What: Candling is a process of running the kiln at a low temperature to remove water from greenware pieces before a bisque fire. Be sure to leave the plugs out of the peep holes so that the moisture can escape. Also, leave the kiln vent turned on at all times during the candling, similarly to as you should during a bisque or glaze firing.
Candling Temperature: Candling is done at temperatures between 180 and 200 degrees Fahrenheit, with the ideal candling temperature being 180 degrees Fahrenheit.
Critical Temperature: The candling temperature should be low enough to provide a buffer below the boiling point of water (212 degrees Fahrenheit), as at the boiling point of water, a clay piece with moisture left inside can explode.
Why:
Candling can help remove excess moisture from clay projects before bisque firing. 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 during the bisque firing. 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.
Why Not Just Place Clay Projects on Top of the Kiln While Firing Another Load?
This method is much safer for the kiln, clay projects, & people in the surrounding area than placing moist projects on top of a running kiln as this can actually cause the clay pieces to crack or even violently explode. Also, having pieces on top of the kiln can cause damage to the lid of the kiln as it is made of fragile firebrick.
How to Know if the Moisture has been Removed: To check to see if the moisture has been removed from the greenware pieces, you can hold up a mirror over one of the peep holes. If the mirror fogs up or becomes wet, there is still moisture left in the pieces and candling should continue.