Using the potters wheeel for model making

Part I. The potter's wheel

The Wheel: Archaeologists, digging into the ancient ruins of Ur near the mouth of the Euphrates River, have uncovered pottery, in layers of the earth just above the level of the Great Flood of Noah's time, which were made on a potter's wheel. At Tell Billa, near the headwaters of the Tigris River, wheel-turned pots, made about 3200 B. C., were also unearthed.

It is believed that the first potter's wheel was a crude balanced stone, which was rotated by hand. With human ingenuity, however, various types of potter's wheels were evolved in different lands, each varying slightly but using simple mechanical principles. The use of an assistant to turn the wheel by hand; the use of a foot pedal in a crank-like action; the turning of the wheel by rope or belt drive were among the kinds developed. However, with the machine age, considerable improvement in wheels is noted.

Fig 33

Types: Of the types in common use today, the kick wheel is probably the oldest and simplest. The kick wheel comprises a "throwing" head (a horizontal circular disk) mounted on a vertical shaft which is supported by a bearing. At the base of the shaft a heavy balanced fly wheel is attached. The seated operator kicks the fly wheel with the foot, while manipulating the clay on the throwing head with his hands (Fig. 33).
An improvement on the kick wheel is the type which is operated by a foot treadle which turns the fly wheel.

The latest improved types are electrically operated, electrically adjustable in rotating speed by foot control, have interchangeable throwing heads, and come with attachments for catching clay drippings, for providing water, hand rests, and other aids.

Throwing heads are made which are removable-so that if several students are using the same wheel, each can put on his own throwing head, or remove it to store his work until a later time when he has an opportunity to use the wheel again. Professional throwers prefer a smooth wood or brass throwing head because the clay to be worked is more easily centred thereon. However, for ease to the beginner, a plaster bat type of throwing head is normally used.

Throwing: The procedure of forming a clay object by hand on a potter's wheel is known by custom as "throwing." It is practically impossible to learn how to "throw" on a potter's wheel from a site. To learn properly it is necessary to study under the supervision of a skilled operator; and proficiency is gained only after considerable practice.
Here we shall describe the general principles and procedure so that you may have an acquaintance with the subject. A plaster bat is cast inside a circular metal hoop which fits into an indentation in the throwing head. A ball of clay is slapped on as near to the centre of the plaster bat on the throwing head as can be estimated (Fig. 34A). This clay is much more moist than normal plastic clay and is frequently sprinkled with water by hand during operation to keep it in the soft moist condition.

Remember that all during the following discussion the wheel is spinning or rotating. The head is rotated at a fairly rapid speed. At the outset, one hand is pressed against the spinning ball of clay forcing it to centre itself. Both hands are then pressed inward against the sides of the clay mass to cause the clay to rise. In throwing, clay is never pulled upward. It is forced upward by squeezing inward with the hands lower down on the clay mass.

Fig 34

As the second step, the clay is wedged on the wheel. This is accomplished by squeezing inward with both hands to force the mass up (Fig. 34B), and then pressing on top of the mass to flatten it down again (Fig. 34C). The clay is forced up and down in this manner several times until the clay body feels smooth and uniform throughout.

After the clay is properly wedged, the next step is to form a hollow object. The thumb of one hand is pressed down on top in the centre of the mass, while on the outside surface the clay is pressed inward with the fingers of the other hand. As the hollow enlarges, the fingers of the inside hand are inserted (Fig. 34D).

Pressing down and outward with the thumb and fingers of the inside hand while opposing the pressure with the fingers of the outside hand is the basis of forming a hollow in a thrown object. If the inside finger pressure is greater than the outside finger pressure, the wall of the object will move outward at the point of pressure. If the inside finger pressure is less, the wall of the object will move inward at the point of pressure. If the pressure of both hands or fingers is equal and opposite each other, the wall will rise straight up in cylindrical fashion.

The student next throws an erect cylinder, making the walls as thin as possible without collapsing the object (Fig. 34E). This practice gives him the feel of wall thicknesses and the possibilities of the clay.

The student then practices to form a simple, wide, shallow, flat-bottomed, straight-sided bowl in order to learn how to form clay out at a distance from the centre of the throwing head (Fig. 34F).

Fig 35

After this, the student attempts an upward outward curving bowl and thus learns the relative inside and outside finger pressures required, and the positions of the fingers of both hands with respect to each other. In curving the wall upward and outward, the fingers of the outside hand press in slightly below the fingers of the inside hand which are pressing outward (Fig. 35A).

In like manner, to curve the wall inward, the fingers of the outside hand press in slightly above those of the inside hand (Fig. 35B).

After the outcurving and incurving bowl shapes are mastered, the student practices in making globe shapes, first curving the lower part outward, then bringing the wall back in the upper part of the object (Fig. 35C). A good proportion of the clay mass is forced toward the top at first as the shape curves outward, so as to provide clay body for the top portion as it is thinned and forced upward and inward (Figs. 35A and B).

