Dissolve 4 oz. of saleratus in 1 quart of water, and boil the files in it for half an hour; then remove, wash and dry them. Now have ready, in a glass or stoneware vessel, 1 quart of rain water, into which you have slowly added 4 oz. of best sulphuric acid, and keep the proportions for any amount used. Immerse the files in this preparation for from six to twelve hours, according to fineness or coarseness of the files; then remove, wash them clean, dry quickly, and put a little sweet oil on them to cover the surface. If the files are coarse, they will need to remain in about twelve hours, but for fine files six to eight23 hours is sufficient. This plan is applicable to blacksmiths', gunsmiths', tinners', coppersmiths' and machinists' files. Copper and tin workers will only require a short time to take the articles out of their files, as the soft metals with which they become filled are soon dissolved. Blacksmiths' and saw-mill files require full time. Files may be recut three times by this process. The liquid may be used at different times if required. Keep away from children, as it is poisonous.