HISTORY OF THE PORCELAIN
"Curiosity always comes first in a problem needing to be solved"
The porcelain of the Chinese
Porcelain has been known as a product of the Chinese since the golden age of West-Chinese cultures (1122 to 770 B.C.).
But Porcelain was not invented in China, but it was the result of a long process of development. Porcelain items reached Europe by way of laborious routes from the 13th century onwards by traders, explorers and globetrotters like Marco Polo.
Porcelain was imported in particular via the Dutch since the 17th century. But production of the precious material remained a secret of the Chinese – it had to be newly invented in Europe.
The definition Porcelain
The term Porcelain is derived from the Italian word “porcellana”, the original name of a sea snail with a translucent porcelain-like shell. Marco Polo is said to have used this name at first for Chinese porcelain products. Like many others, he also thought that the Chinese produced the porcelain from this shell.
The reinvention of Porcelain in Europe
Johann Friedrich Böttger (1682-1719) an alchemist and assistant apothecary is connected with the invention of porcelain in Europe. After failed attempts to produce gold, experiments in the field of ceramics together with Ehrenfried Walther von Tschirnhaus (1651-1708), and Abraham von Schönberg succeeded. The red stone ware, called “Böttersteinzeug” originated at Albrechtsburg in Meißen.
By using white earth (Kaolin) and by the improvement of the procedure the production of the first European hard porcelain succeeded in 1708, whereupon in 1710 the porcelain manufactory Meissen was set up for the beginning of serial production.
The first porcelain manufacturies in Europe
The first European porcelain manufactury was founded by royal decree from 23 January 1710 by the Elector of Saxony “August the Strong” in Dresden. In March of the same year it was transferred to Meissen at the Albrechtsburg.
During the following years other manufacturies were founded in Europe:
Porcelain manufactury Vienna (1718), the French manufactury Sévres (1745), Chelsea, England (1745), Höchst (1746), Fürstenberg (1747), the manufactury Nymphenburg near Munich (1747), the Berlin manufactory (1751), the porcelain manufactury Frankenthal (1755), the porcelain manufactury Ludwigsburg (1758), the royal manufactury Copenhagen (1760), the royal porcelain manufactory Belrin (1763).
Spreading of the porcelain production in Germany
Beside the princely manufacturies private-economic companies were also founded towards the end of the 18th century. In particular in Thuringia, Upper Franconia and Silesia, numerous private enterprises were founded, some of which are still in existence.
In the chosen locations the essential factors for the production startups were available. Raw materials (kaolin, clay) fuel for the stoves (wood and coal) and manpower existed in abundance. As a result of the mechanization of the weaving process, free manpower again found work in the new-founded porcelain factories.
PORCELAIN PRODUCTION IN NORTHERN BAVARIA AND WESTERN BOHEMIA
The Bavarian porcelain production was concentrated in the northern part from the second half of the 18th century onwards, in particular in the Obermainkreis, in the area which later became the governmental district of Upper Franconia. The proximity to Thuringia, where experts with know-how in the area of the porcelain production already lived, was a factor for the location choice of the factories.
With the discovery of kaolin in the surroundings of Hohenberg a.d. Eger, the Thuringian Carolus Magnus Hutschenreuther at the beginning of the 19th century laid the foundation-stone for the centre of the porcelain industry in Northern Bavaria. He founded a porcelain painting workshop in about 1814 from which at the latest since 1822 that porcelain factory arose, which was popular in particular with royal families and statesmen for their etched gold rim decors and the luminous blue cobalt glaze. Factory foundations of Hutschenreuther's son Lorenz, Jacob Zeitler, Philipp Rosenthal etc. in Selb and its surroundings followed.
The Bohemian porcelain industry is concentrated on the river Eger, from Elbogen (Loket) to Klösterle (Klásterec nad Ohrí) in the region of Karlsbad where the required raw materials, kaolin, feldspar and quartz as well as wood and coal, existed in abundance. The first porcelain factories of Bohemia weren’t founded by princes, in fact it was a result of middle-class initiatives. The main impulses for this development came from porcelain traders and porcelain manufacturers from Thuringia, Saxony and Bavaria. For this reason Bohemian and German porcelain are connected in a special relationship.
