After clay is formed into various shapes using different molding methods, further trimming is required. This is also a crucial step in the pottery-making process. Due to the limitations of certain molding techniques, especially those such as hand-molding, hollowing out clay lumps, or assembling clay sheets, the resulting pottery may not have a uniform shape, and the clay body might not be compact enough. To make the body more dense, the shape more regular, and the pottery more durable after firing, strict and careful trimming of the clay body is necessary. Trimming methods include patting, slow wheel trimming, burnishing, and applying a slip coat.
- Patting: This is the most important method of trimming the clay body, aiming to make the structure more compact by patting the walls of the vessel. Tools are used for this process, such as a paddle to pat the walls. Paddles can be made from bone, wood, stone, or pottery, with wooden paddles being the most common due to their broad, flat, and smooth surfaces, which are ideal for patting. During patting, a mushroom-shaped clay pad or smooth pebble is placed inside the vessel to prevent deformation. Potters discovered that wrapping rope around the paddle or carving vertical or crossed grooves on the paddle improved the effect of the process.
The patting process must be even, regular, and systematic. As a result, the textures of the paddle naturally imprint on the surface of the vessel, creating a unique texture that adds visual variety. Initially meant for trimming, this process eventually developed into a method for decorating the clay body. Patterns such as rope marks, lattice marks, and grid patterns likely emerged during this stage, later evolving into intentional decorative techniques that enhanced the pottery’s visual appeal.
- Slow Wheel Trimming: Following patting, this is another important method for trimming pottery. The clay body is placed on a specially designed wooden (or stone) wheel, which is rotated by hand or foot. As the wheel turns, the potter uses a bamboo scraper or other tools to trim the body. Slow wheel trimming is mainly used for the rim, belly, and base, aiming to make the vessel more regular. The slow wheel spins at a low speed, serving only to assist with trimming, not shaping, but it laid the groundwork for the later development of wheel-based pottery making. This method often leaves subtle wheel marks on the body.
- Burnishing: Also known as polishing, burnishing involves using hard, smooth tools made of bone, bamboo, or stone to press and polish the surface of the clay body while it is still slightly wet. Coarse particles are pressed into the body, while finer particles remain on the surface, making it smooth and delicate. During the burnishing process, the clay body becomes compact and dense, and after firing, the surface becomes glossy and smooth. Many Neolithic pots were both burnished and decorated, achieving excellent results.
- Applying a Slip Coat: The slip coat is made by finely washing clay with natural coloring elements, turning it into a fine slurry. This slurry is then applied to the surface of the semi-dry clay body, forming a coating known as a slip. Applying the slip coat can fill larger pores on the surface of the vessel, making it smooth and visually appealing, while also protecting the surface. The slip can be applied with a brush, by dipping the clay body into the slurry, or by rotating the body on a slow wheel while brushing the slurry onto the surface.
Although slip coats come in different colors, they are usually not considered part of painted pottery because they lack patterns. The slip coat can be similar to or different from the color of the clay body, depending on the coloring elements in the clay. Common slip colors include red, brown-yellow, white, and black. For instance, red and brown slip coats are made from iron-rich clay or ochre, while white slip is made from refractory clay such as porcelain clay, which is low in silicon and iron but high in aluminum. Black slip is typically made from easily fusible clay. These three raw materials form the basic palette for slip coats, with other colors achieved through specific techniques and firing processes.
