What thickness is required for the walls of a firebox if no lining is provided?

The specified thickness for the walls of a firebox with no lining is determined by safety regulations to ensure adequate structural integrity and thermal performance. A thickness of 10 inches is required because it provides sufficient mass and insulation to contain the heat generated during combustion, helping to prevent structural failure or fire hazards to the surrounding materials.

Fireboxes are designed to withstand high temperatures, and the chosen thickness reflects the need to resist thermal degradation and to maintain the firebox's function over time. Adequate thickness also aids in the regulation of heat transfer, ensuring that the heat is confined within the firebox and reducing the risk of fire spreading to adjacent areas.

The other choices do not meet the required standards set forth in building codes, which emphasize the necessity of a minimum wall thickness to maintain safety and structural performance in firebox applications.