The materials which go to the making of china or earthenware include china clay, china stone, felspar, flint and bone. The bones and flints are calcined and all the ingredients are ground and mixed before they are “thrown ” on the potter’s wheel and finished by various processes
THE JIGGER is the modern counterpart of the potter’s wheel and is used for forming relatively flat articles such as plates and bowls. A vertical revolving spindle carries a mould of the inside of the bowl. When the clay is placed on the revolving mould a pivoted profile of the outside of the bowl is pressed down on the revolving clay.
THE wheel of the potter has stood from dim ages past as a symbol of craftsmanship, and the making of pottery ranks as one of the most ancient of arts known to man. Machinery now plays an important part in the various processes through which the potter’s clay must pass before yet other machines fashion it into articles of use and beauty.
In Great Britain the manufacture of pottery ranks as one of the most important industries. The materials used in the manufacture of pottery are not confined to clay; other substances that find their way into china and earthenware are china stone, felspar, flint and bone. It is the clay, however, that is the most important of the raw materials of the potter. The best china clay, produced by the gradual decomposition of granite through countless centuries, is obtained from Cornwall and Devonshire. The china clay from those counties is the finest pottery material in the world. The china stone used in the making of pottery also comes from Cornwall. Ball clay, a finer variety of clay, which needs little further treatment, is found in Devonshire.
Among the remaining materials required for pottery manufacture are flints, felspar obtained from Scandinavian countries, and bone. Generally the bones used in the making of china are those of cloven-hoofed animals, and large quantities are imported from the canned-meat factories of South America. The South American bones are used only for lower grade work and are calcined by being burnt on huge wood fires before shipment. The finest bone ash is prepared in Great Britain from the refuse of British slaughter houses.
The calcining of bones is accompanied by unpleasant smells and cannot be carried out in a densely populated district. British engineers have overcome this difficulty by a simple expedient. A bottle-shaped kiln is used for heating the bones, and the resulting fumes are passed through a secondary fire that removes the objectionable smell. The calcined bones are hand-picked, and iron impurities are removed by the use of powerful electro-magnets.
The object of this process is to prepare the bones for the grinding process. The grinding mill may be regarded as the first of a series of machines used in a modern pottery. Among other ingredients that pass through the grinding mill are the flints, and these are first calcined in a kiln similar to that used for bones.
China stone and felspar require no calcining, but are carefully washed and crushed before they are delivered to the grinding mill. A pottery mill is responsible also for the preparation of the glaze or outer coating of the finished china or porcelain. Potter’s glaze consists of flint, felspar, china clay and stone, whiting, borax and certain alkalis. This mixture is first melted in a special kiln and is run in its molten state into a large brick tank half filled with water. The plunging into water splits up the glaze material, which is then ready for the mill without the necessity of further crushing.
One of the oldest types of potters’ grinding mill comprises a series of round oak vats paved with stone. A central revolving shaft in each vat carries a set of strong wooden arms, to which are attached large blocks of chert (flinty stone). Water is added as the millstones revolve, and after eight to fifteen hours’ grinding, according to the contents of the vats, the materials are reduced to a creamy paste. Another machine that is in general use for grinding purposes is the “alsing cylinder”, comprising a large power-driven revolving drum. In the manufacture of earthenware the various ground ingredients are mixed in a moist condition, but for the making of china they are separately dried in a special kiln before mixing. The drying kiln consists of a fireclay floor from 10 to 12 feet wide and from 20 to 100 feet long. Surrounding the floor is a wall of fireclay slabs 12 in. high and underneath are a number of flues that run from a fireplace at one end of the kiln to a chimney at the other. Some materials require a week in the kiln for proper drying, and as the moisture evaporates the immense slab is cut into 1-foot squares for removal to storage bins.
After the refining and grinding processes of the various pottery materials there is the mixing, which is carried out in what is termed the “slip” house. Here the substances are carefully measured into correctly proportioned quantities: the ball clay to impart plasticity, flint to improve colour and facilitate firing, china clay to improve colour and act in conjunction with the flint, and china stone to bind the materials and provide the necessary strength. Finally, there is the bone that makes the finest china translucent.
The materials for earthenware are broken up separately in a machine called a blunger, which comprises a large tank with a central shaft that carries a number of knives. The shaft is revolved through gearing by a belt and power-driven shafting. The separate mixes are measured into containers known as arks and are then finally blunged, or mixed, and after sifting are ready for use in the form of a plastic clay. Ingredients for china are weighed dry and are then mixed as a moistened clay in the blunger.
THE PUG MILL is a device resembling in its operation a large power-driven mincing machine. It is equipped with revolving spiral knives which cut and knead the clay until it is perfectly homogenous. The clay is cut by wire as it is extruded from the pug mill.
In the sifting process that follows the mixing, a series of sieves is used, arranged generally in three decks. The top sieve, of silk or brass, is the coarsest, but even in this instance the meshes are 100 to the inch. The second sieve is finer, with 120, and the last sieve is finer still, with 130 or 160 meshes to the inch. The sieves are constantly rocked backwards and forwards by machinery. In some instances the sieves are arranged in the form of a long box that rotates on a central shaft. The clay is flung out through the series of sieves into a casing and the unwanted refuse gravitates into a box at the open end of the machine.
