[ATM] Tenacious C oating

[Anthony Stillman <atmer@flash.net>]

 Not too long ago, in preparation for refiguring, I began stripping the magnesium fluoride coating from a flint lens.  I considered polishing off the tough pale blue fluoride but opted for etching instead.  Having never removed this sort of coating, I search my books and other literature, I searched the net, and I searched my head for how, all with limited success.     
  Undaunted  by ignorance I began with dilute room temperature sulfuric acid.  The lens sat in a covered pyrex casserole pan overnight.  No effect.  I upped the concentration and tried again.  Still no effect.  I left it in concentrated acid (96%) for a couple of days.  There might have been some tiny change.  The blue seemed bluer, more intense.  Years ago I was told the proper way to strip mag fluoride.  Immerse the element in hot concentrated sulfuric acid for several hours.  I was hoping to avoid dealing with hot concentrated sulfuric acid.  I searched the web again.  This time I found a bit of literature that, though it called for a pinch of boric acid too,  confirmed what I remembered.   I fretted some, thought it through, set it up, placed the lens in the solution, turned the hot plate to 50C, and waited. The boric acid dissolved completely only after the solution was hot.  A few hours later I pulled the plug and let it cool down overnight.  There was a slight but
 noticeable effect.  Toward the center of the lens the blue film was very pale.   Perhaps nearly gone, but the prominent sliver of silvery translucent blue all along the edge remained unchanged.  I repeated the process at 80C. The following day the coating looked clear at the center and faint blue elsewhere.  More so on the top face.  I'd put a small plano-convex lens off to one side in the casserole pan just under the edge of the flint.  It propped it up and allowed for a little circulation.
    
  In need of the space, I poured the three quarters of a liter of acid into a 20 liter plastic bucket half full of water.  It poured through the water and collected at the bottom.  I periodically stirred the solution with a wooden paddle over the course of an hour.  With the bucket?s cover at hand, I began slowly neutralizing the acid by adding baking soda.   Each addition unleashed a torrent of bubbles by liberating a fair volume of carbon dioxide.   My headache subsiding, I poured the diluted partially neutralized professional  strength drain cleaner down the drain and got busy washing clothes.  A week later I set up and etched for a few hours more.   At this point the glass was fairly clean.  Just a faint brown discoloration outside of the 70 percent zone.  Perhaps one more cycle I thought, then suddenly two months went by.
    
  Two nights ago I reassembled the bits and pieces, filled the casserole pan with acid and a dash of boracic, gently added the lens and turned on the hot plate.  The glass sat at 80C for four hours.  Yesterday morning there was a wisp of yellowish brown residue on one side at the edge.  Last night I completed a four hour cycle at 100C. Today the lens is coating free.
    
  I got the sulfuric acid at the hardware store. It was next to the various hydroxide drain cleaners.  Chalk shaped and powered boric acid is used as an ant repellent.  Boric acid is also used as an anti-fungal, an anti-bacterial and an antiseptic.  Some drug stores carry it.
    
  I was careful.  I thought through what might go  horribly wrong and planned accordingly.  I wore safety equipment.  Even pantomimed some steps before taking them.  Still, the process wasn?t nearly as perilous as it is made out to be on the web.  There were no billowing clouds of fuming hydrofluoric acid; life on Earth didn?t come to an unceremonious end and better yet, the lens wasn?t ruined.
    
  Neutralizing the acid isn't necessary to dispose of it.  Following it down the drain immediately with a large load of laundry's worth of water is sufficient.  The baking soda is part of the safety equipment.  I was quantifying  its efficacies.  I was not extinguishing an arrangement of candles with a river of CO2 just for the fun of it.   More like a blanket.
    
    
  Anthony
    
    
  canes in momentum vivunt
  Dogs live in the moment.
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