mill bar

A mill bar is an intermediate form of iron produced immediately after primary rolling or drawing from a bloom or puddled mass. It is not a finished product but a working stock, typically square or round in section, ranging from roughly 1 to 3 inches across depending on the rolling schedule. The bar retains surface scale and irregularities from the initial deformation process and is sent forward to merchant mills for further rolling, drawing, or shaping into finished merchant iron such as rods, flats, angles, or decorative sections.

Mill bars emerge from two distinct routes in ironmaking practice. In bloomery or forge work, a puddle bar, hot and still plastic, is passed through grooved rollers to form a rough bar. In larger mills, a bloom from a reverberatory furnace or mill furnace is roughed down through a succession of passes in the cogging mill to reduce it to manageable section and length. The result in both cases is a bar that can be handled with reasonable safety and is suitable for the next stage of manufacture.

Condition and handling

A freshly rolled mill bar carries heavy mill scale, a thick layer of black iron oxide that flakes off in layers as the bar cools. This scale is partly protective in storage but represents material loss during the next rolling phase. The bar's surface is marked by irregularities: tool marks, minor surface faults, and scale ridges that have no consequence for its use as intermediate stock but would be unacceptable in a finished merchant bar. Mill bars are typically cooled in pits or on floors and may be reheated before merchant rolling.

The mill bar occupies a specific niche in the iron supply chain, neither raw nor finished. A merchant roller expects to receive mill bars in bulk and knows they will be further processed: heated, rolled again, possibly drawn, and eventually cooled, straightened, and bundled for sale to blacksmiths, machinists, and fabricators. The term distinguishes this intermediate form from a bloomery product (which may be much less uniform) and from merchant bar (which has been finished to specification and graded for quality).

Because mill bars are temporary working stock rather than saleable product, their specifications are loose. Dimensional tolerance is measured in fractions of an inch rather than hundredths. Straightness is not required. The bar is simply expected to be sound iron, free of gross porosity or lamination, and of manageable size for the next operation. A cracked or severely faulted mill bar may be cut up and returned to the furnace rather than processed further.