What is the most effective tool for cleaning files?

The most effective tool for cleaning files is the file card. A file card is specifically designed to clean the grooves of metal files, effectively removing debris and metal shavings that accumulate during use. Its bristle structure allows it to reach into the file's serrated surfaces, ensuring thorough cleaning without damaging the file itself.

Using alternative tools such as a knife is inappropriate because it may harm the file's cutting edges by altering their shape or sharpness. Similarly, a scriber, which is used for marking, does not possess the cleaning properties necessary for maintaining files and could potentially scratch the file. Coal, while sometimes used in other contexts, is not suitable for cleaning metal files and has no effect on removing the buildup of materials from the file surface. Therefore, the file card is the optimal choice for maintaining file cleanliness and functionality.