【Japan Classic】Tunnel Kilns (トンネル窯) & Roller Hearth Kilns (ローラーハース窯): Revolutionizing Ceramic Mass Production
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Tunnel Kiln: Modern
The tunnel kiln (トンネル窯) was introduced in Japan around 1920. It has since been replaced by roller hearth kilns and is now somewhat in decline, though further details will be discussed later.
The tunnel kiln’s highly productive design enabled mass production. Products are loaded onto trolleys and moved into the kiln. Its structure allows for the integrated management of the “preheating,” “firing,” and “cooling” processes from the entrance to the exit.
Gas is used as the fuel. Burners expel hot air from one direction, and the hot air drawn toward the opposite wall is returned to its original direction. The flames in the preheating and firing zones flow continuously, as illustrated in the diagram.
Products entering the kiln are first gradually heated in the preheating zone. They are then fired at high temperatures in the firing zone before moving into the cooling zone. Since the firing zone reaches around 1,200℃, the smoke is vented.
In the cooling zone, outside air is drawn in to gradually cool the products. As the heated products are cooled, the originally cool air becomes warmed. This warm air is then recycled back to the preheating zone, thereby operating the kiln efficiently.
In this manner, the kiln is divided into separate sections for “preheating,” “firing,” and “cooling.” In practice, it is subdivided into more specific temperature zones, but these three processes broadly represent its primary functions.
Moreover, because the flame in a tunnel kiln flows in one direction, it does not behave like that in a climbing kiln where the flame strikes the ceiling and then falls (倒炎:とうえん). In other words, there is minimal flame fluctuation or stagnation, which helps maintain a consistent firing environment.
As a result, the kiln produces uniformly fired products with minimal variation, and by recycling the hot air from the cooling zone, fuel is conserved. While there may be limitations on the kiln’s height, a longer tunnel allows for the production of a larger number of products.
Now, despite the tunnel kiln’s exceptional productivity and fuel efficiency, there is one notable drawback: the use of trolleys for loading products.
Because products are loaded onto trolleys, they may not receive heat as effectively, meaning that thermal efficiency can be reduced. Furthermore, stacking products on trolleys requires extra labor to remove finished goods and load new items for firing. This issue led to the development of roller hearth kilns in the 1970s.
Roller Hearth Kiln (ローラーハース窯): Modern
While tunnel kilns use trolleys, roller hearth kilns move products on motorized rollers. The term "hearth" refers to the floor inside the kiln. As the name suggests, this kiln is characterized by a roller-driven floor.
The basic structure for "preheating," "firing," and "cooling" within the kiln is the same as that of tunnel kilns. Both types of kilns also utilize a number of kiln tools, such as protective containers (匣鉢 (さや)) for the products and shelves to display them three-dimensionally.
However, because roller hearth kilns do not use trolleys, one large piece of kiln equipment is eliminated. This allows the heat from the flames to be transmitted more efficiently. With this improvement in thermal efficiency, it is possible to fire one product in about three hours.
Furthermore, since there is less stacking of many products as with trolleys, the finished products can simply be removed from the roller-type floor, and new items for firing can be fed onto the rollers. Consequently, the labor required for loading and unloading is reduced.
Roller hearth kilns began to be introduced in the 1970s, and today the entire production line has been streamlined—with automated temperature settings for each firing zone and roller speeds. They operate in mass production environments as kilns that offer a uniform firing environment and reduced processing time.
Production volume depends on the kiln’s scale, but it is said that tens of thousands of pieces of tableware (such as rice bowls and plates) can be produced in a single day.
Of course, while these kilns do not prioritize the individuality of each piece, tunnel kilns and roller hearth kilns have achieved astonishing productivity with uniform quality. They represent a significant milestone in the mass production of ceramics.
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