#810: The Shochet and his Knife
The Gemara mentions several methods of bedikas hasakin (checking a knife [used for shechitah—ritual slaughter) for smoothness to ascertain that it is free of any pegimos (nicks) that would invalidate it from use. (A shechitah knife must also be extremely sharp, for which there are other criteria not discussed in this halachah.)
Among the ways in which a knife can be examined: It is inspected over the water's surface; the shochet (ritual slaughterer) tests the edges with his fingernail and finger; and it is well-scrutinized b’shimsha (in the sunlight).
The halachah is that the knife must be checked using both the fingernail and finger. Nonetheless, the common practice today is for the shochet to run only his fingernail across the edge of the knife. (Poskim address why checking with a fingernail has become the only accepted method, which is a separate discussion.)
Poskim recommend, however, that an additional bedikah b'mar'is ayin (a visual check) similar to the original bedikah b’shimsha be employed as a hiddur (extra measure). This is accomplished by checking the knife under a very strong light or in bright sunshine to ascertain that the entire edge is smooth and of a consistent hue, since a defect in the chud (sharp edge) of the knife will often show as a whitish discoloration. However, the necessary method remains a physical check via a fingernail feel and the visual light technique is merely recommended in addition.
The visual check process was often utilized in previous generations. The practical significance of a visual check can be great, since even slight discoloration on a knife that is still smooth-feeling will alert the shochet that it will likely develop pegimos within a short time.