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Porcelaine: Fabrication

The work of the designers
Rosenthals artists and designers come from far and wide to the Creative Center at Selb. The Creative Center is part of the Product Development Division, which operates independently of production. At the Creative Center, artists and designers are involved in all stages of a new products evolution: from the drawing board and the first model through to a finished collection of tableware or glassware. The Product Development Division with all its necessary workshops develops new creations for all Rosenthal brands: studio-line, Rosenthal and Thomas.

The work of the modellers
Modellers are master craftsmen. In close cooperation with the designers, they manually construct plaster moulds following their designs. As a modelling medium, plaster can be processed in every conceivable way. Round, symmetrical models are shaped from solid cylinders of plaster on the potters wheel. Reliefs are engraved into the finished models. Handles and spouts are carved separately from plaster blocks. Since porcelain shrinks by approximately 15% during firing and, while still hot and soft, sags somewhat through its own weight, the plaster models have to be made larger. Bulbous shapes, handles and spouts must be shaped straighter and at a steeper angle. As the degree of sagging cannot be calculated, it is the experience and sensitivity of the model maker one has to rely on.

Synthetic resin and plaster cast moulds
Once the delicate plaster moulds have been shaped to satisfaction, they are used to make the master moulds in a durable synthetic resin or in silicone. These are used in turn to make the negative plaster casting moulds. Plaster is used because it is porous and absorbs the excess moisture from the porcelain clay. A plaster cast mould can be used about seventy times. However, if the shape is in relief, the casting mould is discarded after approximately thirty-five castings because a relief pattern wears away slightly with each use. This is why no two reliefs on table or giftware are exactly alike.

Preparing the raw material
Porcelain is primarily made up of 50% kaolin, 25 % feldspar and 25% quartz. These raw materials are crushed in large drum mills with added water and flint until they are finely ground and mixed. This liquid mixture is passed over a magnetic belt, which extracts iron particles, which would otherwise cause brown stains on the fired porcelain. A fine mesh screen removes all other impurities. Most of the water is removed by a hydraulic press, a vacuum press sucks out the air. Now the porcelain is ready for moulding.

Turning
The porcelain clay is portioned into relevant sizes and placed onto the plaster cast moulds for the cups, which is then inserted into the cup machine, a continuous moving track. A metal template is lowered and by turning, the cup is jolleyed. When the raw piece has dried, it shrinks from the plaster mould. The rough edges are trimmed with wet sponges until they are nicely rounded, smooth and even. The cup handles are cast separately. All casting marks are carefully removed by hand before they are attached to the cup.

Isostatic press moulding
The dust-pressing of plates involves a porcelain paste consisting of minute dry granules (spray-dried). These granules are poured into the press mould and bond under high pressure. The pressure necessary is ca. 300kg/cm2. The press tool itself is divided into two parts, the upper and the lower die. The upper die is used to shape the top of the item and has to be rigid. The lower die shapes the back of the article and is fitted with an elastic membrane to isostatically mould the pieces. When upper and lower dies are closed the granules are being pumped into the hollow area inside using compressed air. An article is being moulded under pressure of ca. 300 bar - thereby ensuring it is evenly bonded which is extremely important for the success of the subsequent production process. Dust-pressed articles have then to be smoothed off around the rim only. The normal finishing process follows after that. Press moulding is a more efficient method and among its many advantages is a significant quality improvement in the production of plates and platters.

Slip casting
Porcelain items like teapots, handles, spouts, boxes, oval platters and figures are shaped in moulds of plaster-of-paris. For this purpose a porcelain slip is used. The addition of a little water and other thinning ingredients making sure it can be poured. The two- or multi-sectioned plaster moulds absorb the water from the slip leaving a layer of set clay on the mould wall. Any remaining liquid slip is then poured out. Handles, spouts, cup feet and lid knobs are made separately and then stuck onto the actual body by hand, using the slip as a locating medium. Porcelain figures are also assembled by hand from many separately cast pieces. The casting seams, visible on all pieces once the mould has been opened, are smoothed by hand with blades and sponges.