In making a vase, the lower walls are thinned to their final thickness before the clay on the top is worked inward to form the neck. It follows that considerable clay will have to be forced to the top at the beginning so that there will be sufficient clay mass from which to form the tall narrow neck. It is obvious that once the narrow neck is started, it is impossible to get the hand inside to thin the walls of the lower bulge further; these will have been formed to final thickness prior to commencing on the neck.

Once the feel of the clay, the sense of central balance of the clay mass, the relative pressures and positions of the fingers of each hand, and the knowledge of the amount of clay to force up to complete the top becomes instinctive through practice, the student is capable of throwing any type of shape on the wheel.

But the real craftsmanship is evidenced when the operator is given a drawing of an object with stated dimensions and can fabricate the object exactly. He may make a template (a metal or plastic profile of the shape) or use measuring callipers to check his work as he progresses; but the final result must be accurate and correct. Obviously, this capability is developed with a great deal of practice, and the operator must have an innate feel for throwing.

Turning: While throwing an object, the hands are always kept wet, and the clay is sprinkled frequently. The clay is therefore quite soft, and shows every mark or imperfection, intentional or unintentional. It is necessary, therefore, to "finish" the surface of the object before it can be considered completed. The process of finishing a "thrown" object is called "turning."

After the object is thrown on the wheel to satisfaction, it is permitted to dry to leather hardness. It is only when the object is leather hard that it is "turned."

In turning, a steel scraper is used to cut the surface of the object. These scrapers can be purchased already shaped, although many craftsmen prefer to buy the steel blanks, and shape and sharpen scrapers to suit their own preferences. Broad scrapers are used for slow curves, while narrow pointed scrapers are used to cut sharp corners.

The leather-hard object may be retained on its original plaster bat throwing head and reinforced with plastic clay lumps at the base. However, some prefer to remove the object from the throwing head with a long thin sharp knife, or wire, centre, and remount it on another somewhat moist plaster bat head. After remounting, the object is reinforced with plastic clay to hold it in place.

In turning, the scraper is held so that the cutting edge faces against the direction of rotation, and the plane of the metal cutting surface passes through the axis of rotation of the object (Fig. 35D). The scraper is held against a tool rest to insure accuracy and prevent vibration. The scraper actually cuts the surface of the clay to a fine smooth finish. It takes considerable practice to use the scraper properly. After the surface is polished, the piece is centred on the throwing head upside down, and a footing is cut into the base (Fig. 35E).

The type and quality of the footing of a fine vase are a measure of the care and craftsmanship expended in its construction. Several types of footing are illustrated (Fig. 35F). The turning gives the object the fine finish and ultimate accuracy which is the pride of the best craftsmen. However, considerable experience is required before "turning," as well as "throwing," is mastered.

Part II. Aspects of plastic model making

The Model: Second to the pleasure of creating a brand new ceramic design is the satisfaction of making numerous reproductions of this creation-either for your own use, to present as specially personalized gifts to friends, or for sale!

If the design is complicated, the making of perhaps a dozen replicas by hand methods may prove quite tedious and arduous. Obviously, the use of the original as a model from which a mould can be made and replicas cast is preferable by far. However, there are several principles and precautions that should be observed in making the original model if you intend to make reproductions from it.

Modelling Media: As indicated, models may be made from metal, wood, glass, plaster of Paris, plasteline, modelling clay, plastic clay, bisque, or glazed ware. However, in creating an original model, it is obvious that the plastic media-plasteline, modelling clay, or plastic (ceramic) clay-are the easiest with which to work, are the most responsive, and can be most easily corrected or modified if you wish to change any details. But, if you use plasteline or modelling clay, the model will always remain soft, and may be disfigured during the making of the mould.

The use of plastic ( ceramic) clay is recommended above all others in making the original model. In addition to requiring a minimum of effort in making, a plastic clay model can be fired to bisque, coloured to final design, and smooth-glazed so that you can actually see what the final decorated result will be before you begin mould making and casting.

However, the use of leather-hard or bone dry (green) plastic models directly in mould making is undesirable, as these may collapse during mould making. Models fired to bisque are better because they are more rigid. However, shellacking or other means of sealing the pores of the bisque is necessary before mould making. The most desirable method is to design the original in plastic clay, fire to bisque, and then fire on smooth glaze. The use of smooth-glazed models simplifies mould making and avoids complications which arise in using other types of models.

Principles in ceramic model making: Follow the principles listed below when designing original objects if you intend to cast replicas afterward.

