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Clay tips

Good pot, bad pot

Here’s a way to test bisque (the first firing, when its white) for hidden cracks that can become bigger or even BREAK your pot.

Trouble shooting bisque

Preventing cracks and EXPLOSIONS!

After spending hours, sometimes days on a piece, there is nothing worse than having it come out of the kiln cracked or worse yet exploded into a million pieces which are stuck all over everyone else’s master pieces!
This page looks at problems related to casting and drying.
How you cast or dry your piece is the most common reason for cracking and warping.

Hanging out to dry:
Ceramics contain clay which can absorb and hold water.
Before firing, it is important to remove all of the water so that the piece will not crack or explode when heated.
This is often accomplished in steps with firing being the last stage.

Understanding Drying/ evaporation

When you first make…say a doll head, the clay particles are far apart with lots of water between them. That’s why you can push it around and shape it into stuff.
As your doll head is drying, the water doesn’t want to stay there all trapped between the clay particles. Water wants to be free! So it chances its form and evaporates into thin air! Good trick, huh? Don’t you wish you could do that?
So, with the water leaving the particles settle down and huddle closer together. (that’s why there is shrinkage). Your doll head stops shrinking when all the water has left and the particles are touching each other.

Drying Tips
Sometimes you don’t even know your doll head is ruined till it comes out of the kiln and has cracks or is warped.
They are usually caused by drying too fast or unevenly. If the piece is not completely dried and is heated too fast, the pressure from water vapor inside the piece can cause cracking.

  • Warping can be caused by uneven drying.  If one side is drier than the other.
  • When one surface finishes drying, the piece is now too stiff to recover and the warping becomes permanent and this can lead to cracking.
  • Pottery and ceramics made of very plastic clays or having a high clay content need to be dried uniformly and slowly.
  • Thicker walled pieces will often have a greater tendency to warp or distort because the outside usually dries before the inside does.
  • Laying the piece on its side and using a fan on low will help the piece to dry more evenly.
    Care needs to be taken to allow for uniform air movement around all sides of a piece to avoid drying problems.
  • Sometimes drying must be slowed down to avoid cracking by loosely draping a piece of plastic over the piece, or placing the piece in a cooler room.
  • Handles on cups can have a tendency to pull away from the mug.
  • Doll heads and chest cavities may cave inward.

Reduce Warping and Cracking

  •  Dry slow and even  from all sides.
  • Don’t dry a flat object on a wet or cool surface like a formica, plastic table tops or damp newspaper. If you do, the piece can only dry on one side.
  • Dry objects on something porous like wood or plaster and set them so air can circulate around them.
  • It helps to turn the pieces over during drying for a more even result also.
  • Slow the drying of thick walled pieces and hand built wares.
  • Support areas during drying that might cause stresses to build up, such as a part of the piece that sticks out past the main body of the piece.
  • Slip cast  ceramic pieces, may warp or crack if pulled out of shape when removed from the mold.Even if the piece is gently returned to the original shape, the created stress will ultimately cause the piece to warp or crack.
  • Wheel thrown pieces should not distort during drying unless they are subjected to more shaping once they have started drying, let the piece dry naturally on a bat or shelf and it should be fine.
  • Thick hand built pieces need to be dried for a very long time before they can be fired or it may explode during firing.
  • Several days may be required or a low heat drying in an oven may be necessary to remove all the water.
  • Pieces should not be dried in a kiln, the moisture can damage the kiln.
  • Even drying is particularly important with plates.
  • Warping can cause the center of plate to fall or arch up.
  • Rims and centers must dry evenly to prevent warps, humps and cracks.
  • Drying tiles can present a particular challenge because it can be difficult for the piece to dry evenly.
  • Usually air is passed over the top of the tile.
  • This results in warping because the bottom of the tile remains wet.
  • Drying tiles in tile racks or on screen shelves can help air movement for a more even drying.
  • Don’t be in a rush when drying your pottery or ceramics, a good way to check to see if the piece is dry, is to touch parts of it to your wrist, if it feels cold it is still wet and if it feels warm it is dry.
  • Remember haste makes waste and one wet piece in a kiln load can make junk out of the whole load if it blows up!!!

Stages of Clay
Stage 1-Fresh Clay Stage –  
in the beginning you can add and form clay,

What you can do in this stage:

  • throwing on the wheel
  • hand building
  • molding elements

Things to Remember:

  • pay attention to the space inside of your doll head  –  you are shaping the empty space first, and will make the walls around it match. Keep the walls an even thickness.
Stage 2-  Soft Clay 

Still can make changes and build on to your doll.

What you can still do:

  • changing the doll’s shape
  • adding onto the doll
  • other additions (fingers, noses)
  • texturing surface

Things to Remember:

  • slip and score all joints
  • compress joints with a metal rib or wooden tool
  • perform any bending of the walls or altering of curves

 Stage #3- Hard clay, feels like hard cheese

  • trimming and refining your doll, now is the stage where you can’t add clay to it anymore, only cut off or scrape off clay.

    Processes Supported:

    • trimming
    • rasping away areas of doll
    • cutting away clay
    • carving patterns

    Things to Remember:

    • basic form should not be altered
    • perform subtractive processes to lighten form or add aesthetic elements
      STAGE#4: Hard but not completely dry (feels like stale cheese)

    Only do  dry shaping

    Processes Supported:

    • clean surface up
    • lighten form further
    • soften edges
    • trimming or scraping with rib
    Things to Remember:
    • just before the piece is bone dry, it responds very well to having its surface scraped or trimmed
    • if the work has become bone dry, you can sponge it down to do some of these processes

    STAGE#5: Bone dry

     it feels warm to touch
    What you can do:

    • sponging
    • some light carving

    Things to Remember:

    • sponging the form down reduces sanding, erases unwanted marks, and softens edges
    • don’t add too much water!