Fettle Knife - plastic preferred
Strong Rubber Bands or
Hard Rubber Scraper
The process of pouring ceramic molds is called casting or slip casting. Before you begin, it is a good idea that you know how the mold works. Ceramic molds are made of plaster as opposed to other molds being rubber, sponge, metal or plastic. The reason is that plaster absorbs water.
Slip is liquid clay, it has a lot of water in it (see “What is Ceramic or Clay Slip”) and in order for it to hold a shape, the water needs to be removed. Enter the plaster mold.
There are many kinds of plaster molds, ranging from one piece to two piece to multiple pieces that fit together and form a cavity to hold the slip. Most common, and addressed in this article, is the two piece mold. These molds are designed to fit together tightly to form a cavity with a single opening that is used to receive and expel the liquid clay. See “What is a Ceramic Mold”.
When you fill your mold with liquid clay, the plaster begins to absorb the water and the clay platelets begin to stick to the sides of the mold. This creates the shell that will ultimately form your finished item. The longer you leave the slip in the mold, the more water that will be drawn out and the thicker the shell will be. When the shell is as thick as you want it, you then drain off the excess slip and you are left with a hollow item.
Before casting your mold, you need to make sure it is clean- both inside and out. Debris inside will imbed in your casting and be difficult to remove. Debris on the outside can fall into the cavity or the slip. To clean the inside, use a dusting brush . Brush out all debris, blow into the area if needed. Do not rub or scrub the inside as that could cause damage by chipping or wearing away the detailing. If you have a stubborn detailed area, take a wad of clay/thickened slip and press it into the area and then gently lift away. To clean the outside, use a plastic or rubber scraper.
New molds need to be cleaned before use also. There is a residue in them from their own casting process that should be removed. When dry, a light brushing should do the trick, or you can cast a piece and then discard the casting.
After cleaning your mold, carefully place the pieces together and tightly band them with strong elastic bands or belts. Smaller molds are fine with bands, but with larger molds, belts are best to use. Nothing worse than having a mold split during the casting process and having all that slip spill out on the floor or table. You should use enough bands or belts to ensure the mold it tight on all sides.
Set your mold on your pouring table with the pour hole/pour gate on the top. Whether you use a pump or pitchers, make sure you have enough slip ready to fill the mold completely. It is important that once you begin filling it, you do not stop the flow or it will cause water lines that are not always easy to clean off when the ware is dry.
When you cast your mold, the spot the slip first comes into contact with the mold can create a ‘hot’ spot or a ‘hard’ spot. This can effect future castings, so it is a good idea to deflect the stream of slip as it enters the mold. This can be done several ways. One, you can direct the stream against the side of the pour gate. Two, you can place a stick or other item across the pour gate and deflect the stream by directing the slip to the stick. Three, you can run your belt or band across the hole and use that to deflect the stream of slip. I always keep an assortment of wooden spoons in my casting area and use them to deflect the slip, as well as for many other uses. These hot spots or hard spots will keep your ware from maturing while in the mold and can carry over with the finished piece in creating spots that will be discolored and/or will not retain the paints.
So, are you ready to cast your mold? Make sure your slip has been mixed well and is free of lumps. Let the slip rest for a couple minutes so all bubbles will reach the surface. Fill the cavity of the mold with slip using a nice steady stream. Not too slow, but also not too fast. If you cast the slip too fast, it can cause air bubbles to get trapped in the details and undercuts. Bring the level of the slip to the very top of the pour gate. As the mold draws the water out of the slip, the level will drop. Refill the mold and keep the level above the cavity side of the pour gate. How often you will need to do this depends upon the size of the mold and how thick you want your casting to be.
Most generally, you want the item to be 1/8 to 3/16 inches thick. Larger items will need to be thicker. This process could take anywhere from a couple of minutes to an hour or more. It will also depend upon what kind of clay slip you are using. Porcelain sets up much faster, stoneware takes much longer. Earthenware is in between. A major factor that will effect how long slip is left in the mold is the weather and temperature. High humid, hot days will take longer than dry days, cold days will take longer than hot days and so on. Another thing that will make a difference is if your mold has been cast before on the same day. A dry mold sets up faster than a wet one. So, experience will be your best teacher. One way to check your thickness is to make sure that the pour gate is always full, to the brim. After so many minutes, cut a chunk away to see how thick it is. Most generally a decorative item measuring about 6” tall, will take about 15 minutes; but a cup the same size needs to be thicker, so it can take up to a half hour. Some items need to be cast solid, like plates. These items will usually take much longer. Pay attention and keep notes until one day, you will be able to look at a mold and know it is ready to drain.
EVERY MOLD has its own personality and may cast differently.
