Files that are constantly in use soon become clogged and will not work
properly. The material that clogs them should be washed or dissolved
out. Sawdust can be got out by steeping in hot soap-suds. For iron
filings use a very dilute solution of sulphate of copper. For copper
use dilute nitric acid. For zinc dilute sulphuric acid. Use all acids
very weak, then wash well and dry thoroughly.