  1. Clay objects when fired to bisque shrink about one-eighth in all their dimensions. In building the original model, make it larger by at least one-eighth than the final desired size.
  2. Always use well-wedged clay in model making. The model is to be bisque fired and then glaze-fired. If there are air pockets or the clay is not of uniform consistency the model, like any other ceramic object, may crack or warp on firing.
  3. At no place in the model should the walls be more than 1 inch thick. Upon firing, the chemical water vapour released in the clay may set up internal stresses within too thick walls of the clay and cause them to crack. After establishing the composition, while the clay is still plastic, and before finally finishing the surface of the model, scoop out the clay from the inside of the model with a spoon or modelling tool. Scoop so as to reduce the wall thickness to less than 1 inch. The clay is generally scooped out from the bottom of the model.
    If it will spoil the model, or if the excess clay cannot be reached through the bottom, another method of scooping may be used. With a thin sharp knife cut the model in half in the middle or at the thickest portion, horizontally. Scoop out the excess clay in the upper and lower halves of the model and reduce the wall thickness to less than 1 inch. The pieces are then carefully slip pasted and wedged together. Complete the surface details.
    Never cut the halves vertically or at a slant. Upon firing, the horizontal pieces, because of the weight of the upper half, will fuse together nicely. If cut vertically or at a slant, the parts may separate on firing. If the object was cut across for scooping out, always fire the object very slowly to bisque.
  4. Always design a fairly large base for your model. This is done for support and prevents easy tipping over and breakage of the model and its reproductions. Also, when the object is being fired to bisque and the heat approaches maturity, the chemicals and flux in the clay begin to fuse and melt, the clay may become somewhat soft, and the object may collapse if a very small base is employed.
    If you examine any ceramic object you will note that in most cases the base is quite substantial-generally larger than the top. Figurines are usually supported by tree trunks, columns, or other decorative support.
    Incorporate supporting columns in the dynamic design of the figure. Do not attempt to design a ballerina or other dancing object supported by a toe or one foot, without incorporating a supporting block, column, or other type support. The toe or foot will collapse on firing.
    In the finer ceramic statuettes, the dancing or delicate figures are designed and fired separately with special firing techniques. As an example, in firing a long, thin legged colt, the belly of the colt is supported by a "bisque saddle," which holds up the body and keeps the weight off the legs during firing.
  5. Design your model so that it requires a minimum number of mould pieces. This means that as much undercutting as possible should be avoided. To best understand what is meant, you should make at least one three-piece mould from a selected model. See lesson IX. It will become apparent that if the model is kept simple, and unnecessary undercutting is avoided, the number of mould pieces is reduced, and the difficulty of reproducing replicas is considerably lessened.
    In incising or embossing designs on the surface of the plastic clay object, make sure that the cuts or rises are perpendicular to the surface, or slant to avoid undercutting. Technically, in embossing, the sides of the embossed clay should make a right angle or obtuse angle (in the clay) with the plane of the clay surface of the model, at all points. In incising, the sides of the cut should make a right angle or acute angle (in the clay) with the plane of the clay surface of the model, at all points. Plaster of Paris makes an exact reproduction of the model, does not shrink, and will run into an undercut in incising or embossing, and will make removal most difficult.
    Where an arm or special part of the design extends out at some distance from the main part of the model, this part can be cut off, fired, glazed, a mould made therefrom, and the part cast separately. While the cast part is leather hard, it can be removed from its mould, and slip pasted to the casting of the main part. This is often done to avoid too numerous and complicated mould pieces.
    Note: When making a model for an important composition, the design should not be sacrificed or abandoned because of the mould making aspect. The design should be made in a solid unit, but if possible modified to avoid or reduce undercutting and unusual protruding parts, to minimize the number of mould pieces.
  6. In finishing the plastic model, avoid sharp edges. Sharp edges will not always mould or cast well. Furthermore, glazes, on firing, tend to run away from sharp edges. Gently round the edges of the model while in "bone-dry" or bisque condition.
  • Hint 1. Original, hollow, round models, such as bowls or vases, are best thrown on a potter's wheel. The model can be built by hand methods but it is most tedious and difficult to get an absolutely symmetrical design. While it is true that a student cannot learn throwing on a potter's wheel from a site, the fact remains that the potter's wheel is the best means for making symmetrical, cylindrical, or circular models.
  • Hint 2. In modelling a head or a straight up-and-down type of model, it is often convenient to use a wooden broom handle as an "armature" or support, around which to build the plastic clay. Upon removal of the broom handle, the cavity remaining serves to keep the walls of the model thin, and avoids to a great extent the necessity of scooping out the excess clay in the centre.
  • Hint 3. Always conceive your design in terms of the material you are using. A design that is suitable for bronze casting may not be suitable for a ceramic object. A fine delicate design suitable for metal casting may collapse if cast in clay, during drying, or during firing. Metal walls can be made much thinner than is possible with clay.
  • Hint 4. Metal, or glass hardens after casting. Clay is fragile while leather hard or "bone-dry"; it softens on maturing in the bisque firing. It only becomes hard upon cooling after firing. This must be constantly borne in mind.
  • Hint 5. In designing models for reproduction, commence with very simple items, requiring only one- or two-piece moulds. After practice in designing and mould making, you will have learned the capabilities of the plastic clay medium and the precautions to observe in making the more complicated designs. The more complex models should then be attempted.

Ceramics:
  • Index
  • Coil method
  • Decoration
  • Glaze decoration
  • Glossary
  • Hand pressing
  • Kiln & firing
  • Mould making
  • Plastic & flower
  • Potters wheel
  • Slab method
  • Slip casting

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