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Porcelain Making
Porcelain & Pottery Making :
A Chinese Art
Porcelain making has been a part of Chinese culture for almost 2,000 years. The term porcelain, as described by Funk & Wagnall :
Porcelain (por'lin, pors-, por, ) n.A white hard translucent ceramic ware, usually glazed existing in many varieties, according to its composition and method of manufacture; china, china-ware. it is made from pure clay to which a little of the more fusible feldspar is added. (< Ital: porcellana, org. a cowry ) porcelaneous, porcelianeous. Adj
The key aspects of the description of porcelain is of its composition and the manner in which the product is fired. This aspect of firing has been omitted within the description provided and is an all important part of the process. Simply having the composition of the clay correct will not produce a porcelain without close attention to the temperature at which the clay is fired and to the duration of the firing process.
The exact origins of porcelain in China is an area that has been debated by historians for generations. An exact date for the origin of porcelain depends largely upon what criteria is used to define the definition of porcelain.
The Chinese during the late Zhou dynasty ( 1100 - 221 BC ), of the bronze age, used Kaolin Clay in the manufacture of pottery. This clay contains the mineral kaolinite and is naturally rich in silicas. Certainly examples of "Pottery - Porcelain " from this era exist although they are not generally considered as true porcelain. True porcelain became a feature of China from the middle of the Han Dynasty ( 206 BC - 220 AD ). This was in the form of what is recognized as celadon porcelain.
The descriptions of porcelain provided within most dictionaries will make reference to ceramics and china-ware. In someways this is misleading: ceramics is a general term that applies to articles made of clay and fired.
Ceramic ( ce-ram-ic ) adj.
Pertaining to pottery and to articles made of clay that have been fired or baked.
Also spelled : keramic ( < keramos potters clay )
Ceramics ( ce-ram-ics ) n.
The art of molding. modeling and baking in clay: construed as singular. 2. Objects made of fired and baked clay: construed as plural. - ceramist. n.
China-ware is a more precise term although " China " is not exactly the same as porcelain. The significant difference between the two relates to the firing temperature. China-ware is not dis-similar to Celadon in its characteristics. Both China-ware and Celadon Porcelain are fired at a temperature slightly below that of high temperature porcelain. China-ware and Celadon are both produced as products that are used as dinnerware and for decoration but neither are able to be used for laboratory and electrical fields: this area is exclusively that of porcelain.
China n.
Porcelain or porcelain-ware, originally from China
Also china-ware. - adj. Related to or made of porcelain
There is of course a general term that applies to all of the above " Pottery ".
In general terms the Chinese recognize two form of pottery, low fired and high fired. Pottery and Porcelain respectively. There is also within China a second division that is representative of the products produced. The area of China south of the Yellow River contains clays that are more suitable to the production of high quality porcelain and this is centered around the regions of Jingdezhen and Changsha. The areas around and north of the Yellow River contain material that in more appropriate to the production of Celadon porcelain.
The Yellow River has always marked the division between the north and south of China. The Yellow River is the traditional line that separates northern and southern China when matters such as the dynasties are discussed and it is also a division of a geological nature. The types of raw materials and their compositions are slightly different from north and south of the river.
Defining Pottery and Porcelain by Firing Temperature
Temp. Degrees Celsius Product
700 - 950 Pottery 900 - 1250 Celadon 1200 - 1450 China-ware 1450 + Porcelain
The chemical changes, that take place with the firing process, results in a product transformation from pottery to porcelain. Pottery is usually fired at temperatures between 700 - 950 degrees Celsius. When the firing temperature is increase to above 1,100 degrees Celcius the silicates within the clay begin to melt and fuse with the clay particles which are refractory; that is they retain their form when heated unlike the silicates. The fusing of the silicates and clay material, with feldspar acting as a flux, creates the glassy quality of porcelain. The optimum temperature for high quality porcelain is 1280 Degrees Celsius.
A further differentiation between pottery and porcelain is the translucent nature of the product when it is fired. European consideration is to place an emphasis upon this aspect while the Chinese do not consider it as an essential element of the quality of porcelain. The Chinese consider that porcelain of high quality will produce a high pitch ring when struck.
Categories of Porcelain
The various types of porcelain produced since the first conception of porcelain during the Mid - Han Dynasty are a result of the various trends that developed in the fields of art and culture and the range of products produced are reflective of these factors.