Kaolin + quarz + felspar = porcelain
The product group description ceramics contains those products mainly produced and fired from raw materials containing potter’s clay.
One must differentiate between coarse- and fine ceramics. Coarse ceramics are tiles, construction terracotta, bricks and ceramic pipes, etc. whereas fine ceramics range from simple earthenware to majolica, faience, stoneware, fine stoneware, vitreous china, bone china and porcelain.
Obvious attributes of porcelain are a highly glossy surface, the light sound and the pure white colour. Colour can be changed nowadays by encorporating pigments of nearly any colour. The fractured surface of the porcelain body is smooth, dense and non-water absorbent. In the words of the experts: The body is sintered.
Light gleams through porcelain unless hindered by a body which is too thick. This should be described correctly as translucence. However the term transparency has been accepted within the ceramic sector.
One square centimetre of porcelain can withstand a pressure load of five tons. This characteristic is of hardly any importance for household porcelain but is all the more imortant for the gastronomy sector and technical porcelain, particulary in the field of electrical engineering. Moreover porcelain can withstand high tensile stress. Line-insulators are a good example.
Porcelain is an insulator and is therefore used in low and high voltage systems. It is suitable for electrical resistance because of its insensitivity to heat coupled with its high insulating properties. The electrical dielectric strength is 40,000 volt at a thickness of 2.5 cm.
Within the Mohs-hardness scale porcelain is strength 8 which means that porcelain is able to score any other materials below this value. Despite its delicateness porcelain is physically harder than normal steel.
The difference between porcelain and stoneware
Porcelain and stoneware belong to the large family of fine ceramics and at a fleeting glance look almost identical. One difference is in the different firing temperatures. For porcelain the first firing lies between 900°C and 950°C, whereas stoneware is fired between 1100°C and 1300°C. The second firing for porcelain (glost firing with complete sintering) lies between 1350°C and 1450°C, stoneware (gloss firing) lies between 900°C and 1200°C.
Stoneware and porcelain differ by the type of glaze. Contrary to porcelain, in the firing process of stoneware glaze and body do not combine, so in case of a blow the glaze can splinter. This makes the exposed body absorbent because of its porosity.
Hard porcelain / Soft paste china
Hard porcelain and soft paste china vary by a different koalin content in the porcelain mass. Soft paste china has a lower kaolin content and is fired at a lower temperature whereas hard porcelain has got a higher koalin content and thereby has to be fired at a higher temperature. Special characteristics of hard porcelain is the surface strength of the glaze and a special solid body. Hard porcelain is predominantly produced in Europe, soft paste china in China.
Bone china porcelain
Bone china porcelain varies from other porcelain types by the mixing ratio of the raw materials: 50% to 60% bone ash or calcium phosphate, 15% to 30% quarz, feldspar and kaolin (less than hard porcelain). Bone china is already densely fired at the first firing at temperatures between 1240°C and 1280°C. The glaze is fired between 920°C and 1125°C. The lower firing temperature allows a more versatile colour design within the underglaze and inglaze decoration. The most striking feature of bone china is the transparent body and the gleaming white colour.
Porcelain à fritte
Porcelain à fritte looks on the outside like normal porcelain but is much closer to glass than other porcelain types. For its production a vitreous mass of sand, salpetre, salt, fired alum, soda and plaster is melted together. After cooling down it is pulverised, mixed with 12.5 parts each of chalk and lime marl and sintering process is completed. This porcelain contains no kaolin. The glaze is composed of a pre-molten mixture of quarz, potash and soda and is melted onto fired porcelain parts at relative low firing temperatures.
Porcelain à fritte is not very robust and is only used for special products such as decorative articles and porcelain buttons etc.
Seladon-porcelain describes a light green porcelain. By mixing chromium compounds to the mass or glaze the porcelain appears in a light green jade shade after glost firing.
