[Trade Journal]
Publication: Bulletin of the American Ceramic Society
Columbus, OH, United States
vol. 8, no. 6, p. 135-142, col. 1
PAPERS AND DISCUSSIONS
PORCELAIN MAKING IN CHINA A New Turn to an Old Art
BY J. S. LAMSON
ABSTRACT
A brief description is given of the history of Chinese porcelain and of the methods and materials used by the Chinese at the present time in their manufacture of art and service porcelains Reference is made to a factory in Shanghai, China, manufacturing dry-process electrical porcelain under American methods.
Introduction
History According to Chinese history, the art of pottery making was highly developed in China some three thousand years before the Christian era, although porcelain as it is known today, dates back to the Han Dynasty from 206 B.C. to 25 A.D. It was during this period that the Chinese discovered the necessary ingredients and invented the process of working them to make the ware called porcelain.
No authentic specimens of Chinese porcelain of a period earlier than the Sung Dynasty (960 to 1279 A.D.) are known to exist, but even at this period it is most remarkable how the Chinese, working with such crude and variable ceramic raw materials, were able to manufacture a product of such translucency and artistic quality that today it is to be marveled at and it is reproduced only with great difficulty.
The early porcelains were made almost exclusively for the Imperial families, although as far back as 1280 A.D. when Marco Polo visited the southern part of China he reported that porcelains were being exported to various parts of the world.
Porcelain manufacture was given its greatest impetus during the Ming Dynasty from 1368 to 1644 A.D. when great strides were made with new colors and new designs. Up to this time there were no decorations, all porcelains being covered with a plain white glaze having a bluish tinge, due evidently to firing under reducing conditions.
Additional impetus was given during the Manchu Dynasty by the emperors who took a fancy to porcelain so that we find extraordinary strides made during the reign of Kang Hsi from 1662 to 1723 and during the reign of Chien Lung from 1736 to 1791.
In 1920, some 2000 years after the birth of porcelain, the materials used in the manufacture of the precious Ming vases were finding a new use in the manufacture of electrical wiring devices in Shanghai.
Present Manufacturing
Porcelain is manufactured today in a number of provinces throughout China, the most important districts which I had the opportunity of visiting in 1920 being in Kiangsi, Fukien, and Hunan Provinces.
With the exception of one factory, the Hunan Porcelain Company at Liling, Hunan Province, there was no semblance of any mechanical power employed in any of the porcelain factories. Unfortunately, I did not have the opportunity of seeing this Liling factory in operation as only a short time previous, several wings of the factory were destroyed during the fighting between the Northern and Southern Chinese soldiers. I did see, however, the remains of a small steam power plant, jaw crushers, ball mills, lathes, etc. The ware manufactured by the Hunan Porcelain Company was by far the best quality manufactured in Hunan Province although it did not compare with the best grades manufactured in China.
In the district of Tehwa, Fukien Province, ware of excellent quality, comparing favorably with the best Chinese porcelains, are now manufactured only on a small scale although the industry flourished here at one time. There are several other districts in Fukien where porcelain is made, but the ware is of rather poor quality.
By far the most important district for the manufacture of Chinese porcelain is that in the city of King-Teh-Chin in Kiangsi Province, the porcelain center of China for the past thousand years. Of its 200,000 inhabitants, practically 150,000 are engaged either directly or indirectly in the manufacture and marketing of porcelain. With possibly the single exception of the Kiangsi Porcelain Co., no single manufacturer in King-Teh-Chin includes all the operations of forming, firing, and decorating in his own factory. As a general rule one manufacturer will form and glaze his ware and then rent space in a kiln belonging to another for the firing. He will then sell his plain white glazed porcelain to the decorators.
Kaolin Used
Chinese porcelain is made essentially from a primary kaolin and a material known as pe-tun-tse, formed by pulverizing a white firing felsitic rock. Searle gives the molecular formula of Chinese porcelains as:
0.40 to 0.5RO(Al2O2) 5.5 to 6Si02
with an average chemical composition as:
The kaolin used in the finest King-Teh-Chin porcelains originally came from the district of Kaoling in Kiangsi Province, and it was from the name of this district that our English word "kaolin" was derived. With the growth of the industry, however, new deposits were located and today most of the kaolin for King-Teh-Chin comes from Sin Tse in Kiangsi Province.