Celadon Porcelain:
Celadon of the Han Dynasty is often considered as the oldest true form of porcelain. While the use of Kaolin clay is found in earlier products the accepted generalization is that the products produced were not true porcelains. This point could be argued either way to some degree of acceptance. Celadon from the Han period is also sometimes referred to as Yue-ware.
Sancai Porcelain:
Sancai is commonly known as Tricolour Porcelain or Three Colour Porcelain. The use of three colours is misleading as many more colours were used. The early works from the late Han Dynasty and the Sui Dynasty were usually of a low fired porcelain. During the Tang Dynasty, Tricolour Porcelain was produced at a higher temperature.
Jian Porcelain:
Jian or Black Porcelain became prominent towards the end of the Tang Dynasty and was popular through the Song Dynasty. Black porcelain is produced from clays with a high iron content and are fired at between 1,100 and 1,300 degrees Celsius. Jian- ware was used extensively in the production of tea sets.
Qingbai Porcelain:
Quigbai Porcelain is in real terms the origin of the " Blue and White " porcelain of the later Ming Dynasty. Quigbai - ware was produced within the region of Jingdezhen and has a clear white and polished stone appearance.The porcelain was produced from a fine paste containing very small amounts of iron. This produced a product that was semi - translucent with a slight green tint. The firing temperature was usually between 1,200 - 1,450 degrees Celsius. Thus, they are in a sense accepted as China-ware.
Blue & White Porcelain:
This Blue and White Porcelains are a carry-over from the Qingbai Porcelains of the Song Dynasty. They are produced from very fine clay paste usually of a white colouring, free from contaminants, and are fired at temperatures above 1,450 degrees Celsius. The blue colouring that characterizes the porcelain from the Yuan and Ming Dynasties is produced from the addition of cobalt to the glaze.
Raw Materials : The Essentials for Porcelain
The making of porcelain depends upon two main factors. The firing process and the temperature at which the product is baked and, the composition of the clay paste used. While an understanding of porcelain and the techniques required for its production were known to the Chinese since the late Han Dynasty they were maintained as a closely guarded secret. The Arabian states had first come in contact with porcelain during the Tang period and were perplex as to its composition.
The existence of porcelain within Europe was not known until the late Yuan Dynasty and like the Arab States they were perplex by from what material it was made and exactly how it was produced.
Many experiments were conducted in an attempt to produce porcelain. Finely ground glass was added to the clay pastes but this failed to produce at desired effect. In the early 18th century, two German potters began to experiment with feldspar as an ingredient and this proved successful. The later part of the 18th century brought about further changes to the recipe with the addition of ash obtained from the bones of cattle being added to the mixture; which had the effect of increasing the translucent character of the porcelains produced. Hence the name " Bone China " came into being though it was never a Chinese concept.
Feldspar ( feld-spar, fel'- ) n.
Any one of a group of crystaliine rock-forming minerals which consist of silicates of alluminum with potassium, sodium, or calcium: Sometimes spelled felspar. Also feldspath ( -spath ) ( Partial trans- of G feldspat < feld field + spar) - feldspathic, feldspathose. adj.
Production of Porcelains
The secrets of porcelain producing are almost entirely confined to the two areas of the raw materials and the relative percentage of the components and to the firing process. The making of the body of the porcelain is an art practiced for centuries with pottery.
The process of producing high quality porcelain requires that an exacting and close attention be paid to the preparation of the raw materials. If you are in China then some of the preparation process of the paste is simplified. The addition of additives such as ash and feldspar may not be necessary as the appear naturally within many of the clays used. Clay from the Yellow River regions of Central China, used in the making of Celadon porcelain are frequently taken directly from the clay pit, washed and used in the natural state without any additives and often with out any screening. But, not everyone is so fortunate.
The clay base material firstly needs to be screened to remove any particles that are over-sized. This is done by mixing the clay with water to produce a slurry which is then passed through a fine mesh screen; At this stage the slurry mixture will be subjected to a magnetic screening that is designed to remove as much as possible of any iron based components embedded within the mixture.
Once this initial process has been completed the additives are mixed into the slurry at the desired amount. This amount will vary depending upon the nature of the finished product that is desired.
The clay slurry is next passed over several fine screens to remove the excess water from the slurry and then pressed to remove further excess moisture until the desired consistency is achieved. The clay is then ready to be formed which may be by either hand and traditional wheel methods or, by casting, and pressing techniques that are commonly used in industrial manufacture.
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