Kiln Furniture

Kiln Furniture

The definition and function of kiln furniture

Kiln furniture, in simple terms, is a tool made of refractory clay that serves as a spacer, support, cushion, and protector for the blanks during the firing process. In the ceramic firing process, kiln furniture is used to support and protect the product, and is a special type of refractory product. From the functional point of view, it can be divided into support type, isolation type, etc. For example, kiln plates, shelf plates, kiln columns, etc. belong to support type kiln furniture. They bear the heavy responsibility of carrying the blanks or other kiln furniture and adjust the height of the multi-layer space in the kiln. Among them, the kiln plate is placed flat on the kiln car and is the most basic load-bearing plate on the kiln car. For example, a tunnel kiln for firing toilets may only have one layer of kiln plates; the shelf plate is similar to the kiln plate, but it is lighter and thinner and can carry more blanks. The sagger, as an isolation type kiln furniture, can be used to load the blanks into it, separating the flames and the atmosphere in the kiln, and effectively preventing the blanks from being contaminated or deformed.

Classification of kiln tools

Classification by function

Support: In addition to the kiln plates, shed plates, and kiln columns mentioned above, there are also kiln racks, etc. They cooperate with each other to build a support system for the green body in the kiln, ensuring that the green body maintains a stable position and shape during the firing process.

Isolation: Mainly saggers, different types of saggers are suitable for different ceramic firing needs. In addition, there are some auxiliary kiln tools, such as temperature cones, which can be used to measure the temperature in the kiln to help kiln workers master the firing temperature; kiln cars are important tools for carrying kiln tools and green bodies in the kiln.

Classification by material

Clay-clinker: It is composed of clay and clinker (such as waste saggers, waste clay, etc.), which can be used at around 1300℃, has certain cost advantages, and is more commonly used in some firing scenes that do not require particularly high temperature.

High aluminum: The aluminum oxide content is more than 46%, and it can be used in an environment less than 1450℃. It has good high temperature performance and is suitable for some ceramic firings that have certain requirements for temperature and material stability.

Mullite: The main crystal phase is mullite, which is composed of 10% kaolin and 90% synthetic mullite. The service temperature is 1400-1500℃, the strength is high, and the shape and structure can be well maintained during high-temperature firing.

Cordierite-Mullite: Cordierite and mullite are the main crystal phases, and the service temperature is below 1310℃. The composite preparation of the two materials forms microcracks at the interface of the two phases, which can effectively reduce the thermal stress caused by the rapid temperature change, thereby improving the thermal shock stability.

Silicon carbide and recrystallized silicon carbide: Silicon carbide kiln furniture has excellent thermal shock resistance, high strength, and a service temperature below 1400℃; the short-term service temperature of recrystallized silicon carbide can reach 1650℃. The long-term service temperature of the “silicon carbide combined with silicon nitride” refractory material formed when silicon carbide is used as a binder can be as high as 1550℃. This type of kiln furniture performs well in high-temperature firing and places with high thermal shock requirements.

Development of kiln tools

The spacer kiln tools were originally in the shape of balls, also known as “beads”, which were kneaded with white refractory clay containing more alumina. They were found in many dragon kiln sites in the Spring and Autumn Period and the Warring States Period in Shaoxing and Xiaoshan, and were often used for stacking small objects such as primitive porcelain bowls and plates. Due to its strong fire resistance, it rarely collapses and sticks, and some of the less deformed ones may be reused.

In the middle and late Eastern Han Dynasty, three-legged pancake-shaped spacer kiln tools appeared in celadon kilns in Shangyu and other places. They were made of ordinary refractory clay, but their melting temperature was close to that of the porcelain blank, and their feet were conical. During the high-temperature roasting stage, the toes were easy to sink into the blank, affecting the product quality. Therefore, they were replaced by toothed bowl-shaped spacer kiln tools in the late Three Kingdoms period. The toothed bowl-shaped spacer kiln tools are made of ordinary refractory clay, have more than six toothed feet, and the foot ends are rectangular. The pressure on the contact surface with the porcelain blank is much smaller than that of the three-legged pancake-shaped spacer kiln tools, which can prevent the foot ends from sinking into the blank and improve product quality. At the same time, a little high-temperature resistant white refractory clay is applied to the foot end during use, which not only prevents the kiln tools from sticking to the objects, but also facilitates reuse. However, because its own weight limits the number of stacking, it is no longer popular in Yue Kiln since the Eastern Jin Dynasty. The ball-shaped spacer kiln tools are widely used for stacking bowls and plates, and the volume becomes smaller and the number of uses increases. During the Tang and Five Dynasties, the volume of the spacer kiln became smaller and the shape became more diverse.

From these developments and changes, it can be seen that the evolution of kiln tools reflects the kiln workers’ concept of making full use of the kiln space, and the efforts to pursue output and product quality under certain conditions, which is driven by factors such as energy saving and improving production efficiency.

The importance of kiln tools

The quality and performance of kiln tools have a significant impact on the quality, output, energy consumption, qualified rate, grade rate, production cost and the economic benefits of the entire enterprise of fired ceramic products. High-quality kiln tools can withstand harsh environments such as high temperature and thermal shock, ensuring that the green body is not damaged or contaminated during the firing process, thereby improving the qualified rate and grade rate of the product; at the same time, reasonable design and selection of kiln tools can also improve the space utilization rate of the kiln, increase production, and reduce energy consumption and production costs. For example, in modern industrial production, the production of lithium battery positive electrode materials requires a large number of high-performance kiln tools to meet the special needs of high-temperature sintering.

With the continuous advancement of science and technology and the development of industries such as ceramics, kiln tools are also constantly innovating and improving. In the future, it is expected that kiln tools with better performance, more energy-saving and environmentally friendly will appear, and continue to contribute to the development of industries such as ceramics.