1. Clay, Quartz, and Feldspar: The Materials Used to Make Ceramic Plates
The soil used for ceramic plates is not just clay. It is a mixture of clay and powders made from quartz and feldspar. According to materials, clay is essential for holding the shape, and at least 30% is needed. [1]
Feldspar is said to melt slowly over a wide range of temperatures when heated, creating a glass-like substance that helps the shape stay firm. [1] Because the mixing ratios differ slightly depending on the source, it is best to just remember that "clay is the main player." [1]
In Mashiko, Tochigi Prefecture, soil from the mountains around the Otsusawa area is introduced as a local resource. This soil is sandy and has strong stickiness. It is dried, crushed, and then put in water to remove impurities in a process called "suibin" (water sieving) before being kneaded by hand. [2]
2. Kneading the Soil to Remove Air Before Shaping
Before shaping, the soil is kneaded to remove air. This is called "datsu-ki" (degassing). It can be done with a vacuum machine or by hand. Ceramic explanation sites state that if air remains, it can cause cracks from the inside during firing. [3] It is good to think of it as the item breaking from the inside due to air or steam. [3]
Methods for shaping include the potter's wheel, hand-building, casting into molds, and using slab clay. [3] In all methods, it is said to be important to make the thickness even. [3]
Thick parts hold more moisture. They shrink more when drying, and if moisture remains during firing, it can cause damage. [3] This is similar to how a wet towel dries unevenly, with thick parts taking longer to dry. [3]
3. Why Ceramics Must Dry Slowly and Evenly to Avoid Cracks
Once shaped, the piece is dried. The ceramics course at Kyoto University of the Arts explains that it is important to dry it slowly and naturally, arranging the placement so that it dries evenly throughout. [6]
If it is fired before it is dry enough, the moisture left inside turns into steam at high temperatures. This can cause the piece to crack or warp. [3] The time this takes varies depending on the size and thickness. [3]
4. Bisque Firing: Why a Shaped Piece Is Fired Once Before Glazing
It is said that soil that has been shaped and dried may still contain about 20% moisture. Bisque firing is a step where this is fired down to a few percent to reduce damage during the final firing. [4]
The temperature is said to be around 600 to 800°C. The Kyushu Ceramic Museum states it is about 800°C. [5] When bisque fired, small holes form in the vessel, making it easier to absorb the glaze applied later. The vessel also becomes moderately hard and easier to handle. [5]
Another reason is shrinkage. A ceramics terminology site gives an example: even if soil shrinks 20% from shaping to final firing, if it shrinks 8% during bisque firing, the shrinkage in the final firing only needs to be 12%. [4] This is a numerical example; shrinkage varies by shape and soil. It is not the same value for every soil. [4]
5. Glaze: How It Is Applied and How It Becomes a Glass Coating
Glaze is applied to the bisque-fired vessel. There are various methods, such as dipping, pouring, or spraying. [7] The Kyushu Ceramic Museum explains that glaze becomes like glass when fired. [5]
After applying the glaze, the vessel is dried one more time before entering the final firing. [3] The mechanisms of patterns and colors are handled in a separate article. [3]
6. Final Firing at About 1200°C: How the Plate Becomes Hard
The final firing for earthenware (tōki) is approximately 1200 to 1300°C. The Kyushu Ceramic Museum explains that when fired, it becomes hard and tightly fired, with the surface becoming like glass. [5] Some sources set the final firing for earthenware at 1100 to 1200°C, so it is good to think of it as "around 1200°C." [5]
In the climbing kilns of Mashiko, it is introduced that they fire with firewood for 48 to 72 hours. [2] Earthenware has many small holes in the body, and the water absorption rate can be close to 10%. [8] Considering these holes makes it easier to understand why the glaze coating acts to block water. However, this point is an interpretation connecting the wording of the sources. [8]
Porcelain (jiki) is fired at higher temperatures than earthenware, approximately 1250 to 1350°C, and its water absorption rate is said to be nearly 0%. [8] With drying, bisque firing, glazing, and final firing, the vessel is fired in a kiln twice. It is labor-intensive work. [3]
7. Ideas for Investigating Ceramic Making Further
You might try investigating or testing the following things. [5]
Turn over a ceramic plate or tea bowl at home and compare the part without glaze to the smooth, glazed part. Handle it gently on a table, as it is easy to break. [5]
Read the "How to Make Ceramics" pages on public facility sites, such as the Kyushu Ceramic Museum, and look for differences from this article. [5]
If there is a kiln or ceramics class nearby, it might be good to ask if they can show you bisque firing and final firing on separate days. [5]