The kaolin is transferred to a pit, mixed with water and passed through a series of settling troughs to remove coarse particles. The partially refined clay slip is then run into settling tanks where the supernatant water is drawn off, and the thick slip transferred to drying pits lined with porous brick. When dried to the proper consistency, the kaolin is molded into bricks weighing about five pounds each, stamped with the name of the miner and shipped to King-Teh-Chin.
The pe-tun-tse rock is mined both in open quarries and by driving tunnels into the hillsides, depending of course on the extent of the overburden. The best grades of pe-tun-tse are jade-green in color, brittle, and transparent on thin edges while the poorer grades are found in various stages of decomposition down to a soft white rock than can be crushed between the fingers. A chemical analysis of pe-tun-tse is given in G. P. Merril's Non Metallic Minerals as:
The selected rocks are reduced by hand hammers to about two-inch cubes and delivered to the pulverizing plants located along the hillside following the course of a stream. The pulverizing is accomplished by stamping for 24 hours in a mortar and pestle pulverizer with power derived from a water-wheel. The mortar is made of granite holding a charge of fifty pounds, while the pestle which drops about forty times per minute is made of wood and capped with a porcelain disk. An average pulverizing plant contains 24 mortars and turns out about 1200 pounds of crushed rock per day.
The pulverized material is made into a thin slip in an open air pit, stirred by hand with wooden paddles, passed through a settling trough and finally molded into brick similar to the method employed in the preparation of kaolin brick.
Due to the lack of adequate rail transportation, the Chinese depend almost entirely on their network of waterways for transporting clays. In certain districts the streams are so shallow that the Chinese resort to bamboo rafts capable of holding a load of only 1000 pounds of clay brick.
In other districts, where waterways are not available, it is not uncommon for the clay to be transported by wheelbarrow or suspended from a bamboo pole on the shoulder of a coolie on a journey lasting two or three days.
At the factories the kaolin and one or more varieties of pe-tun-tse, or perhaps one or more varieties of pe-tun-tse without any kaolin, are blended into wooden vats, the coarse particles are permitted to settle, and the slip containing the fine particles transferred to another vat. This refining is repeated a number of times, depending on the quality of ware desired, after which the refined mix is allowed to settle, the clear water drawn off, and the thick slip transferred either to saggers for drying to the proper consistency for the potter's wheel, or into cloth bags weighted with stones so as to form a filter press.
The plastic mix is then kneaded with bare feet, permitted to age for a day or two, and finally delivered to the potter for spinning his ware.
Manufacture of the Ware
The potter's wheel is mounted on porcelain bearings and receives its momentum from the potter, who either by placing a stick in a hole near the circumference starts the wheel spinning, or else kicks it around with his feet. After spinning, the ware is air dried, placed again on the potter's wheel, and turned down to the desired size and shape.
The glaze, which the Chinese refer to as the flesh of the porcelain, the body representing the bones, is applied by dipping, spraying, or painting with a brush, depending on the shape of the piece to be glazed, and the effect desired.
The atomizer most commonly used for spraying is nothing more than a small bamboo tube covered at one end with a layer of fine cloth. The covered end of the tube is dipped into the glaze so as to allow a small amount of the liquid to filter into the tube, which is then held close to the work and the glaze is blown out as a fine spray.
For the crudest types of ware, children are employed to paint a simple underglaze decoration on the raw ware. For the better class of ware, men are employed in decorating.
With large vases of complicated design, it is not unusual for a skilled craftsman to spend weeks and even months completing one piece.
Naturally, the formulas of bodies and glazes are carefully guarded, and handed down from generation to generation by father to son.
All the colored oxides were formerly obtained in China, but now the more modern manufacturers resort to the purer oxides obtained principally from Germany and Japan. Due to the absence of mechanical grinders, the colors are ground by hand with a mortar and pestle, a process requiring several days for the grinding of a few pounds of colored frit.
White glazes are made from the best grades of pe-tun-tse, softened by additions of limestone and alkalis from the ashes obtained by firing certain varieties of ferns.
After being glazed, the unfired porcelains are carried to the kilns where space is rented for the ware to be fired. As might be expected, the choice parts of the kiln where there is the least danger of being overfired or under-fired, command the highest prices.
