On older boilers, the masonry work that surrounded the furnace was termed:

The correct term for the masonry work that surrounded the furnace in older boilers is known as the boiler setting. This part of the boiler design is essential as it serves to support the furnace structure and provide a safe and stable environment for combustion. The setting is made from materials that can withstand high temperatures and effectively contain the heat produced during the burning of fuel, ensuring that the furnace operates efficiently while also maintaining safety standards.

The function of the boiler setting is not only structural; it also plays a role in insulating the furnace, thereby minimizing heat losses to the surrounding environment. Properly designed settings help in directing the flow of gases and optimizing the combustion process.

In contrast, terms like firebox, masonry, and casing refer to different components or concepts within the broader context of boiler design. The firebox typically refers to the combustion chamber of the boiler where fuel is burned. Masonry generally refers to the materials (like bricks or concrete) used in construction but does not specifically define the entire assembly related to the furnace. Boiler casing usually pertains to the outer shell or enclosure of the boiler that protects inner components and minimizes heat loss. Hence, the specific role of the boiler setting makes it the most accurate answer in this context.