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Attaching Clay Pieces
Attaching Clay Pieces
Myth: When joining two leatherhard pieces of clay together or two pieces of clay together that are at different stages of dryness, simply using water or slip is enough to keep the two pieces together.
Reality: As seen in the example below, even lightly scratching (scoring) the pieces along with adding slip is NOT sufficient. Scratch lines should be 1-2 mm deep and have a liberal amount of slip added over the lines when joining the pieces of clay together.
Tip: When sticking 2 pieces of clay together after scratching and applying slip to each, apply pressure to the piece of clay you are attaching beginning at either
(1) the center of the clay piece and working your way to the edges, or
(2) one side and working one's way to the other side.
This helps to reduce the change of air pockets and helps make the join more secure. When attaching 2 pieces of clay together, do NOT apply pressure starting at the edges of a clay piece and then working your way to the center as this can cause an air bubble and, thus, an unsecure join.
Hollow Forms & Venting
Hollow Forms & Venting
Myth: Hollow projects explode when fired if they are not vented due to air being trapped inside.
Reality: Air on the inside of a hollow, unvented, form does NOT make a sculpture explode when it is fired. It is the water molecules trapped inside of the sculpture that can cause an explosion. If the water molecules have no easy way to escape the piece, they will try to emerge with great force when the piece is heated up, if the piece is heater too quickly. Generally, if the piece is not well vented, moisture can remain trapped inside of the piece, even if the exterior of the piece appears dry, thus, causing an explosion.
It is possible to fire a completely hollow form without any vent holes provided that the moisture has been sufficiently removed and the firing is done slowly.
Air Bubbles
Air Bubbles
Myth: Air bubbles in clay projects cause projects to explode in the kiln during firing.
Reality: On their own, air bubbles in a clay project do not cause explosions. It is the water molecules (steam) trapped inside of the sculpture that can expand within an air bubble and cause an explosion if the steam has no easy way to escape the piece and tries to emerge with great force when the piece is heated up too quickly. Projects with air bubbles and even completely hollow, enclosed forms can be fired successfully and safely if done slowly and the moisture has time to escape.
Why Stress Over Air Bubbles?
Stressing to students that air bubbles in the clay are "bad" can encourage students to properly wedge their clay which can result in the clay being at a more consistent moisture level throughout the piece as well as to promote better compression and alignment of the clay molecules, resulting in clay pieces that have greater structural strength.
If there is a tight timeline required to turn around clay projects and the teacher is not aware that there are air bubbles in the clay, the project can explode if they are fired too quickly as the steam becomes trapped in certain parts of the project. Thus, especially at the elementary school level, it is a good idea for the art teacher to always assume that their students' projects may have air bubbles, and to fire slowly, preferably even with candling session before the bisque firing.
Clay THickness
Clay THickness
Myth: Clay explodes because it is too thick.
Reality: While clay can and often does explode if it is too thick, it explodes due to the moisture left inside, NOT simply because of its thickness.
A project that is 3/8" thick will also explode in the kiln if it still has moisture in it and is heated up at a rate faster than the moisture can escape.
It is actually possible to fire solid bricks without an explosion if they are completely dry and fired slowly.
This sculpture, pictured at right, was several inches thick, and although it dried on a wire shelf for over a month and had one vent hole, moisture was still trapped inside in other interior parts. Sometimes, no matter how long a clay piece is sitting out to dry, the ambient moisture in the environment can be enough moisture to cause cracking and, sometimes, explosions, if the piece is heated up too quickly. As such, it is a good idea to candle a load of greenware before bisque firing and/or to ramp up the kiln slowly, especially if the pieces are thick.
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