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The 5 Stages of Finishing Clay

Updated: Jun 12, 2021

Many people who come to my class have the same question. How long does it take for my piece to dry and when will it be dry enough for me to pick it up?


To get a finished piece from rough wet clay you must both dry it fully and vitrify it in a very hot oven called a kiln.

1. Drying

The first step after a piece is formed is to be sure that it has enough time to dry fully to a stage of clay called bone-dry. This means that the piece must sit undisturbed for 3-7 days until all of the moisture has evaporated.


2. Candling

Once the piece is finished drying we place them in the oven (kiln) and prepare it by heating it up just under the boiling point of water 100C (212F) to be absolutely sure that there is no water left inside the piece, this process is called candling. This is extremely important to ensure that the piece does not explode or crack due to moisture inside the oven. Some potters even practice this candling procedure overnight.


High-fire clay

We use a high-fire clay in class which is basically clay that requires a final glaze firing temperature above cone 10 to turn into ceramic, which is 1300C (2372F).


3. Bisque Firing

So after the candling process is complete the oven is heated to a temperature called cone 06 which is 1000C (1832F). This is the first of two times that it will enter the kiln. This process takes around 10-12 hours if done properly. During the bisque firing, the organic compounds begin to burn off, the water that is chemically bonded to the clay has an opportunity to evaporate, calcium carbonate decomposes, and finally sintering occurs when sulfur in the clay decomposes. During this process the piece shrinks as the state of the material changes.


4. Glazing

Now your piece is what we call ‘bisque’ which is a very porous and partially vitrified stage of clay that is ideal for glaze to adhere to. During this stage, your piece is either dipped, sprayed, splattered, poured over or brushed with a mixture of minerals that we call glaze (look out for my next post explaining why we don’t ‘paint’ our pieces). Inside this glaze compound is silica, which is glass, among many other minerals. We want to be sure that none of this glaze is on a surface of the piece that will come into contact with the shelf inside the kiln. If the glaze is on the bottom it will fuse to the shelf and well likely have to smash it off with a hammer and break the piece and damage the kiln shelf in the process which is costly and emotional. So after the piece is covered in the glaze mixture and the bottom is wiped clean we put it into the kiln for the glaze firing.


5. Glaze Firing

During the glaze firing we heat the kiln to a temperate called cone 10 or 1300C (2372F). This process takes around 24 hours generally. 12 hours to reach the high temperature and 12 hours to cool back down. During this time the mineral makeup of the glaze matures and fused to the clay underneath which becomes fully vitrified. This is when the clay fully turns from clay into ceramic.


Conclusion

So it is by this process that this material forms into a useable semi-permanent mass. After we vitrify clay, it is impossible to return it back to its original state of clay.


Fun Fact: Vitrification is a process that happens inside of a volcano!


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