Japanese Potteries(Explanations to the technical terms of ceramics No.17)
There are various technical words on potteries. Here are some of those, for which I'd like to give you some remarks.
Plaster Mold
The image of manufacturing potteries that you may recall would be the process that a workman is changing the shape of clay speedily on a potter’s wheel. The appearance of skilled workmen using a wheel is even beautiful and attractive. One more thing that we can’t miss on mass-producing tableware or so is a plaster mold. Plaster had been largely used since the ancient period in Egypt or Europe as a material of construction or sculpture, while it is also used for potteries thanks to its nature to absorb water very well. It can be said that there are mainly 3 types of plaster mold, which are inner mold, outer mold and casting mold.
Inner mold (above), outer mold (middle) and casting mold (below)
Inner molds are reflecting the inner shape of vessels. When using, you shape clay roughly in the size on a plate or a wheel and put it on the mold, and then extend the clay along with the mold slowly from the top of the mold to side. The clay may be cracked if you hurry, so make it even gradually. You can make various shapes by using molds such as octagon, square or maybe like shells that can’t be made with a wheel. However, if it’s too late to remove the clay from the mold, the clay may be shrunk and not be able to be taken off the mold. Outer molds are reflecting the outer shape of vessels contrary to inner molds. You shape it by pressing clay into the mold. Or, there is another way called sludge casting to pour muddy clay into a plaster mold. You take the mud off the mold about 10 minutes later, and see clay that is about 1 cm in its thickness remaining in the surface of the mold. When the clay starts to dry, it makes a gap naturally between the clay and the surface of the mold, therefore you may easily know the timing to take it off the mold. Accessories like brooches or cameo-style decorations are often shaped in this method. Casting molds are for shaping complicated forms or thickness. Inner molds and outer molds are good for shaping a form like a vessel but not for shaping something like an apple ornament. If you want to make something which inside is empty or stuffed on the contrary, casting molds are suitable. You make an entire original form and then draw a parting line according to its angle for pulling out and divide the plaster mold into a few parts. For making something which thickness is even, you press a clay board to the mold or pour clay from a casting hole of the mold. For making something which thickness is uneven, you cast muddy clay into the hole from a higher position so that the casting pressure works to spread the clay all over the complicated parts inside. Although the works of making casting molds from making its original shape are precise and tiresome, once it is finished, you can manufacture plenty number of products in an exactly same shape, no matter how its shape is precise and complicated. However, several points in the process such as the timing of taking clay off the mold and handling of the muddy clay require a certain skill though it looks easy. Techniques of plaster molds have been developed in Europe since the beginning. In the delicate and sophisticated shapes of the dolls of Lladró or graceful decorations of Wedgwood, a lot of technologies to which the above skills are applied are actually used. Such disadvantage that they couldn’t locally obtain a sticky type of clay had, on the contrary, contributed to develop the techniques of casting molds and create world-famous potteries, which is a domain to which the Asian could hardly reach. Difference of clay’s nature makes difference of products—it is true even in the scale of the world.