forestone
A piece of cast-iron which lies across the hearth with its ends resting between the keystones, and which can be moved toward the front or back of the hearth as required. It is a part of the small rectangular furnace called the "ore-hearth," used in the smelting of lead.
forestone: movable hearth bar in lead smelting
A forestone is a cast-iron bar that spans the width of an ore-hearth in lead smelting, resting on the keystones at both ends. Its primary function is to contain and direct the molten lead and slag within the small rectangular furnace, and it can be shifted forward or backward to adjust the effective size and working depth of the hearth as the smelt progresses.
The ore-hearth itself is a compact rectangular furnace, typically 3 to 4 feet wide, designed for lead reduction from galena and other lead ores. The forestone sits just behind the front opening or tuyere area, and as ore is charged and material melts, the operator moves it incrementally backward to maintain proper working conditions. This adjustment prevents bridging of unmelted charge, controls where matte settles, and helps manage the slag blanket that floats above the molten metal.
Foretstones are always cast iron because the material must withstand direct contact with molten lead (melting point 327°C) and the corrosive action of flux and slag. Cast iron resists penetration better than wrought iron in this service. The bar eventually erodes and must be replaced after weeks or months of operation, depending on ore composition and smelting intensity. A worn or sagging forestone creates voids and poor thermal distribution, leading to incomplete reduction and excessive fuel consumption.
The term reflects its position: it stands at the fore, or front working area, of the hearth proper, distinguishing it from the back stone or rear elements of the furnace. Foretstones are specific to lead smelting operations; copper and iron furnaces use different internal geometry and dam systems. The keystone mounting is critical because it allows vertical movement without requiring the entire furnace structure to be altered when the bar is withdrawn for replacement or cleaning.