Because clay body is wedged or kneade.d to insure uniform consistency before forming operations begin, the process usually starts with a ball or lump of clay. It is useful to distinguish three main stages of conver sion: primary forming, secondary forming, and surface modification. The first two arc sequential; the third may occur during either of the other stages, after them, or both.
During primary forming, a lump of clay is con verted into a form resembling the finished vessel. Usu ally one part, cither the upper section and rim or the base, is completed and allowed to dry to some extent, commonly to the leather-hard stage. The remaining part is completed when the drying is sufficient to sup port the additional weight. Techniques commonly used for primary forming are throwing, coiling, prepar ing and joining slabs, pinching, and molding.
During secondary forming, the shape of the vessel is defined and completed and the relative proportions of various parts are established. Techniques used in secondary forming include turning, scraping, beating, trimming, throwing, coiling, and joining.
During the third stage, surface modifications change the texture and enhance the esthetic character of the vessel. Techniques include scraping, smoothing, polishing, burnishing, applique, incising, impressing, and carving. These decorative techniques are part of the forming sequence because they are tied intimately to the varying properties of clay bodies as they dry and may be completed before secondary forming processes begin. For example, the rim of a vessel may be con structed during primary forming and a rouletted deco ration applied to it before secondary forming begins. Surface modification does not involve materials other than the clay body from which the vessel is made. This differentiates it from decorative processes that employ different materials (such as pigments, slips, glazes, and organic coatings).
shape of the base. Flat, wide bases can rest on any Hat area and, provided the lower walls are sufficiently thick, the vessel will be adequately supported. While the base is in a plastic condition, it will take im pressions from the surface on which it rests. If a vessel is cut from its support, cut marks will be evident. This cutting is essential for wheelmade vessels, which must be removed from the wheelhead before the next vessel can be formed. Support impressions will be obliterated by finishing techniques such as smoothing and polish ing, and by secondary forming techniques such as scraping to thin the base.
Round or pointed bases are an indication that spe cial techniques were used; otherwise unacceptable de formation of the plastic clay would certainly occur. Three major types o f support are commonly used:
Flat-surface Supports. Examples include a fiat mat, a level part of the ground or house floor, a piece of wood or stone, or any other material suitable for
Fig. 42. Two kinds of supports for forming vessels, a, Flat-surface support in the form of a fragment of matting. Waiwai, Guyana,
b,
Mold support improvised from the top of a broken jar, which is rotated with the left hand while the rim is pinched and smoothed with the right hand. Boera Village, Papua New Guinea.common because the clay shrinks away from the mold as it dries; with a convex mold, the clay shrinks onto the mold and may crack. Examples of molds include baskets, the half coconuts used by the Mailu of Papua, broken vessels (Fig. 42 b), and special molds, such as used by Pakistani potters to form round-based vessels on the wheel. In the latter case, the mold is supported in a ring of cloth placed on the wheelhead (Fig. 43).
Chuck Supports. These are associated with the potter's wheel. The chuck is usually a ring, inside which a vessel can be set, or a dome over which it can be placed upside down. The function of the chuck is to hold a partly finished vessel in position so that its shape can be modified and to insure that the vessel is re-centered axially on the wheelhead. If the chuck is newly made from soft clay (Fig. 44), the vessel will stick to it. To prevent this, the chuck is usually cov ered with a thin cloth or dusted with a parting agent, such as fine sand. Where it has not subsequently been
64 POTTERY TECHNOLOGY
Fig. 43. Mold support used on a potter's wheel. It has been placed on a cloth pad on the wheelhead and centered, Pakistan.
removed by smoothing, a "contact line” impression is left on the vessel where it made contact with the chuck. If significant pressure was applied, the walls may be distorted at the contact area.
Eccentricity in thickness of the wall is indirect evidence that a chuck was used. If a vessel is removed from a wheel, replaced upside down in a chuck, and not accurately recentered, operations such as turning will remove more clay from one side than the other (Fig. 74 f).
Vessels made without using the wheel are at times supported on chuck-like devices, such as a ring made from plant fiber or cloth. Since the principal function of the chuck used on the potter's wheel is accurate recentering, whereas similar devices employed with non-wheel techniques serve only to hold and support the vessel, the latter should be termed supporting rings rather than chucks.