In China the first Seladon-porcelains were produced during the era of the Sung dynasty under the name of pi-si (secret colour). The green shade was created by mixing in small amounts of iron oxide under deoxygenation.
Porcelaine noire – black porcelain
Dark, so-called Chimú-ceramic was produced in Peru as early as the 11th century. Under the name Black Basalt Josiah Wedgewood in 18th century England devised stoneware and stoneware-similar products with a black colouring. Rosenthal GmbH produces black porcelain under the tradename Porcelaine noire.
This porcelain is created through either an admixture of manganese salts or the addition of gold alloys to the mass. Rose or Rosé porcelain is fired at the usual glost-firing temperatures.
To achieve brown porcelain either the mass or the glaze has to be coloured brown. The glaze is most often coloured by the admixture of iron oxide, manganese oxide or chrome oxide. The coloration of the mass by using clays which turn brown when fired is less common. Brown porcelain is most often used for cooking- and ovenware.
Biscuit porcelain is fired unglazed. Its most conspicuous charcteristic is its water impermeability in spite of the coarse surface structure. This type of porcelain was very popular in the neo-classical period because of its similarity in appearance with marble.
Cobalt decoration is one of the oldest porcelain refinement processes. For this, a mixture of glass frit (glaze) and cobalt oxide is applied to white porcelain and then fired for a second time between 1350°C und 1450°C. The authenticity of a cobalt decoration is indicated by an ill-defined transition betwen the decorated and undecorated areas.
The beginning gave it to earth, the earth to the hands, the hands to fire, the fire to light for play: hard however delicate, opaque however clear: that's accomplishment.
from the Chinese
Types and forms
The basis for all the steps in the production process is the drawing. A clay or a plaster mould is then produced from this drawing. In the production of the mould the designer must allow for shrinkage, which is about 16 percent, and thereby make the mould 16 percent bigger. Subsequently the so-called master mould is cast from plastic or metal. This master mould is for the production of working moulds made from plaster from which the porcelain parts are formed or cast.
Processing the raw materials
In the terminology of the porcelain workers the mass is the established term for the ductile, fluid or powdery mixture of the individual raw materials. One distinguishes between the moulding body (ductile), the casting compound (fluid), and the moulding material (granulate). For the production of the fluid casting compound, which the porcelain worker calls the slurry, the raw materials are mixed together in large drums, and then finely ground using water and grinding balls. After that the liquid mass travels across a magnetic band which retains all ferrous particles which would otherwise produce visible brown stains on the fired pieces. A very finely-meshed vibrating screen removes further impurities. A hydraulic filter and a vacuum press remove most of the water and the air from the ductile moulding body. The mass is now mouldable.
Porcelain parts are turned using mantles or by moulding. With mantles the mass is worked either manually using models or by using plaster moulds and machines with heated rollers. For many years rotationally symmetrical forms (plates, etc.) were obtained by using this complex process, which is now being replaced by a press moulding technique. During moulding the mass is turned into plaster casts (cups, bowls, etc.).
For plates, platters, and uncomplicated bowls produced using this process tiny pellets serve as the porcelain mass. The press mould is filled with this granulate material and compacted at high pressure (from 270 bar to 300 bar). Articles produced in this fashion need only be trimmed at their rims and they are more strongly formed than when produced using other production techniques.
Jugs, pot handles, bowls, oval platters or figurines are cast in plaster moulds using a porcelain mass (slurry) which is made mouldable by the addition liquifying agents like soda. The slurry-filled moulds, which are either bipartite or multipart, absorb water from the slurry so that the plaster wall of the mould is covered with a firm and uniform coating thereby creating the porcelain body. The desired thickness of the body is a result of the retention period of the slurry in the mould. The remaining porcelain mass, which is still fluid, is poured out of the mould.
After predrying, the porcelain is fired for the first time at around 1000 °C, after which it is no longer soluble, but is porous and water absorbent. On the burning tray the porcelain is transported through an open fire. This firing process is more effective and the firing time can be reduced to 4.5 hours which is in sharp contrast to the old tunnel kiln process which had a firing time of from 36 to 40 hours, this considerably reduces energy consumption.
