Why did horizontal zinc retorts have high energy and labor costs?

Horizontal zinc retorts, such as the Belgian-type used in the 19th century, were characterized by high energy and labor costs primarily because they relied on small-scale batch operations.1

The process was physically demanding and inefficient for several reasons:

• Small-Scale Batch Processing: Each individual retort produced only about 40 kilograms (90 pounds) of zinc per day, requiring hundreds of retorts to be banked together and fired simultaneously.1
• Labor Intensity: The operation was described as physically arduous in the extreme, as the manual handling of numerous small units necessitated significant human labor to charge, fire, and empty the retorts.1
• Energy Inefficiency: Because the process relied on external heating of these many small, individual units, it lacked the efficiency of later, continuous methods like the vertical-retort process, which utilized high-conductivity materials and larger-scale production to improve output.1

Would you like to know more about how the vertical-retort process or modern electrolysis improved upon these limitations?