First, perhaps it would be a good idea to discuss what IS a ceramic mold and how does it work.
A ceramic mold is a plaster form, with the image or pattern of an object imbedded. A mold can be comprised of one, two or more pieces that fit together with an open access to fill with ceramic slip. This open access is called a pour gate.
Slip is a liquified clay and chemicals formulated to solidify when the water is drawn from it.
The mold, being made of plaster, draws the water from the slip creating a shell of clay platelettes along the walls of the mold cavity. The longer the slip remains in the mold, the more liquid will be absorbed and the thicker the shell. The more water a mold draws, the wetter it will become which will affect the casting process. See section on care of a mold.
The pieces of a mold are banded tightly together for both its casting and storage.
Let’s begin to pour or cast one of your molds.
Handle your mold gently. Take apart sides and pieces carefully so as to not bang them together and chip the internal edges. Using a stiff bristled brush, lightly brush or blow out all debris, dust or foreign objects from the cavity of your mold. Please use care as to not rub or gouge the edges or design. It is important to do this with a new mold also as there is usually a coating of what I call mold soap that needs to be dusted out.
Re-band your mold tightly, fitting parts together carefully. Many molds are easier to band using specially made belts that are spring or lever released and can be tightened so as to not allow slip pressure to expand them as can happen with rubber bands – a must with the larger molds. Nothing worse than having a mold split during the casting and have all the slip spill out onto the table and floor. Besides a waste, it is a pain to clean up.
Fill the mold with the slip using a steady, smooth flow-not too fast. Do not stop or hesitate during this process or you will have ‘water rings’ appear on the item which require added attention during the cleaning process and are often overlooked. Do not fill too fast or you risk air bubbles becoming trapped at some of the undercuts and sharp detail areas as well as the bottom of the item (which is the top during casting). As water is drawn into the plaster, your slip level will drop. Make sure the slip level does not drop below the pour gate. Until you know your molds, it is best to keep the level to the top of the pour gate.
After the slip has set in the mold for the desired length of time (depending upon how thick you want the shell), you will cast off the excess. To do this, you will want to tip the mold over at an angle and let the slip drain off into the container/vat for future use, upending completely as most has drained off. If you cast it off too fast, you can cause a suction that can pull the form away from the mold and it can collapse, or it will not harden. In come cases, you may need to upend the mold and tip it again as sometimes slip becomes trapped.
Depending upon what the item is, determines whether the mold is set aside with the pour gate at the top or bottom. If you are casting a figure, leave the pour gate down to continue draining until it is ready to be removed from the mold. If the item is a cup, or bowl, or vase, or trinket box (anything that you will be able to see in when finished). Drain off the excess until it stops dripping and then upend it so the pour gate is on top. This will cause any excess slip to slide back into the bottom of the piece and smooth out. If left with the pour gate down, it can cause drips or bumps in the surface causing difficulty in the cleaning process to smooth them out.
As a rule of thumb, most decorative pieces are cast between 1/8 and ¼ thick. The larger the item the thicker it is cast. Utility pieces, bakeware, dishes etc., are cast heavier for added durability. Ornaments are cast much lighter. Ultimately, with the exception of the very large pieces and utility items, it becomes a matter of preference.
Casting time is determined by several factors – thickness of the slip, wetness of the mold, weather and humidity to name a few. A nice tool to have, is a set of slip rings. They look like rubber washers. Set the one for the desired thickness on your mold, near the pour gate. Fill it with a small amount slip after you fill your mold. When that is solid, you know it is time to drain the mold. If you do not use slip rings, keep your slip level to the top of the pour gate and you can ‘test’ it with a slight fettle cut into the side to see how thick it is getting.
After the mold has been drained, the item needs to remain in the mold until it reaches leather hard stage. During this set-up time, the mold will continue to draw off the water until ultimately, the item will pull away from the mold. At this point, your mold will easily open and the parts lift away. Please note that some molds are more temperamental and need to be handled differently. If allowed to stay in the mold too long, some ware may suffer stress cracks. This is because during the maturation process the item shrinks (pulls away from the mold). If there is no way for the item to pull away (areas that bend, like arms), the stress will cause the item to split. To prevent this, you can open your mold a little earlier. You will quickly learn which ones that will need to have the mold opened early. Just take off one side of the mold and let the item continue to mature. There are cases where even then, stress cracks can occur. In those cases, hold the open mold with the item and tap the edges lightly until you see the wetware lift slightly. Again, leave it in the mold until it hardens sufficiently enough to allow you to carefully lift it out. Another note – if a mold won’t open, it is usually because it has not matured enough to pull away from the plaster. Give it a little more time and try again. A mold is usually ready to open after an hour or two from casting it. (Some less and some more)
Before opening a mold, you should remove the spare (from the pour gate). This should be done by using a fettle knife. Slip the knife (I recommend a plastic one) between the clay and side of the pour gate. Insert it far enough into the gate to reach the bottom of the item. Cut away the spare. Throw the spare away (unless you use it to help clean your mold)
After you have removed your wetware from the mold, you need to clean your mold again. This time you need to be extra careful because the mold is wet and soft. Use a barely damp sponge to wipe away any clay from the seam lines. DO NOT press down along the edge, but rather use a lifting motion. Blow out any loose crumbs of clay. If anything is stuck in the details, take a wet piece of trim/spare and press it against the detail. You should be able to then lift it out. If not, then leave it be. Close and tightly band your mold. Using a hard rubber scraper, scrape away any slip spills from the outside of the mold. It is best to store your mold on wooden shelving with the pour gate down to prevent intrusion of bugs, critters or debris.
To give your mold a long life, it is recommended that you not cast it more than twice in any given day and not more than three days in sequence before giving it a few days to rest and dry out. Each time you pour your mold it gets softer and the detail will wear away faster. Letting it dry thoroughly and not over pouring it will extend its life and productivity.
Please feel free to discuss this article or any other ceramic matter on our blog.