Jiggering is a process of forming pottery that employs a spinning plaster mold and a tapered template to press soft clay or porcelain against it.
A process for mass-producing simple shapes on a mechanized pottery wheel having a solid swing-arm with a template. For plates, a profile describing the outside shape of the ware is used to force the soft clay against a rotating plaster mold describing the inside shape. For vessel forms, the profile forms the inside shape. After drying against the plaster, the piece shrinks and releases and can be removed from the mold. After removal, fix-up generally needs to be done to smooth any irregularities on the surface that was against the mold.
Typically, clay used for jiggering is much softer than that used for throwing on the potter's wheel. The work required to set up a jiggering process is significant and a number of mechanical and mold making skills are needed. RAM pressing is a better alternative for many shapes (but is much more expensive).
Medalta still manufactures ware, albeit on a much smaller scale, in a dedicated facility. Visitors to the exhibits see this operation when they do the standard tour. Most ware is transparent glazed and fired at cone 6 oxidation in electric kilns.
Notice the inside of this large transparent glazed cone 6 stoneware bowl. There is a concentration of specks on one part because that area was sponged at the leather hard and dry stages to smooth surface problems that happened during the jiggering process. These specks are normally driven below the surface during forming.
These are not cracks. They do not go through to the inside. During forming the clay was folded over itself rather than compressed against the mold.
The cup-head was lathed from a block of aluminum and it attaches to the shaft the same as a regular wheel-head. Plaster molds simply drop in and sit on their shoulders. The shoulder is the only point-of-contact, this prevents chattering while the mold spins when under pressure. I am using these molds for a casting-jiggering process. I cast a mug in the mold first. After pouring out the slip I wait a few minutes and then, as the wheel spins, finish the rim using a 3D-printed template and rib the inside surface. I do not actually use the jigger arm, it is easier just to hold the template in hand.
The grey outer shell mold on the left was printed in two parts and glued together (at the shoulder). It's vertical split enables me to open it a little. The center model of the outside contour of the mug (on a two-step base) was made by casting the plaster inside another two-piece 3D-printed form I had made (we had to use a heat-gun and scissors to get the PLA printed form off of that plaster). It smooth the surface on the wheel using a metal rib and trimming tool. Then I stretched a rubber band around the first step at the bottom (because the shell was a little lose-fitting). Now the outer shell mold fits perfectly and clamps tightly in place. To cast a jigger mold it is just a matter of soaping the plaster model and the inside of the shell and pouring in a mix of 1300 pottery plaster and 900 water.
A process in ceramic production where items are slip-cast first and then finished using a jigger wheel.