Iron Making: Making Flats and Nail Rod

NPS Photo / Will Whearty

Merchant bars were further worked to create other semi-finished products that blacksmiths could use. Contrary to the blast furnace and forge, little was recorded about the archeological foundations of the rolling and slitting mill in the early 1950s. Much of what is known about the rolling and slitting mill is based upon inventories and accounts of the original iron works and 17th and 18th-century engravings of similar machinery.

While the blast furnace and forge were wonders of chemical and metallurgical engineering, the rolling and slitting mill machines represent a relatively new application of precision in mechanical engineering. Sometime in the 1580s the use of gears (similar to what might be seen in grist mills or saw mills) was applied to rolling mills for the purposes of flattening iron.

The rolling mill consisted of a pair of cast iron rollers supported in a heavy-duty wrought iron framework. The machine was linked to waterwheels with iron couplings. The top and bottom rollers turned in opposite directions so that the bar iron could be pulled into the machine.

Wrought iron merchant bars were preheated in a cord-wood fired reverberatory furnace to bring the iron to a red/orange heat. When the iron was malleable, it was fed into the rollers. The torque of the waterwheels on the rollers created a high pressure and flattened the iron bars. There was likely a mechanism for adjusting the distance between the rollers so that flats of varying thicknesses could be made. Flat bar was shipped out so that blacksmiths would have the wrought iron stock to make wagon tires, axes, saw blades, and hinges.

Some flat bar might also be processed through the slitting machinery. Archeological finds provide evidence that the slitting mill made ¼" X ¼" iron rod for the purpose of making nails. The slitter machinery was made up of two square iron bars with cylindrical bearings. In the case of the ¼" slitters, ¼"thick steel [?] cutting disks and ¼" thick iron spacer disks were alternately stacked upon the square shaft and bolted together. A similar but interlocking set of cutters and spacers was assembled upon the other square shaft. These were also coupled to waterwheels and turned in opposite directions. Water was fed over the slitters to keep the precision cutters cool and properly heat treated. Iron flats were heated to a red/orange heat and fed into the slitters. The flat bars were pulled through the slitters and sliced lengthwise. Thus, a quarter inch thick flat bar passing through quarter inch slitters produced ¼" X ¼" slit rod.

It is possible that slitters may have been larger sized like 1" thick. A 1/4" thick flat that passed through 1" thick cutters would produce ¼" X 1" slit flats that could be useful for making horse or ox shoes.

Both flats and nail rods were semi-finished dimensional iron that helped a blacksmith save a lot of time. In a previous time, flat bar and slit bar would have been pounded into dimension by using a series of water-powered hammers in a "battery" or perhaps more commonly, using hand hammers.