Drying Pottery

21 Sep

Drying your newly made pottery pieces may seem like the least of your worries, but it’s not as uncomplicated as it looks. Not terrible, but there are some tips that will help you to get through this stage safely, and I’ll share some of them with you here. If any of you reading have additional insights, please feel free to share. Likewise, if you have questions, please post them. Your questions and insights will make this post more useful to everyone concerned, including me. 🙂

It is important not to dry your pieces too quickly, and the more complicated they become, the more slowly you’ll want to dry them. A cheap plastic painter’s drop cloth is your best friend here. Buy one and unroll it bit by bit, cutting off pieces of the width you need. Drape them loosely over the pieces you want to dry slowly. For pieces you’d like to hold at their current state of wetness, it’s good to wrap them in the plastic bags they give you at the grocery store–two or three of them–and then drape them with the plastic drop cloth.

Coil built and puzzled pots in particular need slow drying. Pieces with appendages such as handles or applied decorations also require slow drying, but not so slow as for the coil built pieces. Pots made all in one piece may be dried more quickly, but you should turn them upside down if possible as soon as they’ve hardened up a bit. If they can’t be turned, set them on a piece of drywall (sheetrock, gypsom board, wall board) or plaster to allow their bottoms to dry evenly.

If your pots are cracking as they dry or during bisque firing, they are drying either too quickly or too unevenly. Keep them out of drafts (cover with plastic if necessary) and away from heaters or air conditioners. If you still have problems with cracking, you might need to change your clay body.

Clays go through different stages during drying. Really, they pass along a continuum from soft plastic to bone dry, but there are a few stops along the way, which I’ll describe for you as well as explain what sorts of things might be done to a pot at that stage.

  1. Soft plastic: At this stage, the clay will hold its shape, but it is easily altered. You can join pieces, such as handles or knobs, at this stage, but the softness of the clay makes it difficult to do so without deforming the vessel walls. You can alter the vessel walls, stretch the clay, compress the clay–this is the time to set the shape for your pot. It will be more difficult or impossible later. Clay at this stage is sticky, and if you attempt to carve it, you’ll have a bit of a mess, with the pieces you’re trying to carve away sticking to your tool, each other, and the pot itself.
  2. Soft leather hard: The clay has begun to firm up, but it’s still a bit sticky and you can still alter the shape of the vessel. If you want to put scallops in the rim of your bowl or plate, this is a good time for that. The clay is firm enough to hold the shapes you’re creating, but you must work gently or you could cause cracks to form. If you have decorative or functional bits you’d like to add to the pot, you can do that now. The clay is still a little too soft to easily add handles, but it’s a perfect time to add little molded pieces for decoration–say pine cones or seashells. Be sure that you don’t trap any air under these appliques, or the expanding air during firing could blow the applique off the pot. If you have any thick parts (more than 1/2″ or so), poke some holes through them with a needle tool to help them dry and avoid later explosions in the kiln. You can heal these holes on the outside of the pot if you like, and just let them have an opening on the inside of the pot. It will be filled by glaze later. If you have any enclosed hollow spots, they’ll need holes poked in (one or two are plenty) to allow expanding heated air to exit. Be careful when you’re poking these holes, as the residual stickiness of the clay may fill them in as you’re removing your needle tool. If you try to carve the clay at this point, you’ll still have it sticking to your tools, though not as bad as before, and it will tend to form jagged, sticky edges on the parts from which it has been cut away.
  3. Leather hard: The form of the vessel can’t be altered without cracking the clay. At this point, you can add handles, etc. (Remember our earlier lesson, though–as always, scratch and moisten or add slip before attaching). You can still add decorative bits as described above. This is the optimal time for trimming the bases of wheel-thrown pottery (a process I’ll get into when I start to talk about throwing). This is also the best time to do any carving you’d like to do. Ribbons of clay will fall cleanly away from the pot, making little curls that sometimes still stick to the body of the pot. Leave these fragments. When the pot is dry, you can easily brush them away.
  4. Hard leather hard: The pot is just about to start losing it’s dark, wet color. If you carve away a little strip, it won’t hang together as at leather hard, but will fall into crumbles. Still doable, but not as nice as it was at leather hard, and it will dull your tools more. You can still add stuff to your pot if you absolutely must, but it’s more likely to crack off. At the hard leather hard stage, your pot has shrunk about 6.25%. The bits you’re adding on still have that amount to shrink, so you can see where this could cause some pressure. If you have to add stuff at this stage, be sure to moisten, scratch, slip well, and keep the pottery under plastic for a week or so before you allow the drying to complete. You can still trim on the wheel, though it will be more difficult and will dull your tools more.
  5. Bone dry: The pottery is room temperature to the touch, not cool. Coolness indicates continued evaporation and an incomplete drying process. It has lightened in color and feels absorbent–like old dried out bones. This pottery is past the stage of working with, unless you break it up and dump it back into the slop bucket and let it dissolve and return to the plastic clay state. You can sand it (wear a NIOSH approved mask), but that’s pretty much it. Bone dry clay in Florida isn’t the same as bone dry clay in South Dakota because the relative humidity in these two places is very different. Clay will always be as moist as the air around it until it’s been fired.

Take care of your bone dry pieces. They’re hard, but very brittle. Don’t hold cups by the handle or lids by the knob, and be careful not to bump delicate appendages. If you do break something, you can attempt to fix it by moistening both surfaces and trying to smooth the two pieces together. Sometimes it works. Better not to break it.

Next I’ll talk about your first firing. In the meantime . . .

God bless and happy potting,

Cindy

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: