Plaster plate molds for large ceramic plates
by Douglas Kenney
Plaster plate molds have been around for hundreds of years. Douglas Kenney has been making and using his own plate molds for over 30 years. This blog article describes how to make a wide plate mold. To make a 22” diameter finished ceramic plate on the potter's wheel the mold should be about 24 ½” in diameter at least.
One needs a large bat: a plywood base that has holes in it to correspond to pins fastened to a pottery wheel head. The bat should be larger than 24” diameter.
An added a layer of foam around the 24" bat with duct tape makes it 25 ½” diameter. In a school/college situation it helps to sign your name on one's bats. This is a ¾” plywood bat, painted and varnished, drilled at 14” for bat pins. Make sure the bat/wheel is totally level.
After double checking everything is level it is time to start centering a 25 lb bag of clay. This is a stoneware clay and it is a good choice for this process. If one is going to center a bag of clay it is best to try and hit in the corners first. Use a lot of water and arm strength to bring the clay up and down, while it is spinning until it is centered on the bat/wheelhead.
From a manual up and down centering effort the clay needs to transform into a symmetrical, beehive/dome shape.
Once it is centered another 25 lb. ball of clay is thrown down onto the first ball. Again, using an up and down motion with the hands and water bring the clay into a centered form. After this 50 lbs is centered it is time to add another 25 lb. clay ball for a total of 75 lbs.
With a forceful up and down motion this 25 lb. ball needs to be centered on top the previous 2 balls and made run true. Once this form is centered, the clay needs to be pushed down and out on the bat. Douglas Kenney recommends that you do not take the clay out the edge of the bat, instead keeping the shallow dome about an 1/2 in or an inch from the edge of the bat. This will provide some thickness of the plaster at the top of the mold. The finished master mold will be flipped upside down and one casts the plate molds off the master mold.
Once the shallow dome of clay is totally centered, it is time to use a corrugated cardboard dike, around the perimeter of the bat, to hold the liquid plaster and to keep it from leaking. Use clay coils around the bottom where the bat and cardboard meet. Smooth them out. Shipping tape or duct tape wrapped around the cardboard dike works good to keep the form from leaking plaster. Before pouring the plaster, coat the form/walls everything that will come in contact with plaster with mold soap, petroleum jelly, or cooking spray so the plaster will not stick to the form/bat/cardboard walls. In order to bypass this whole process a perfect shallow dome shape could be purchased or another potter could help make the form. Restaurant supply stores sell extra large plastic salad bowls and woks. A plastic dome or giant ball might work
Next, it is time to mix the plaster to make the large plate master mold. I recommend the book Mold Making for Ceramics, by Donald Frith to get the correct plaster/water ratio. Or online you can get it here: https://ceramicartsnetwork.org/daily/pottery-making-techniques/making-ceramic-molds/how-to-mix-plaster-for-ceramic-molds/ After one pours this very large mold, it is best to wiggle the whole thing a little to get the plaster air bubbles to come to the top. Then wait about 20 minutes or so to remove the cardboard dike around the perimeter of the bat. Once the cardboard is removed, it is good to trim the excess plaster and sharp edges of this mold, while it is spinning on the wheel. A pottery loop tool works fine for this trimming process. Give this plaster mold/form at least 45 minutes to set up and harden before trying to separate the plaster, master plate mold from the form. Use a putty knife if it will not release
Once it has released from the form flip it over, center it on the wheel and trim the top of the plaster mold with a loop tool. Do this to remove any sharp plaster parts and round it off to help prevent chipping. Sharp edges on large plaster mold tend to break when to move it around, etc. Wait two weeks or so to let this master mold totally dry. Once it is dry one can start to cast the plate molds.
Make sure the wheel and bat are still level before starting to make the first plate mold. Use a pencil to center the mold on top of the bat/wheel head. Hold the pencil in one place as the master mold spins and wherever it makes a mark the whole mold needs to come towards the mark. It is important to get it as centered as possible.
Once the master mold is centered on the wheel, reuse the cardboard dike to wrap it around this new plate mold. Here it is shimmed a little on one side because the mold is not totally perfectly round. Next, use some clay coils to squeeze in between the cardboard and the mold. Add as much as needed to make it smooth all the way to card board wall. Use a rib and sponge while the wheel is spinning to make clay perfectly smooth and continuous to the edge of the cardboard wall.
The master mold should look something like this before casting a large plaster plate mold. Make sure the cardboard dike is taped together well to hold the heavy liquid plaster. Next, it is time to make a form to save plaster and make the back of the mold follow the contour of the form.
Use different diameter bats duct taped together and cover them with a trash bag. This form will be used to push the liquid plaster out to edges of the master mold and create a convex back to the plate mold. The reason for this is to conserve plaster and make a lighter mold when making plates and moving them around. Also when one flips a jiggered, finished clay plate/mold on to another bat - less weight is better.
The form to use on the back of the plate mold should look something like this. Duct tape can be used to patch holes or hold it into a convex shape. Coat it with petroleum jelly, cooking spray or mold soap before depressing it into the back of a liquid plaster plate mold.
Work quickly to get all the plaster out of the bucket with gloves and wash it before using the plastic bag, concentric circle form to depress into the back of the plate mold. Push the plastic bag form down and hold it down for a while. A bucket of water or a couple of bricks can be put on top of this plastic bag form to hold it down. Or, use your hands. It will want to float. That is why you need to push it down and weight it or hold it down. It is going to take about 20 minutes or so for the plaster to start setting up. The plastic bag form will be stuck when the plaster starts to harden. One will see this and can start pulling the edges of the black plastic towards the center. If it does not cave in (still liquid) wait a little longer. As it hardens, but the plaster is still kind of soft, pull the black plastic form out.
It should look something like this. Work quickly with your gloves to smooth the semi liquid plaster into a smooth concave form while the wheel is spinning. Next, use a trimming tool to smooth it out in to a contour that matches the other side of the plate mold. Best practice is to leave about two or three inches of plaster near the edge of the plate mold. Trim a 1 inch high 90 degree edge on the inside of the mold to help with mold strength and make the mold easy to grab when moving or flipping. It also helps to keep the mold weight on the bat from moving around when one hand jiggers a plate. Next, pull off the cardboard dyke and remove the clay coils. Proceed to trim the edge of the new plate mold while the wheel is spinning. Round off the edge with the trimming tool and sponge etc.
The plate mold/master should look something like this. Next try to wedge a putty knife between the two plaster forms all the way around. Be very careful not to chip the plaster plate mold because the plaster is still semi wet and has not have time to harden. Slowly, and carefully dislodge the new plate mold from the master mold.
It should look something like this. Make sure the plywood bat is level on the wheel head. Now, carefully flip the new plate mold and then center it on the wheel head/plywood bat. To center it one can tap it into the center while the wheel is spinning with your hands or hold a pencil in one spot, when the wheel is spinning, and make a mark. Where the pencil mark is it needs to go to the center of the wheel. Once centered the mold should be trimmed using a loop tool and sponged/smoothed off. Any imperfections in the surface should be ribbed or wet plaster added and filled. Use a thin stainless steel rib to scrape off any imperfections while it is spinning on the wheel. When one is finished with the tooling it is good to mark the mold with a black sharpie or marker with a perfect circle around the edge.
The finished plate mold should look like this. Here's are link to some pics from 1997 from my Santa Barbara studio making a huge plate with a similar mold but larger created as described in this blog article: Douglas Kenney making a large plate with a clay slab and coils