Last week I said I was determined to show more process, so here is a first attempt: a pictorial tour of the process of slipcasting the Ooi cups. Slipcasting is a wonderful way of mass-producing ceramics that works by using a liquified clay (called slip) and pouring it into a plaster mold. The plaster sucks the moisture from the clay, hardening the part. It’s so simple it feels like magic, and it becomes highly addicting!
The process starts with a positive, which is the blue thing in the photo above. I made the positive by turning a piece of balsa foam on a lathe and finishing it with joint compound finely sanded and paint. Because of shrinkage in the drying and firing process of the clay, you generally make positives about 10% bigger than your final design.
Next you need to find the high point, which is going to be where the mold’s parting line will be. This is a simple two-part mold but if you have very organic, complex design, this process can be very tricky because you may have 3 or 4 or more parts to your mold. In the example above, if you split the mold on either side of the true accent of the cup’s profile curve, you won’t be able to get the finished piece out of the plaster mold. For a simple piece like this, the best way to find it is to set the piece on a flat surface, lip side down, and bring a vertical surface up next to it. The vertical will be tangent to the high point.
Next you prep the piece for making the mold, which means making a setup where half the piece is covered. I did this with pink foam, and filled the gaps neatly with clay (clay not shown).
Then you set up the mold boards and pour the plaster.
Once you have half the mold poured, you cut keys, which are indents in the surface of the mold. Those indents will be matched with “outdents” on the other side, which will ensure the mold is always lined up perfectly. Once the keys are cut and the first half is dry (this can take a few days to a week, if humid), you soap up the surface of the mold, applying with a brush and washing off with a damp sponge.
This is essential, as it’s the small amount of soap that absorbs into the mold surface that keeps the two halves of the mold from sticking together. This is my favorite part of the moldmaking process. The soap smells like the stuff you used to wash your hands after painting in elementary school art class, and it reminds me how natural this process is in comparison with other types of moldmaking. For example, when you make a silicone mold, the equivalent product is a chemical called mold release that comes in an aerosol can. Of course, silicone molds have their place, and you can’t do everything with plaster and clay, but it does feel good to be working in a way that is relatively non-toxic and wholesome.
Once the mold is done there is a lot of trimming and scraping to do to clean up the edges and make it “mold beautiful,” which is my professor Irv Tepper’s term of highest praise. An ugly mold will do the job as well as a beautiful one, but you’re using this tool a lot and it deserves some care in craftsmanship. Even if you don’t go all the way to mold beautiful, the scraping is necessary to remove the “skin” that forms from the contact between the mold and the mold boards to allow the plaster to breathe. Every so many castings, the mold gets scraped again to keep its performance up. One mold could make 80 or so castings before needing to be remade.
Now you start casting! You pour the well-mixed slip into the opening and set a timer. The magic number for the Ooi cups is about 14 minutes, but of course this will vary with temperature and humidity. What you’re waiting for is the right amount of moisture to be absorbed from the slip so that the wall thickness is the way you want it. Too thin and the piece may warp during drying or firing. Too thick and it just won’t look right.
After the right amount of time has passed, the slip is drained back into the bucket to be reused, and the mold is left to sit for a half hour or so.
Eventually the part will start to pull away from the mold, and you can easily remove it. This is what top half of the cups look like when they come out of the mold. That extra clay around the lip will be trimmed with a knife when the piece gets to the leather hard stage. At that point you can handle the piece without denting it.
Here are some pieces in varying stages of greenware. The two in the front are firm enough to handle, so I’ve trimmed the “spare” around the lips, but they are not dry enough to sand off the parting line yet. Parting lines are notoriously difficult to remove and once you’ve slipcast a piece you start noticing them on all kinds of ceramic pieces. I don’t mind the ghost of a parting line as it reminds me of the way the piece was made, much like the navel on the bottom of the Ooi cups is a reminder of the live center of the lathe that the original positive form was turned on. In an industrial age, these marks of mass production are like the fingerprints of workers; they are the only provenance our goods can offer, the only clue that they were not just dropped here, perfectly formed, from outer space.
These cups are bisqued, which means they’ve been through the kiln once. It never fails to amaze me how the color changes in the kiln, and how creamy and pure a set looks when they come out.
Glazing. I glaze the Ooi cups by dipping them because I think it creates a smoother application than brushes. The glaze goes on bluish, but the dye is just to help you see where you put it. The glaze is actually white. The bottoms of the cups are unglazed to highlight the eggy form and create a textural contrast, both for the eye and the hand.
These guys are glazed and waiting to go in the kiln. Soon they will be real cups…
Final, glaze-fired Ooi cups. If you look very closely, on some you can see the faint trace of a parting line, but otherwise they are all identical. Just waiting around for good homes!