|Monthly Tech-Tip |
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.
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A process for mass-producing simple shapes on a mechanized pottery wheel having a solid swing-arm with a template. The process is, of course, very automated in industry but it is also very accessible to potters.
For plates, a profile describing the outside shape of the piece is used to force the soft clay against a rotating plaster mold describing the inside contour. For vessel forms, the profile forms the inside shape. After drying against the plaster, the piece shrinks and releases from the mold and can be removed. After removal, fix-up generally needs to be done to smooth any irregularities on the surface that was against the mold and finish the edges and lips of pieces. Industrial jiggering machines use a rotating mechanism and can press the clay against the form with much greater force, this enables the use of stiffer material and produces a smoother surface that does not need fixup.
Typically, clay used for traditional jiggering with a fixed template 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. Small scale jiggering is most practical for plates and bowls. Upright shapes, like mugs, present many more challenges, both mechanical and material, it can actually be faster to throw these on the wheel than jigger them. Molds for plates are much bigger and therefore more difficult to make on typical 3D printers.
RAM pressing is a better alternative for many flatter 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.
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.
Print this using PLA filament and it will be plenty strong enough to use (even if not printed solid). The knife-edge will print very precise and smooth, giving a good profile. After making several of these I discovered that I could hold them in-hand to finish a mug I had already slip cast in a jigger mold (a jigger arm was not even needed). The profile contour was easy to copy from my drawing of the mug itself. This template is fairly narrow, this was needed so the backside cleared the rim of the spinning mold as the jigger arm is lowered. But now that I use it in-hand, I could increase that dimension so it finishes right across the base.
Showing dimensions to fit a Shimpo RK2 wheel, its shaft is 0.983 in diameter. The molds fit down inside as shown, only contacting the cuphead at the shoulder. This is machined from aluminum. We do not sell this of course, but you could take this drawing to a machine shop and have them make you one (the numbers are readable when this is zoomed to full size).
The head is lathed from aluminum. It fits on a potter's wheel (adjust the inside diameter to fit the shaft). The plaster mold drops into it this cuphead and stays in place by friction. The only point of contact between the mold and aluminum should be at the top inside corner of the cuphead.
The arm is heavy, made from 1/2 inch plate steel. The counterweight at the end have 1 in plate weights. The swivel mounts are machined to fit the custom cone-ended bolts.
We have mechanical drawings for this (and the cup heads). This is very sturdy and useful. The arm is relatively short compared to industrial jigger wheels and is thus useful for only small shapes. There is an advantage: The template contacts vertical walls at a more perpendicular angle. But the disadvantage is that the trailing edge of the template hits the outside edge of the lip on taller shapes. The pointed bolts hold the arm securely and their tightness enables varying the friction of movement. They have enough length to also position the arm horizontally. We 3D print templates and block masters for making working molds that drop into the cuphead.
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 shoulder. 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 (or just casting). For example, I can cast a mug in the mold, then pour out the slip, wait a few minutes and then, as the wheel spins, finish the rim and inside sure using a 3D-printed template/rib. I do not actually use the jigger arm, it is easier just to hold the template in hand. I can finish the rims on any round pieces made in these molds.
This is machined out of aluminum. We also have drawings of a 3D-printable shell-mold for making molds that drop into this. And methodology for printing the outside contour of pieces to be jiggered.
This was part of my nursery plant-pot mold-making project. I held previous shell-molds down with a brick on top. But that was not working well, putting the mold out of shape, making it difficult to pour the slip inside and agitate the mold after pouring to surface the bubbles. I found that the 3D printed mold can be held down by simply using a sticky clay slip (Plainsman Polar Ice)! I printed this one with a flange at the bottom for this purpose. I am pouring the mold on an arborite bat on the wheelhead of a potters wheel. This enables turning the wheel slowly and pouring the plaster into the perimeter slowly through a funnel, that produces a much better surface. If you don't have a super sticky clay you can make one by adding 4% Veegum to a Grolleg porcelain. Actually, Grolleg and New Zealand kaolins are both very sticky, so they could also work.
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.
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.
Casting plates, is it practical?
Project to Document a Shimpo Jiggering Attachment
Make your own jiggering attachment for your potter's wheel. Here are the plans and lots of help to get you going with molds and techniques.
2019 Jiggering-Casting Project
A process in ceramic production where items are slip-cast first and then finished using a jigger wheel.
|By Tony Hansen|
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