After the shell has formed to your desired thickness, it is ready to have the excess slip cast off or drained. There are a couple things to consider when doing this. If there is a large pour gate, tip the mold to its side an let the excess flow back into the storage vat. Hold it on its side for a minute or so giving plenty of time for the slip to slide to one side, then up-end the mold with the pour gate down to continue draining. Depending upon what your item is, you will leave the mold with the pour gate down until it is completely drained. NOTE: If it is a box, vase, utility item like a cup or similar where the inside will be seen when in use, turn the mold over as soon as the dripping stops. Return it to the position with the pour gate on top so that the slip will smooth out on the items ‘bottom’ and not leave little drip bumps. If the item is a figure or such, it can be left a while with the pour gate down. If the mold is a cup or something with a handle, turn the mold so that the handle is on the down side and the last little bit of slip can fill the handle. Handles, especially cups, should be solid with no ‘dimples’
If the mold has a small pour gate, it is very important that you not let the slip gurgle or glug as it drains out. Carefully tip the mold and watch the flow so that it exits smoothly. Do not up end it until you are sure it will not gurgle. If you let it glug out, it can cause a suction that could pull the shell away from your mold casing and will cause it to not dry or mature properly. It can also cause the entire casting to collapse.
Once your slip has been cast off, you will need to let the mold rest and continue to draw water from the item. How long again depends upon: thickness, item, size, weather, etc. Most generally, a medium item can be removed in about an hour. Smaller items can be removed after about 15 min. It is all a matter of trial and error at this point. As mentioned above, every mold has its own personality. A mold will not open if it is not ready, so don’t force it. Some molds need to be opened and then the item left in one half a while to continue the drying process before removing it from the mold. The length of time again depends upon the item.
Before opening your mold, you will most often want to remove the ‘spare’. That is the clay shell that formed in the pour gate. This is usually done with a plastic fettle knife. Metal fettles can cause damage, so if you use them, be careful to not scrape or ding the mold. There are some molds that removal of the spare should be done after the mold has been opened. IN ALL CASES, cut the spare away, do not pull on it or break it off. The breaking or tearing away can cause hairline cracks to radiate out from the pour hole. Often these cracks won’t show up until AFTER the item has been fired.
When your item is leather hard, or firm, it is ready to be carefully lifted from the mold and set on a level surface where it can continue to dry and harden. Using a bat cut from plaster wallboard is a great surface to set your wetware to dry. Be sure to not leave the wetware where there are drafts as it can cause the ware to tilt and warp. This is especially probable with tall slim items.
When you first remove the item from the mold, in its leather hard stage, it is a good time to do any cuts, like windows in houses, and to punch holes such as for lights in Christmas trees. Sometime before the ware dries, it is also best to make any attachments. Some molds cast several different items which are to be joined together. Sometimes these can be immediately attached once the pieces are removed from the mold, but others, like wings on Pegasus, it is best to let the ware harden a little longer; however, they should be attached while the pieces are still damp. Smaller attachments can be kept wrapped in plastic as they will dry faster than larger items.
To attach items together, you will need to create a vent by punching one or two small holes in the main item where you need to make the attachment. Next, apply some slip to the pieces with a brush and press them together with a slight twisting motion. Again, pieces need to be wet/damp. With a clean, wet brush, wipe away any excess slip from the joined area.
So now you have your casting. Fun, wasn’t it? Well, you are not done yet.
The most important thing about casting is to make sure your mold is clean before you cast it again or put it away. Primarily you just need to blow away and loose crumbs of clay, and if any stuck in any of the detailing, use a wad of trim/spare and press it over the area then lift it away. If the clay does not remove all of the slip from the details, leave it. You will cause too much damage trying to pick it out.
If there was any flashing, it is most likely your mold was not banded tightly enough. Flashing is a thin line of slip that formed along the seam lines. Removing this is a very delicate process. You DO NOT want to create any pressure along that seam line as it will wear away the plaster and create even more flashing on subsequent castings. So, take a barely damp sponge and wipe across the flashing from the outside edge of the mold toward the cavity and LIFT the sponge as you get to the cavity so there is little or no pressure along the seam edge. Make sure keys are clean to ensure a tight fit (these are the knobs and sockets that lock the pieces together).
The exterior of the mold should always be cleaned before storage. It is easiest while the slip is still damp, by using a hard rubber scraper or plastic trimmer. See “How to Store Molds”
Want to cast another mold? Go right ahead, but remember that when the plaster gets wet, it gets soft and when it gets soft, it will wear out. Over casting your molds will shorten their life expectancy. A good life for a mold is about 40 castings; however if you follow a few guidelines, you can get over a hundred castings and still have good detail in your mold for more.
Do NOT cast a mold more than twice in one day - Once a day is best.
Do NOT cast a mold more than three days in a row - give a good 2-3 days resting time before casting again - longer if possible.