|Monthly Tech-Tip |
A process in ceramic production where items are slip-cast first and then finished using a jigger wheel.
Key phrases linking here: casting-jiggering - Learn more
The slip-casting process is well adapted for producing non-functional ware, especially having thinner walls. However it does not do well for functional pieces that require thicker walls and contoured lips (heavier cast walls have less regular wall thickness). However these issues can be dealt with by adding a jiggering step to even out the wall and round and contour the lip.
For the jiggering step to work a casting slip of adequate plasticity is needed so that clay does not tear when subjected to pressure from the template (more plastic slips need more ball clay and less kaolin, this can mean that longer casting times must be tolerated).
To jigger the lip of a piece it has to be cast in such a way that the clay extends upward from the top of the mold. The key to achieving this is 3D-printing a pour spout that and carefully placing and gluing it to the top of the mold (using slip). After draining the molds, the spout can be removed and the jiggering process done.
There are a number of complications to effectively creating this process in a studio, but with the advent of 3D printing many of these have been removed.
An interesting aspect of this process is that "weekend warrior" potters can employ it to produce ware. Within a two or three hour time window molds can be filled, poured, jiggered and pieces extracted ready for handle attachment and drying. Since these bodies normal shrink very little, drying can be done without any need for covering ware.
I am creating molds for a 2019 casting-jiggering project to reproduce heavy stoneware mugs manufactured here 50 years ago. I have a profile drawing I want to match (upper left). The solid plaster model on the left was my first attempt at manual tooling. The metal template was time-consuming to hand-make, its contour was difficult-to-match to the drawing and the plaster surface turned out rough and difficult-to-smooth. To make the plaster model on the right I printed a shell (using my 3D printer), poured the plaster in, extracted it after set and then smoothed it on the wheel using a metal rib and trimming tool. It matches the drawing perfectly and the round is very true. 3D-printing is revolutionary for this type of thing! The drawings: I hired someone on Upwork.com to make them for me (using Fusion 360). The shell-mold (to cast the model) on the upper right: I printed that too, in two pieces.
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.
I prepare to attach these by simply touching them to the top of the slip (it is sticky and coats the underside evenly). Then it is just a matter of setting it in place and and it glues down in seconds. I make these a little larger diameter than the top of the opening (e.g. 2mm) in the plaster mold, producing a small overhang on the cast piece. During the time in the mold, the clay slurry creates a thickness against the plaster walls but also extends upward above the top of the mold (inside the spout). A while after pour-out I am able to make a cut and lift the spout away leaving an over-hang, over-height and thickness that all play to enable jiggering the rim to the exact contour I want.
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.
Casting plates, is it practical?
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.
2019 Jiggering-Casting Project
|By Tony Hansen|
Follow me on