The trouble caused by “iron-mould” must be avoided in the making of pottery. The sieved mixture or “slip” is therefore passed over a series of powerful electro-magnets that remove any minute particles of iron that may have escaped the earlier refining processes. The slip is in too liquid a form to be made up into pottery ware and it is therefore passed through a clay press that extracts most of the moisture.
The clay press consists of some three dozen grooved hardwood trays held together by steel straps and long bolts. Between every pair of trays is placed a finely woven cotton cloth to form a bag, through which water will pass while retaining the clay. The slip or liquid clay is pumped under great pressure through a pipe fitted with a number of gunmetal taps, one to every bag. As the bags fill with clay the water is forced out and flows away down the grooves in the trays. When the bags have been filled, the securing bolts are slackened and removed, the press is opened and the clay is scraped from the cloths.
Even at this stage, however, the clay is not quite ready for use and requires attention from a machine known as a pug mill. This device is really a large power-driven mincing machine equipped with revolving spiral knives that cut and knead the clay until it is perfectly homogeneous and ready for use. For the best china this kneading is done by hand and is termed wedging. A slab of clay from the press is cut in halves by a wire and one piece is then forced down on top of the other. This cutting and reuniting are repeated many times.
The carefully prepared clay is made into earthenware and china by various methods. The potter’s wheel is still used in the fashioning of clay, but the primitive spindle worked by the feet has given place to a power-driven machine. This consists of an iron framework carrying a deep tray with a rounded front. At the back of the tray is an iron standard carrying a number of fingers or pointers that indicate the required diameter of the work. Through the front portion of the tray rises the end of a vertical spindle, on top of which is a wheel or disk between eight and twelve inches in diameter. The spindle is driven from a vertical shaft by a double coned clutch, operated by a foot pedal, that controls the speed in addition to providing a means of stopping and starting the wheel. The making of a teacup provides a typical example of the wheel’s use. A piece of clay, accurately weighed, is thrown on to the revolving disk in the tray. For this reason the wheel is generally known as a thrower’s wheel and the working of the clay on it is termed throwing. The clay is first worked roughly into the required number of cup shapes on the disk, which is then replaced by a brass “chum” or mould. This fitting is hollow and the interior is the exact shape of the finished cup. The roughly shaped cups or “linings” rapidly revolving are pressed into contact with the mould, which is then replaced by an empty mould ready for the next cup.
Throwing and Turning
The throwing operation is carried out quickly and the moulds, with their clay linings, are taken to a drying room. Here the clay dries to a certain extent and the linings are removed easily from the moulds.
After having been thrown, the cups are turned on a lathe resembling the machine used for wood turning. This process is necessary to finish the outsides and the edges. The lathe is worked by foot treadle or machinery and the revolving spindle is equipped with a shaped chuck or “chock” on which a cup can be fitted. The turner supports his hand on a rest close to the revolving work and lightly pares the clay with a thin strip-iron knife. The complementary processes of throwing and turning are used also for the making of eggcups, vases, teapots and similar ware, often without moulds.
The fixing of the handles on tea cups, or handling, as it is called, is a trade in itself, involving the exercise of great skill. The handles are made from liquid slip, several at a time, in moulds. When dry the handles are attached to the turned cups by the use of a fettling knife (for cutting off the surplus material) and of a soft sponge that finishes the joint.
ELECTRO-MAGNETIC SEPARATOR for removing particles of iron from potter’s slip. The separator is set on an incline and the slip flowing down passes over and between a series of high-powered electro-magnets. The bars of these magnets are arranged with serrations, thus creating magnetized edges to which the iron particles are attracted.
The cup cannot be regarded as complete without its saucer, and this serves as an excellent example of what is known in the pottery industry as flat-making. This process also covers the making of plates, round dishes and similar articles. Two power-driven machines are used in flat-making. One is called a spreader, the other a jigger, and both are generally arranged on one bench or framework. The spreader resembles the thrower’s wheel, but in place of the standard with its pointers is a column that supports a pivoted lever. With a ball of clay on the disk of the spreader, the machine is set in motion and the lever is pulled down. Attached to the lever is a “batting out” tool which presses out the clay to the required thickness, ready for the other machine, the jigger.
This machine has a vertical revolving spindle and a tray. On the upper end of the spindle is attached a removable brass or gunmetal head that is profiled to the contour of the inside of the saucer (or plate) to be made. Behind the jigger head is a standard, with a pivoted lever that carries a profiling tool coinciding with the outer side, or underside of the saucer to be made.
