Shingling was a stage in the production of bar iron or steel, in the finery and puddling processes. As with many ironmaking terms, this is derived from the French - cinglage.
The product of the finery was a bloom or loop (from old Frankish luppa or lopp, meaning a shapeless mass); that of the puddling furnace was a puddled ball. In each case, this needed to be consolidated by hammering it into a more regular shape. This was done manually with heavy hammers; later by a waterwheel or steam powered hammers, leading to modern power hammers. The result was an oblong-shaped iron product similar in appearance to shingles used on roofs. In the finery, this was part of the work of the finer; during puddling, it was done by a special workman called the shingler. The iron (or steel) then had to be further shaped (drawn out) under the hammer or rolled in a rolling mill to produce a bar. In more recent times, the process was carried out using mechanical jaws to squeeze the puddled ball into shape.