The saggers in which the porcelain is fired are made in special factories and sold to the ware makers. The sagger wall is made by cutting a strip of the plastic sagger mix from a block, wrapping it around a collapsible circular form, and kneading the ends of the strip together on a potter's wheel. The bottom of the sagger is pressed into a steel ring and joined to the sagger wall by kneading with the fingers.
Wood is used as a fuel and of the 125 kilns in King-Teh-Chin, about 100 burn the wood from pine trees. The kilns are about 50 feet long with a combination wicket and fire box at one end and a chimney about 40 feet high at the other end. The width of the kiln at the fire box end is about 25 feet, but this narrows down to about 7 feet toward the chimney end. Firing is completed in about 36 hours, at a temperature of approximately 1400 °C. The Chinese fireman depends almost entirely on his eye to determine the kiln temperature, and as a result, kiln losses are frequently very high. Evidence of kiln losses is seen in the mountains of scrap and in the porcelain lined river beds around porcelain manufacturing districts.
At the unloading of a kiln, the various ware makers are around to claim their porcelains which they identify by marks on the saggers and also by the trademarks on the porcelains themselves.
Electrical Porcelain in China
With this background of China's five thousand years of pottery production and the excellence of Chinese art and service porcelains, the writer was called upon to introduce the production of electrical porcelain, a new phase of an old art brought into existence by one of the most modern and gigantic of industries, electrical apparatus manufacture. This introduction of electrical porcelain was started in Shanghai in 1920.
Naturally the specific requirements of electrical porcelain production are somewhat different from those of the art and service porcelains made for so many years in China.
After being accustomed to thinking of electrical porcelain in terms of ball clay, china clay, flint, and feldspar, it was somewhat disturbing at first to be reduced to practically nothing but kaolin and pe-tun-tse as body materials. The Chinese materials, however, were found suitable for the manufacture of electrical porcelain and satisfactory dry-process bodies were developed using approximately 20% kaolin and 80% pe-tun-tse. The fine grinding of the pe-tun-tse gave it sufficient plasticity to correspond to the plasticity of a mixture of plastic ball clay and nonplastic quartz and feldspar so that the final porcelain body was found to mold quite well in steel dies. Due to the numerous disturbing elements throughout China cutting off sources of supply, it was found necessary to change formulas rather frequently.
The porcelain mixture was prepared in a blunger, passed through a 120-mesh vibrating screen and then filter pressed at a pressure of 80 pounds per square inch as compared to the Chinese method of stirring the mix by hand with wooden paddles, removing the coarse particles by repeated settlings, and removing the excess water by drying in saggers.
The filtered cakes were dried in a steam drier, moistened with the necessary amount of water required for dry-process work, and then passed through a pulverizer, a method quite unknown to the Chinese.
For pressing the porcelain parts, screw presses operated by men were used for the heavier porcelains, while lever presses operated by women were used on smaller work.
After drying in a steam-heated drier, the dry porcelains were cleaned and then glazed by spraying in mechanically-driven glazing booths. Although the atomizers were operated with compressed air, the principle of spraying was the same as that of the old Chinese atomizer made of a bamboo tube previously described.
The glaze formula corresponded to the formula of Seger cone 4, using Chinese limestone, kaolin, and quartz, and Japanese feldspar, ground together in a ball mill.
The ware was fired in 36 hours to cone 10 in an updraft potter's kiln using Japanese coal as fuel. Temperature measurements were taken with Seger cones in addition to a recording pyrometer with thermocouples in three parts of the kiln. The introduction of a pyrometer was quite an innovation in the history of porcelain making in China as the usual practice of the Chinese fireman in determining temperatures was to spit on a porcelain disk built into the wall of the kiln and to observe how the spit rebounded. This disk was exposed to the heat in the kiln and reddened as firing progressed.
With brass parts imported from America, wiring devices of American design were assembled on the Chinese-made porcelains primarily for Chinese consumption.
GENERAL ELECTRIC COMPANY SCHENECTADY, N. Y.
Presented at the Annual Meeting, AMERICAN CERAMIC SOCIETY, Chicago, Ill., February, 1929. (White Wares Division.) Received January 4, 1929.
