied as an immersion glaze on the outside, slightly irregular finishes also occur at the edges of the mouth.

Slight chipping on floors & wells

The "real porcelain" coated with a coloured glaze is fired above 1,320 °C. Despite the utmost care and cleanliness in a factory, the tiniest particles of sand and residues of the greenware or raw cullet can lead to minimal colour chipping on the setting rings and base edges, even on the kiln car covered with fireclay brick. Another reason for such chipping - especially on the bases of cups, bowls and dishes - is the coloured glaze itself. Once the item has been dipped into the coloured glaze, it must be removed from the shelves by hand using a sponge. The tiniest residues of the glaze on the setting rings act as a kind of adhesive and thus bond with the oven plates and also lead to minor chipping. The chef would say, "... a little bit set, but far from spoilt...". If these break-outs occur sporadically and only as "small spots", they are not to be considered faulty in the context of mixed sorting.

Pollutants: lead, cobalt & cadmium

Low-fired ceramics, which contain pigmented clays instead of kaolin, can fix coloured glazes far better than densely fired porcelain. This is the reason why there are far more fully coloured ceramics on the market than porcelain. On the other hand, the risk of pollutant emissions is much higher with coloured ceramics than with porcelain. Incidentally, every solid colour from Holst Porzellan is officially tested in a neutral laboratory for its harmlessness with regard to the emission of lead, cadmium, cobalt, zinc, antimony and barium.

Conclusion

Porcelain is made to last forever and is free from any ageing caused by use or time. It is made from 100% natural mineral raw materials and fired at temperatures above 1,320 °C. The true quality of porcelain does not lie in a perfectly flawless surface, but in its unique functional and usage properties.

Minor tolerances and natural differences are the special quality characteristics of porcelain. Only synthetic goods are 100% identical. This applies in particular to full-coloured tableware.