Sort and Dry Your Clay Scraps
As you work, you are likely to have a number of scrap pieces of clay accumulate. This is true of hand building, and even truer of throwing. In hand building, if the scraps haven’t dried out too much you can re-work them without having to do much more than compress them back together and work the air out. If you are throwing, your scraps are likely to be quite wet and will include slurry.
Don’t throw these scraps away; you can recycle them back into usable clay again.
The first thing to make note of is that like should go with like. This is especially important in regards to the clay bodies’ maturation range. Other similarities you may want to sort by including working characteristics (throwing clays vs. hand-building clays) and color.
White clay bodies, if you want them to remain white, should only be mixed with other white clay body scraps of the same temperature range. You will also want to make certain that the bucket, towels, working surfaces, and tools used are all clean and free of other clay bodies or ceramic colorants.
When your scrap bucket is halfway full, discontinue putting new scrap into it (switch to a new bucket for new scraps). Allow the scrap clay to dry completely, which may take several days to over a week. Make sure that any large pieces are broken into bits. Smaller pieces will slake down faster and more thoroughly than large chunks.
Slake Your Scraps
Once the scraps are thoroughly dry, fill the bucket with water, covering the clay by several inches. If the clay absorbs too much of the water and re-emerges, add more water to cover the clay completely.
The clay scraps will slake quickly, giving you a bucket of slurry. Let the now-slaked clay settle, at least for several hours or even a couple of days. If you have a layer of water on top, gently pour as much liquid out as possible. We suggest doing this outside; it's not as potentially hard on your plumbing.
If you have one available, transfer the slurry into as wide a container as possible for this step. The point is to give the slurry as much surface area as possible for evaporation to remove water from the clay. Something like a big flat plastic storage tub will work well. Otherwise, continue in the same bucket.
Place the container somewhere where it won't get knocked or spilled, and let it rest for several days. Check on the tub every day or so, stirring every day in dry climates, or every other day or so in humid ones. The clay will slowly dry and thicken to a sludge-like consistency, becoming thick but not quite solid.
Dry the Clay to a Working Consistency
At this point, the trick is to get the clay to dry out, but not dry it too much.
Optimally, if you are going to do this a lot, you will want to make a big plaster bat, a large plaster surface that you could work the clay on. If this is a one-shot deal, you can use the same idea without the plaster bat. Find some towels that you don't care about saving; you can hose out the clay and wash them clean again afterward, but the clay may irreversibly stain the material, especially if you are working with red clay.
Working outside, dump the clay onto the plaster bat or the spread out towels. Spread the clay thinly, about 2 to 3 inches thick. Let the clay sit for a time, checking on it periodically. In hot, dry weather, this step may only take an hour or so.
When the clay can be formed into a ball without sticking to your fingers, begin gently scraping it off the plaster or rolling it up off the cloth. Putty knives work well for this, especially the cheap plastic kind. If you used plaster, be certain no plaster particles get into the clay.
The clay will probably still be soft; work it in your hands or on a piece of canvas to dry it out further. If you are planning to hand build, just work the clay a bit to compress it and remove any trapped air. If you are planning on throwing the clay on a potter's wheel, then you'll need to wedge it, to ensure that all air is removed and that the clay is homogeneous.
Store Your Clay
After the clay has dried to the right consistency and compressed or wedged, it is ready to use. If you don't want to use it right away, store it in heavy-duty plastic bags. Freezer bags are good for small quantities.
Note: Most plastic is not truly airtight; air does move through the plastic, just slowly. Some plastics are much more air permeable than others.
If it will be several days before you use the clay, place the plastic bags of clay into an airtight container, such as a plastic tub with a tightly fitting lid. Clay can be kept in a workable state indefinitely if it is kept in an air-tight container with little to no air in the container with it. Clay is not like food in that it can't "spoil;" it's already decomposed—decomposed rock. What it can do is dry out.