Old kilns primarily worked by using a controlled fire to heat pottery or other materials, often utilizing a specific two-chamber design to separate the fire from the items being fired.
The Basic Principle
At its core, an old kiln was an insulated structure designed to contain high temperatures necessary for processes like baking pottery. The fundamental idea was to generate heat in one area and direct that heat to another area where the objects were placed, ensuring even heating and preventing direct contact with flames or fuel.
Structure and Operation
A common design involved two main compartments, as described in historical accounts:
The Combustion Chamber
- This was the area where the fire was built.
- It was often dug into the ground for stability and insulation.
- Its shape could vary, typically being oval, circular, or rectangular.
The Firing Chamber
- This was the upper compartment where the pottery or items to be fired were placed.
- It was positioned directly above the combustion chamber.
The Perforated Floor (Eschara)
- Separating the two compartments was a crucial element: a perforated floor, known as an eschara.
- This floor had holes or perforations.
- The eschara allowed the heat from the lower compartment to circulate within the firing chamber.
- This circulation of hot air effectively baked the pottery without direct contact with the flames or ash.
How Heat Transferred
- Fuel (like wood or charcoal) was burned in the lower combustion chamber.
- Hot air and gases rose from the fire.
- These hot gases passed through the perforations in the eschara.
- The heat then enveloped the pottery in the upper firing chamber, raising its temperature evenly to the desired level for firing.
This separation allowed for better temperature control and prevented damage to the items from the raw fire, enabling the creation of durable ceramics.