Press casting
This method is used in the production of square and oval platters. The porcelain slip is pumped into a two-part, porous plastic mould and then put under pressure. The water escapes through the open pore canals of the plastic mould while the remaining slip, the actual porcelain paste, stays within, shaping the desired item. When finished, the mould opens automatically and a suction pad lifts out the pieces.

Biscuit firing
After the raw porcelain has been pre-dried and biscuit fired at about 1000 degrees C, it is no longer water soluble but still porous and water absorbent. The latest development in firing technology, for biscuit firing as well as for the subsequent smooth firing, is the so-called fast fire kiln. Here the articles are transported on fireproof support slabs made from silicon carbide on a conveyor belt throughthe furnace. Saggers and the stacking of items now becomes obsolete. The firing is much improved and the firing time reduced to 4 - 6 hours, thus considerably reducing the consumption of natural gas. (The old tunnel kiln requires a firing time of 36 - 40 hours).

Glazing
The biscuit fired articles are stamped with the logo of the company and then glazed. The glaze is immediately removed from the foot of the plates and the rim of the cups with wet sponges to prevent the porcelain from sticking to the base during the second firing. The glaze is a mixture of quartz, feldspar some kaolin and a high proportion of different thinning agents. The glaze melts when fired and fuses with the body before its pores "close", creating an inseparable bond between body and glaze. The glaze is applied to the porcelain either by hand or machine dipping. Because of its high content of quartz it produces a very hard surface when fired. With particularly delicate pieces (e.g. "Zauberflöte" and Limited Edition Art), the glaze is applied using a special spraying process.

Sharp firing
In the smooth or sharp firing process at temperatures of up to 1400 degrees C, the paste shrinks, becomes non porous and waterproof. The intense heat causes vitrification and turns the porcelain into a hard, delicate, translucent substance. This process not only effects a chemical change in the porcelain article but also changes its contours. Therefore it is virtually impossible to produce identical porcelain articles. Even the sharp firing is done in fast fire kilns nowadays.

Supporting props
To keep distortion of the soft, hot porcelain to a minimum, many pieces are fired using props. Props are supports that are made from the same porcelain paste, so that they undergo the same shrinking process during firing. Once a porcelain piece is fired, the prop cannot be reused.

Hard-paste porcelain
After the two firing processes at intense temperatures, the porcelain has become hard, impact resistant and translucent despite its extreme fineness. Hard paste porcelain has the highest scratch resistance to stainless steel cutlery and is unaffected by acids, except hydrofluoric acid. Porcelain is weatherproof and does not change after the last firing, no matter how old it gets. It is suitable for use in the microwave oven with the exception of items decorated with precious metals (gold, platinum). The white, sometimes slightly bluish hue of the porcelain is achieved by reduction firing: that means it is fired in an atmosphere of reduced oxygen.

Porcelaine noire
Black porcelaine is dyed with metal oxides in the body as well as in the glaze. As precious a porcelain as Porcelaine noire should never be put in the dishwasher. It is best to wash it by hand in warm water and to dry it immediately with a soft cloth. A sustained contact with food containing lemon or acetic acid should be avoided as staining could occur. Black porcelain should also never be used in the microwave oven.

Grinding
The unglazed contact surfaces of the porcelain are still rough after the second firing. By grinding and polishing repeatedly they become smooth and dirt resistant.

Sorting
The undecorated porcelain, also called "white ware" is carefully inspected and sorted by highly skilled personnel. The intrinsic characteristics of the materials and the ceramic production process, especially the firing, inevitably cause minute deviations between individual pieces within pre-set standards. The sorting process, painstakingly executed, eliminates the items with an unacceptable level of deviation and discards altogether pieces with obvious defects.

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It is 15th novembre 2018 17:41:32