The skills of fine trimming is also called “trimming” or biscuit throwing. It is one of the most important working processes in porcelain shaping. It is a key link both to make the final of the ware and to smoothen the surface of the ware. Its form should be coherent. The trimming of the inner and outer surface is one of the most important part of the skills of porcelain making in Jingdezhen which has much better shaping skills than other kilns. It is a technological guarantee to establish the Jingdezhen style of porcelain with distinctive characteristics. Potters for trimming are not only required to have a good knowledge of the qualities of clay and the thickness of biscuit in different parts, but also quite experienced with changes of lines in from and the degree of shrinkage of every part when firing. Generally speaking, different parts of the same ware have different thickness because each part of the ware has its own degree of shrinkage and endurance in high-temperature firing. As a result, potters should have a good command of different degree of thickness in different parts when making fine trimming in order to prevent the ware from being distorted when firing. In addition, trimming potters are also required to file trimming knives and tools in accordance with different shapes and forms and adjust the radian and angle of knives and tools. They should also learn to prepare the biscuit bats (locally called litou or linao). The size and appropriateness of the biscuit bats are directly linked with the quality of trimming. Therefore, the first lesson for all of the apprentices engaged in the trimming is to learn to file the trimming tools and the next step is to prepare various types of biscuit bats based upon the sizes and forms of biscuits. If not so, they cannot meet the requirements concerning the trimming quality. The speed for trimming on the potter’s wheel should slower than that of throwing. Most of the blades of the trimming tools have distinctive picot edge which is filed on purpose. The very purpose is to make the blades sharper so that the blade will not slip and will better the trimming. The trimming tools are usually made of iron. Before filing, first of all, put the tool in a stove. When it turns red, take it out quickly and put it into the water for a quick cooling, i.e. quenching, before filing again. The purpose of quenching is to improve the strength and hardness of the trimming tool and its corrosion stability. As a result, the trimming knife will not be worn and blend less rust with the biscuit due to the corrosion of the tools. After a period of time, the tools should be quenched and filed once again to make them sharp when they become blunt. It is the first lesson for trimming apprentices to learn to file the trimming tools. When filing the trimming tools, first, sharpen the tool edge to make it flat and straight, which is favorable for trimming vases. After this, file the inner edge of the tools and, finally, try hard to file picots over the tool edge. Trimmers usually use more than a dozen of tools and knives. Each has its large and small types at the same time.
When engaged in the fine trimming, the way to control and distinguish the thickness of the biscuit is very important in application of the trimming skills and guarantee of the quality of biscuit trimming. All of these depend upon the skills and practical experiences. In addition to bettering the outer appearances of the wares, it will reduce loss of fuels while firing and the amount of raw materials. However, too thin biscuits are inclined to become distorted so that more attention should be paid to the clay accumulation for different parts and different forms. If the clay accumulation is not appropriate, such distortions as convex bottom and soft breakdown will take place when the wares are fired. According to the common knowledge, the method to measure the thickness of the biscuit is to feel it up and down with fingers and pat it gently in order to distinguish the different sound of different sections. Therefore, it is necessary to clear out the excess clay spares out of the biscuit and listen to its sound time and again. If the biscuit is thick, a rough “gege” sound will be heard and the “dongdong” sound comes from the middle thickness. When the high-grade porcelain is trimmed to an appropriate thickness, there comes a clear and melodious sound of “ pupu” . When trimming the egg-shell porcelain ,in addition to the above-mentioned method , another method is to dip water on the porcelain with a painting brush for the last stage. We should have a close observation as the water flows straight down the biscuit . At this time when the water dips , the biscuit become wet and have a distinctive wet mark on its surface . If the biscuit is trimmed evenly ,then, the water mark on the surface is well-distributed , or it will have different shades, which means that the biscuit has not met the standards and need to be trimmed again .
before trimming , the thickness of the biscuit is much more than that of finished wares as we have had a thicker biscuit for fine trimming.
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