herman by niusheng11

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									      Iron Artifact Conservation:
         Science on the Cheap
             Rhett Herman, Radford University


•   Background/history of Saltville, Virginia
•   Iron relics
•   Rust process
•   Conservation reaction
•   Equipment/setup
•   Reaction time calculations
              History of Saltville
• Salt was important as the main preserver of food
  and a necessary part of livestock diets
• 1748—first surveyed by expedition returning from
  Cumberland Gap surveycalled ―Buffalo Lick‖ when
  natural salt springs were seen
• 1782—first small-scale commercial ―saltworks‖ begun;
  used iron kettles to boil natural briny water
• ~1800-1856—production expanded but limited to southern
  states by transportation along nearby Holston River
  (~250,000 bushels/12.5 million pounds by 1856)
• 1856—railroad service established in Saltville
• By 1860, Virginia was main mineral-producing state in the
  south (salt, lead, iron, saltpeter, coal)
             History of Saltville
• mid-summer 1863—Saltville and nearby areas were
  only salt source left for most of Confederacy
  (Alabama works cut off from all but Gulf Coast)
• 1864—Saltville produced 4 million bushels (200
  million pounds) of salt at its peak, becoming a major
  target for Union Army
• October 1864—U.S. General Burbridge attacked
  unsuccessfully from Kentucky with 5,000 troops
• December 1864—U.S. General Stoneman and others
  attacked again, this time successfully after more
  bitter fighting
             History of Saltville
• After Civil War, Saltville returned to twice pre-war
  production (~400,000 bushels per year now)
• Late 1800s and throughout 1900s, Saltville produced
  salt and a variety of salt byproducts (baking soda,
  etc.)
• 1969—hydrazine made in Saltville used on Apollo
  rockets to go to the moon

• Some small production of salt and byproducts
  continues today
                     Iron Relics
• Salt kettles—over 2,600 in 1864 in
  38 multi-kettle furnace structures
  (lotsa Bernoulli action to control &
  equalize temperatures!)
• Large and small kettles, depending
  on time of manufacture
• Natural brine is 22% salt by
  volume in Saltville
• seawater~2.6% by weight (r~1,026
  kg/m3) for bulk density of salt
  ~1,150 kg/m3
         Iron Relics
Current condition
         Iron Relics
Current condition

                    Pieces falling off…
         Iron Relics
Current condition—very unstable
 and brittle (still rusting)
                   Rust Reaction
• Freshman chemistry redox reactions for the iron
anode (+):     2 H2O + 2 e–  H2 + 2(OH) –
               Na+ + OH –  NaOH

cathode (–):   Fe+ - 2e–  Fe2+
               Fe2+ + 2(OH) –  Fe(OH)2
               4Fe(OH)2 + O2  2 Fe2O3 •H2O (one possibility)


Fe2O3 • H2O ≡ hydrated ferric oxide ≡ ―red-brown rust‖
                 Rust Reaction
• Other reaction products formed (rust-related)
Fe2O3 • H2O ≡ hydrated ferric oxide ≡ ―red-brown rust‖

Fe3O4 • H2O ≡ green hydrated magnetite
Fe3O4 ≡ black magnetite (also FeO • Fe3O4)
FeO2 ≡ ferrous (Fe2+) hydroxide
FeO(OH) ≡ ferro-hydroxide
FeCl2 ≡ ferrous chloride, anhydrous (very damaging)
FeCl2 • H2O ≡ ferrous chloride, hydrated
Fe2O3 ≡ ferric oxide
FeCl3 ≡ ferric chloride (Fe3+), anhydrous
FeCl3 • H2O ≡ ferric chloride (Fe3+), hydrated
               (chlorides are all very damaging)
                   Rust Reaction
• Common layering with high-salt (i.e., marine,
  Saltville) environments:

Outer layer: hydrated ferric oxide (common rust, Fe2O3 • H2O )

Middle layer: hydrated magnetite (Fe3O4 • H2O)

Inner layer: black magnetite (Fe3O4)
           Conservation Reaction
                  e–



     Artifact    Electrolyte (e.g. NaCO3)    ―mild steel‖
    (cathode)                                 electrode

 Fe2+ + 2e–  Fe                            iron oxidation
                        Fe, eventually
 Fe3+ + 3e–  Fe                            oxygen evolution
                                            ―sacrifice electrode‖
iron reduction
hydrogen evolution 
        mechanical cleaning!
            Conservation Reaction
                   e–



      Artifact    Electrolyte (e.g. NaCO3)   ―mild steel‖
     (cathode)                                electrode

• Start with current just at the point where H2 is released for
  maximum iron reduction ~0.001-0.005 A/m2
• When resistance R=V/I goes down slightly, turn current up
  to ~0.05 A/m2 for maximum chloride removal without H2.
• When resistance R=V/I goes down dramatically, turn current
  up to ~0.1 A/m2 for hydrogen evolution cleaning.
    Equipment/Setup
First homemade power supply



                 RC > 1/120 sec
                     R~2W
                  C~20,000mF
            Equipment/Setup
               First artifact—railroad spike
                 (formerly size of thumb)


                              Reaction vessel (anode)
                                  was coffee can
Hammer indentations
revealed, indicating it was
―smithed‖~1700s
       (forging~1800s)
             Equipment/Setup
                        Variac              Artifact
 voltmeter                                 (covered)



       dc supply
 (will be cheaper to use
battery charger in future)       ammeter

  • Larger setup for larger currents
       current densities > 0.001 A/cm2
          Equipment/Setup
  Anode CO3
                                   Cover to keep
crustation can
                                  fumes contained
 be a problem



                    Artifact is
                     cathode
     Reaction
  products/refuse
    build up in
     container
       Reaction Time Calculations
                   dq         dq
               I       dt 
                    dt         I
                    1 dq species
               dt                   total mass
                     I species mass
                    1     1      1 NeM
               dt  Ne M 
                     I    m      I m
 Ex. Reduction only 10kg of rust: Fe2O3•H2, m=2.95 x 10-25kg
     @ I=10A                       19
                     1 3(1.6 x10 C )10kg
               dt                25
                                         ~ 19 days
                    10 A 2.95 x10 kg
This assumes 100% efficiency, no hydrogen evolution/cleaning
stage, no other species being reduced. 
         Reaction Time Calculations
Expect e.g. a Saltville kettle to take ~1.5-2 years from start to
 finish:

• Reduction:
   Initial slow reduction stage
   Chloride removal stage
   Hydrogen evolution/mechanical cleaning stage

• Alternate dipping in boiling and cool de-ionized water to add a
  layer of ―museum-ready‖ black, controlled oxide.

• Finally, cover in thin layer of microcrystalline wax to seal
  artifact. Done.

								
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