The annealed parts are marked on the bottom with the company brand and glazed. The glaze itself consists of quartz, feldspar, a little kaolin and a high level of diverse fluxing agents. It is applied to the porcelain by immersion, either manually or by machine, in a bath of glaze. To avoid adherance during the firing process, the glaze is immediately removed from positioning rings and supports. In the glost firing technique the glaze melts before the porcelain body compacts, thereby creating a strong bond between glaze and body.
In the process of glost firing or high-temperature annealing at temperatures up to 1450 °C the porcelain mass shrinks and becomes impermeable. It vitrifies to a hard, fine translucent porcelain substance whereby the chemistry and form of the mass is changed and this process is different for each individual piece. The porcelain is now 16 percent smaller than it was during moulding. Glost firing today takes place in a conveyer kiln.
After the second firing the unglazed positioning rings and supports are still coarse and are ground and polished several times to make them smooth and stain-resistant.
The undecorated porcelain, which is also called completed whiteware, is carefully graded by skilled personnel. This requires a great deal of experience to sort out flawed pieces. The typical material characteristics and the ceramic manufacturing process itself inherently encompasses slight variations between individual pieces. The work of sorting meticulously differentiates and grades these variations and removes flawed pieces.
Richly decorated porcelain pieces go through six firings: annealing, glost firing and four onglaze firings.
A wafer-thin, wet coloured foil is placed over the whiteware. The decorative foils themselves are generally produced in a screen-printing process. In the past they were only used to decorate services but today they are also used for costly and limited series which cannot be produced using a manually painted technique.
Decorated chandeliers, gold and platinum bands, etched gold decorations and gilded reliefs are applied by hand. The layer of gold is polished to a high gloss with hairfine glass-fibre brushes or agate polishing stones after firing. A hand-painted gold application consists of up to 95 percent pure gold (22 carat). Apart from that there is decoration with bright gold with a lower concentration of gold but appearing already glossy after firing. All figurines are hand-painted. They are either painted onto the glaze or on the sintered porcelain body, which is then glazed and glost-fired.
In order to cover large areas of a porcelain piece with a colour, this background coloration is applied to the porcelain uniformly with a paint-spraying pistol. All the areas that are to remain white are insulated beforehand with varnish which has to be removed after the spraying process. After peeling off the layer of varnish, and before firing, the white areas of the porcelain are carefully cleaned. After firing even miniscule remnants of paint would form visible stains.
For the very costly gold-etched rim, parts of the decoration are etched out of the glaze with hydrofluoric acid, the only acid which attacks porcelain. The parts which are not to be etched are first covered with a protective lacquer. The longer the porcelain is exposed to the acid, the deeper the decoration is etched. Subsequently the decoration is twice gilded by hand. Each coating of gold must be separately fired. After the second firing the gold is polished with with hairfine glass-fibre brushes.
Underglaze decorative firing
After the first firing, underglaze decorations are frequently applied to the still porous porcelain by hand. They are subsequently glazed and glost fired up to 1450°C. The maker’s mark is also stamped onto the base before glazing so that it lies under the glaze. Only a few colours are suitable for such high firing temperatures. Among these are cobalt blue, green, brown, a matt yellow as well as mixtures of these colours from gray to black. Underglaze cobalt decorations are often combined with etched gold decorations.
High-temperature decorative firing
Such decorations are applied to the finished porcelain by means of coloured foils, hand-painting or decor spraying. In high-temperature firing considerably more shades of colour as well as gold and platinum can be melted into the glaze compared to the underglaze firing process. Within 90 minutes the porcelain is heated to 1250°C so that the decoration sinks into the molten glaze and is protected by the glaze. High-temperature decorations are dishwasher-proof and immune to surface influences.
Onglaze decorative firing
These decorations are also applied to the porcelain by means of coloured foils, hand-painting or decor spraying. Intensive colours unable to withstand high firing temperatures such as red and orange as well as rich gold, platinum and luster decorations are smelted onto the glaze at temperatures between 800°C and 900°C. Onglaze decorations are less smooth than the glaze and can therefore be felt.