When knives are being made, iron forging often leaves a spherical or flaky byproduct known as forge scale on the blade. It typically forms when hot iron rapidly oxidizes in the area: when iron is heated, it develops an outer layer of iron oxide, which may separate from the original piece after being hammered or subjected to differential thermal contraction.
Additional forge scale is produced from the heating and hammering required to shape the piece. Due to its high iron oxide content, the scale produced at this stage tends to be slimmer and darker in color.
If you make knives using the forge method, you’ll want to remove these scales, which can impact the aesthetic quality of the finished blade. In this blog, the team at Red Label Abrasives goes over some commonly-used methods for dealing with these imperfections.
A vinegar soak will remove scale and speed up the finishing process. Soak the knife overnight in household vinegar -- the entire workpiece needs to be immersed or it will etch where air meets vinegar. Use a baking soda bath to neutralize, then dry. The surface should now be ready for sanding.
Although they work well, vinegar soaks take time. If you’re on a tight schedule, angle grinding will get rid of the scale faster.
Hand-sanding tools like sandpaper and files can be damaged by forge scale. With an angle grinder, you can use a flap disc to remove scale and decarburized steel from the blade. This leaves the blade ready for further shaping and finishing.
Start with a coarse abrasive such as P36 or P40. Whenever you are machining through scale, you need to cut deep enough to get under it. The tips of your cutter should be cutting steel, not scale. Scale is hard, but brittle and not very well bonded to steel. Taking away the steel beneath it will cause it to shatter and fall off.
To minimize scale buildup, brush the workpiece frequently during forging. When it's out of the forge, go over it using an angle grinder with a knotted wire cup brush to remove scale while maintaining the forged finish if that’s the visual outcome you’re looking for.
Other tips include: