Wild Clay Slip Casting

I'm a creature of habit (in all but practice); I can certainly purchase clay, use my wallet to solve my problems, but this habit, this modern way of solving problems, it creeps into every part of life, and what we miss, what we skip to subdue our urge to create in haste, is a fascinating amount of knowledge that parallels in ways only those of us, who take the risk, who dive down the well, can understand If you want to know a bit of chemistry, a bit of physics, and a whole lot about yourself, then this thing, that I call “wild clay” might appeal to you as well.

We have talked a lot about the material properties of wood, and as far as we know, it is unique in that it exists only on this one rock we are lucky enough to live on. On the other hand, base minerals, like feldspar, silica, alumina, and yes, clay, something that comes from a long process of these basic minerals interacting with eachoter, we have a pretty good idea exists on every planet that we know of, in one way or another, makeing it a common fundamental building block, So I want to know and understand how it functions, just like any other material.

It's all really just material properties. It may seem as though I delve into every detail, every nuance of a subject; for me, and I hope you as well, it's quite simply that exploring how material properties work helps me find a fundamental understanding of how to make things, and combine experience and understanding, in an attempt that sometimes fails to communicate what I have learned to you folks.

If you didn't catch the last one, I have been working to create a casting slip from wild clay I found locally, processed using levigation to remove rocks, sand, and organic material, and dried; now I have a bone-dry, base raw material to begin testing and amending. Now It's time to see what all this work, all this effort, that I thoroughly enjoyed, to refine out of this piece of ground, can do, or in our case what we can make out of it, and how it might need to change, based on these tests.

Let's keep it simple to start. I test in 500 Gram batches in an old rock tumbler, where I incrementally add water, track viscosity and specific gravity, and watch for anything that might cause problems down the road, while keeping notes on the sodium silicate additions and how they affect the material's slip-casting properties. There are plenty of things to add that can improve plasticity, improving a short clay, and also modify bisque and glaze firing temperatures, and adjust the shrinkage and absorption properties, all to dial in something that hits a target firing temperature, in this case cone 6, and performs well as functional ware. Still, I'm trying to see what this raw material can do with only water and sodium silicate additions to get it to the ideal, or as close as possible to ideal casting slip consistency. After we fire these, we are going to test for shrinkage and absorption to gather more information about what to add to make a really excellent casting slip using only found materials. I hope. But that's in the next one.

In this batch, I started with 500g of raw clay and added 218g of water, about 45%. I added another 50g of water, and then another 25g of water, followed by two separate additions of sodium silicate, about 2.5g each, that I thinned with 5g of water each, and it really surprised me how thin it went, with a specific gravity of 1.59, not perfect, at least by the book, but let's give it a chance to do its thing before we start throwing books around.

I'll use this three-piece plaster mold to cast the test pots; it's a lot less stressful to unmold with these, and they are pretty simple to make, using 3d printed molds (molds for molds, not confusing at all, is it?) This one is a bit worn but still kicking, so let's see if we can make something of it.

One interesting property of casting slip is that it tends to gel when not continuously agitated, and there are many devices to do this, which vary widely in both cost and size. With my small-scale, small-batch testing, this old rock tumbler does the trick. I can, in just a bit of time, because the casting slip is ready to go, pour a mold, tracking the time and carefully watching the walls build up, adding more clay when necessary as the plaster absorbs the water, before I pour off the excess slip, let any leftover drip away, and then add a bit of clay to thicken the base, letting everything set up slowly, while covered, and in a few hours, unmold and start cleaning up the seams. I let the greenware go bone-dry and shape the rim and base, which are a bit ragged from the casting process- the downside to a three-piece mold- on my granite plate to leave crisp, clean, flat edges. These turned out fantastic, and I am really happy with this clay so far. The plasticity is better than I expected, and it doesn't seem short at all. I didn't have any cracking during drying, so the shrinkage rate from wet to bone-dry isn't bad either. I'm excited to get these fired, but the fire may tell another story. By the third, I really have the process down. I have a good understanding of how to cast with this material, what it will do, and a bit of what it won't do, but overall I'm really happy with it. I know what the timing should be, at least for these-sized shapes, and it's all repeatable, so I’ll need to plan a trip to bring home a bit more next time and tell the story of how I found it and how I harvest it as well, if that's something you would like to see.

I made three of these and varied only the casting time, keeping everything as similar as possible between them. The first was 20 min, the second 10 min, and I felt like I was both over and under, so I went to 12 min on the third. It seemed perfect, at least to me, but I will have to bisque-fire them and see if any problems arise. It takes a day or so to dry out the mold completely again. If I made them with thicker plaster walls, I could do more than one in a day, but I'm not in a hurry, and the short breaks that break up my day are a welcome change. Ceramics, at least functional ceramics, seem to me to be something that we as people have done, learned from, and passed down for a really long time; it's no surprise to me that it's satisfying to practice this functional art. Don't get me wrong, our modern lives are great; I'm not about to give up every convenince, but I do dream of a day when I can make the functional ware used in my home, something that in the past might have been a lot more common.

And there they are, three ready to fire. I will make sure they are completely dried out, and I will probably also go easy on them with the firing schedule, but more on that in the next one, which I'm sure you will want to see. If you are interested in a way to fire in a small space at very little cost, in an easy, quick method that I don't think I have seen in a video to this point, follow along; it should be out next week.