In operation, a disk of clay is taken from the spreader and thrown on to the mould or head of the jigger. The mould is then rapidly revolved and the profiling tool is brought down on to work, thus forming the underside of the saucer. Excess clay is cut from the mould by a knife. The mould (as in cup-making) is then removed, with the clay attached, to a drying room. On contraction the saucer is removed from the mould, which is then used, successively with many others, for the making of yet more saucers. The casting method of making pottery is extensively used for the large-scale production of all kinds of earthenware and china. The moulds used in pottery casting are made from plaster of Paris and are maintained in a warm condition. Liquid clay or slip is poured into a mould, the inside of which becomes coated with clay as the moisture in the slip is absorbed by the plaster. When the clay has dried out to the required thickness inside the mould the surplus slip is poured away. The mould, which may be built in one piece or in a dozen sections if required, is then removed from the cast article, which is finished by the use of soft sponges and india-rubber pads.
The casting method of making pottery has a wide range of utility and is used for such widely divergent articles as china teapots, earthenware lavatory basins and similar fittings. Casting has been greatly facilitated by the discovery that silicate of soda and soda ash will make the clay slip as liquid as water, so that it runs readily into the interstices of the most intricate moulds.
Pottery made by any of the foregoing processes is first dried in special stoves before being passed to the great “biscuit” and “glost” ovens that respectively bake the clay and set the final glaze.
A biscuit oven, as it is called, consists of a round firebrick structure from 12 to 20 feet high and about the same diameter, with walls some 18 in. thick. The oven is encircled by a number of iron bands and is provided with a domed roof. The oven is entered through an iron-framed doorway that can be sealed up with bricks and clay. In the foot of the oven wall are several mouths, or grates, about 4 feet long, 3 feet high and 2 feet wide. These mouths contain the fires, the heat from which is led into the oven through short brick tunnels called “bags”. A natural draught is used to maintain the fires. The whole oven, with a firing space round it, is covered by a “hovel”, an immense bottle-shaped structure over 60 feet high.
GRINDING MILLS reduce to the correct consistency the ingredients from which pottery is made. The mills comprise a number of round vats, each fitted with a central revolving shaft. Attached to the shaft are strong arms carrying large blocks of chert (a flinty stone). Water is added as the millstones revolve and the contents are gradually reduced to a creamy paste.
China ware must not be exposed in the oven to the flames and direct heat from the fires. The work is protected while in the oven by fireclay receptacles known as saggers, about a foot in diameter and of varying heights from 2 in. to 3 feet. The china, after inspection and the removal of any dust, is loaded into the saggars, an operation that requires skill and experience if crooked ware is to be avoided. The china is packed between layers of powdered flint, which is infusible at the temperature of the oven.
Cups and similar articles are packed in pairs with their rims joined by adhesive compound to maintain their shape. An oven may contain as many as 3,000 dozens of china articles and the time allowed is about sixty hours. After some forty hours, numerous “trials”, or test pieces, of china are removed from the oven to keep a check on the progress of the articles in the saggars. Observation is also maintained on temperature by viewing, through a small sight hole, a number of fusible cones placed within the oven. The cones are arranged in sets, each affected by various degrees of temperature.
After cooling, the baked china or earthenware is taken to the “biscuit” warehouse, where it is again inspected and cleaned. Flat china ware is cleaned in a machine known as a rumbler. This consists of a cylinder, 5 feet in diameter, with the interior divided into sections fitted with cages. The china ware is placed in the cages and the cylinder also is loaded with a large, quantity of finely broken unglazed china. The doors of the cylinder are closed and it is then revolved by machinery. After about twenty minutes the continual shower of broken pieces has cleaned the contents of the cages and the dust is extracted by electric fans.
This cleaning and dusting prepares the china for the next operation, which is termed dipping. The dipping is another process that requires great skill and comprises the immersion of the ware in a glaze solution.
Next follows the final phase of pottery making, the firing of the glaze in glost ovens. The pottery is placed in saggars similar to those used for biscuit firing, but the receptacles are glazed on the inside to prevent the drawing of glaze from the dipped china. The china is placed in the saggars so that the various articles are separated by spacers, or “cranks,” or by “thimbles” that touch the work only at three points. A glost oven is fired for some twenty-four or thirty-six hours, with a similar period for cooling, before the final withdrawal of the finished work. The decoration of pottery is largely a matter of highly skilled craftsmanship. There are two methods of applying coloured ornamentation to pottery. One is used before, the other after glazing, In the first of these processes the china is painted in oil colours and the work is then fired at a low temperature to remove the colour mixing oils. This firing is necessary so that the water-mixed glaze shall adhere to the work when dipped. In the large-scale production of china ware the pattern is generally printed on the china and is later filled in with colours. Decoration after glazing, or enamelling, is
done by hand with enamel paints, or alternatively is carried out by an interesting lithographic process. The design is made with grease paint on a stone surface, a separate stone for every colour. The painted surfaces are then printed in turn on the same sheet of thin paper, which is allowed to dry. Next the paper is cut to the outline of the pattern and is attached to the china by a coating of size. The paper is pressed on to the work with a rubber roller and is then washed off with water, leaving behind a coloured design in enamel. Finally the enamelled ware is fired in a kiln at a temperature of about 900 degrees centigrade.
HYDRAULIC CLASSIFIER in a Staffordshire pottery. The classifier ensures that an unvarying quality of ingredients goes into the mixture from which the “slip” is made. The vortex action in the patent classifier separates coarse from fine grades and maintains an equal consistency for every batch.
