caption: Drain cleaner with 93 percent sulfuric acid, diluted with water to a 10 percent solution, can sharpen a file in about an hour. When the acid is poured into water it generates heat so I prefer to use glass instead of plastic containers that might get softened when warmed. I bought these glass flower vases at Goodwill for 69 cents apiece. I prefer the vases over glass baking dishes because I can suspend the files and keep the tangs away from the acid and dry for safe handling. The vases also give the acid less surface area to emit vapors and I can clearly see the acid at work. In a baking dish the bubbles would obscure the view through the surface of the liquid. I marked the acid vase with levels for the water (9) and the acid (10) so I could easily make solution. The other vase gets filled with water and baking soda is added until no more can be dissolved. The third vase, to far right is water for rinsing.
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