Clay for the Slip Casting Process

Video Transcript

"So in this segment we're going to look at the slip casting process a little bit, because it's another kind of clay making that is related to what we've already done, but really different in a lot of ways. Now slip is liquid clay, and we want it to be liquid because we're going to be pouring it into these molds and creating hollow forms just in the same way we were absorbing moisture from the clay bodies that we were making before. We pour the liquid slip in and the moisture gets absorbed into the wall of the mold, and then you end up with a hollow casting like little lamb here. Slips are generally fine grained clays. Really, they don't have any gritty particles like some of the materials that might go into the clay bodies we were using on the wheel for instance. So they're either low fire talc based bodies, or high fire porcelain bodies. Now, there are several properties of slip that are real specific to slip because they don't apply to the clays that we want to be firm, like the other ones we were making. One of them is the viscosity, which is the measure of the thickness of a liquid, you've probably heard that term before. You can measure the thickness of a liquid with a gizmo called a zhan cup, which is a cup with a hole in the bottom of it. Now, you can pretty much use any container you want to with a hole in the bottom, and since this is a relative comparison, you can figure out what the ideal viscosity is for the slip that you're using, and just adjust the viscosity based on your baseline comparison, so the, we're measuring the amount of time it takes for this liquid to drain out of this cup. That's how you measure viscosity. The other critical factor for slip is its specific gravity. Which is a term that means the weight of a material relative to that of water. Now, one milliliter of water equals one gram, and so I have a one hundred milliliter graduated cylinder here. If we were to fill up this graduated cylinder to the one hundred milliliter mark with water, it would weigh one hundred grams. If we fill it up with slip, it will weigh more because there's some solid material in that water. Now the ideal specific gravity for slip is somewhere around one point seven for instance, so if we were to fill up the graduated cylinder with this slip, it would weigh about one hundred and seventy grams. So, the viscosity is really important because we need it to be fluid enough to flow in and out of the mold easily. And we adjust the viscosity by not just adding a bunch of water because we don't want to super saturate these molds, we want to add just enough water to make it liquid and then we add a material called a deflocculent, which expands the water molecules, and makes it more liquid without adding water, and the most common deflocculents are sodium silicate, soda ash, and a material called darban seven. So, you add just a tiny bit of these deflocculents once you've got some water mixed in with the powdered clay, and blend it up so that it's really smooth and then it becomes extra, extra liquidy. It's almost like it just instantaneously becomes liquid after having been kind of a pudding consistency. So, there's a whole lot to slip casting. It could be a, you know, whole other lecture series in and of itself, but I just wanted to touch on that because it's another common clay making process that people may have a use for, and those are critical elements of slip casting."