The porcelain manufacturing process is complex and demands a wealth of experience and patience. The porcelain paste is roughly composed of one half kaolin (china clay), one quarter feldspar and one quarter quartz. It is the purity of these materials that guarantees the quality of the final product.
The exact formula – the arcanum – is still a well-kept secret. Quartz in combination with other materials provides the porcelain with its strength and stability. Feldspar enables the body to harden in a thermal process known as sintering and is also the ingredient that provides porcelain with its translucent quality. The basic materials are mixed with water and drained through sieves to remove any foreign particles. After the water is removed from this “porcelain slurry”, the remaining paste solidifies into what is known as a filter cake, which is stored in a room with a humidity of close to 100 percent. This part of the process is called ageing.
Working from an artist’s sketch, the modellers produce an initial freehand actual-size plaster model, known in the trade as a maquette. A model scaled up by 16 percent is then produced to compensate for the shrinking that occurs while the porcelain is being dried and fired.
This is also modelled by hand - a process requiring great technical skill. A modeller is then able to create a working mould in plaster made up of several parts. The first porcelain models are produced using this working mould, and the next step is to produce a master mould from the final approved model. This master mould provides the basis for subsequent moulds made out of plastic.
Complex pieces may comprise up to 80 individual components. The plastic moulds make it possible to copy each part of the master mould as often as required.
Hollow pieces, such as vases, are slip cast. Water and deflouccant are added to the filter cakes and the mixture is stirred into a large agitator to make a casting slip. The slip has to meet certain strictly defined criteria before it is then slowly poured into rotating plaster working moulds.
As plaster is able to absorb large amounts of water, the mould draws moisture out of the slip, causing an increasingly hard layer to form around its sides. Once the body has reached the desired thickness, any excess slip is removed from the mould. The “green body” now begins to dry slightly while it is still in the mould. This causes it to shrink slightly and detach from the sides. At precisely the right moment, the working mould is opened and the green body removed.
The porcelain paste required for shaping and throwing flat pieces, such as plates, has a more solid consistency than the slip used for hollow pieces. It is produced by putting the filter cake into a machine that mixes it, removes any air pockets and compacts the material. A great deal of skill and experience is required to bring the porcelain paste to the right consistency. Unlike in pottery, plates are not moulded freely by hand on a potter’s wheel. Instead the material is poured into a special rotating hollow mould.
The mould caster assembles the individual pieces produced by the plastic moulds into a plaster working mould. For example, a coffee pot may be composed of a main body, spout, handle, lid and lid handle, which are all produced separately and assembled later. An item can easily require 11 to 15 mould pieces. Such items which consist of several components have to be produced quickly and put together in a moist state. The seams between the different components and finally the piece as a whole is carefully “fettled” or smoothed with paint brushes and small sponges. Any imperfections produced by the plaster mould are also corrected at this stage. The porcelain can be cut so long as it is still wet. This requires a great deal of skill and patience. For example, to produce the filigree lattice work on the SCHINKEL BASKET, modellers have to make around 3,000 precise cuts by hand.
After quality control, the pieces are fired for the first time at 980°C. During this firing the remaining water evaporates from the porcelain. The organic components take on a softer appearance and the body becomes much stronger. The porcelain is then dipped in a blue solution that burns off completely during the second firing. This makes it possible to spot any small imperfections that would not otherwise be visible to the naked eye. Only faultless porcelain pieces are then stamped with the KPM trademark - the cobalt-blue sceptre and the KPM lettering.
With the exception of flat dishes, such as plates, all porcelain pieces are hand-dipped in a tub of glaze. The strength and distribution of the glaze is determined by the porousness of the porcelain body and the amount of time it is dipped for. If the design stipulates that certain areas be left unglazed – such as with biscuit porcelain – these areas are carefully covered with wax or a special coating before dipping. The kiln burns off this protective layer to reveal the unglazed areas.
The second firing, known as glost firing, permanently fixes the glaze to the porcelain body. Fired at a temperature of 1,420°C, the porcelain sinters and shrinks to the size of the maquette – the original actual-size plaster model. The porcelain has also now obtained its final physical characteristics.
A special characteristic of KPM porcelain is that the rims of cups are glazed a second time, adding an additional stage to the process. The cups undergo a second glost firing. The glost firing process takes around 19 hours. However, complicated pieces susceptible to cracking may be fired for up to 55 hours. After firing, the underside of each piece of porcelain is smoothed in the grinding shop.
Floral decoration in a naturalistic style helped to establish the manufactory’s artistic reputation. The décor for each piece is hand painted, giving every item a unique quality. Only the theme and the arrangement of the motif is predetermined; the rest of the design is created by the artist. Since 1832, hand-painted pieces have been stamped with a special trademark, the imperial orb.
Porcelain pieces are gilded with burnished gold, made from finely powdered 24-carat gold. This is either polished millimetre by millimetre with an agate stone tool and, for more intricate decoration, often also elaborately engraved, or it is given a matt finish with beach sand or a fibreglass brush.
Since 1803, painted porcelain has received an additional mark in on-glaze colour. Like the KPM trademark, these marks changed over the course of time. Today, it takes the form of an imperial orb that is usually stamped alongside the sceptre. The colour of the orb differs according to the type of decoration.
A red orb indicates all floral painting as well as figurative scenes and landscapes. A green orb is used for all non-floral decorative elements. Items with grand feu decoration are given a blue orb. Grand feu colours are fired at very high temperatures and fuse with the body and glaze, ensuring they are protected within the glaze and able to be placed in the dishwasher. A black orb is used to mark items with printed décor – for example, pieces featuring company logos.
As well as the imperial orb, the underside of each piece of porcelain also features a small hand-painted signature. KPM porcelain painters all have their own individual signature, which they paint on every piece they decorate. Each piece of hand-painted porcelain is characterised by the unique and individual style of the artist.