herman by niusheng11


									      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,
• 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

     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

      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.
First homemade power supply

                 RC > 1/120 sec
               First artifact—railroad spike
                 (formerly size of thumb)

                              Reaction vessel (anode)
                                  was coffee can
Hammer indentations
revealed, indicating it was
                        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
  Anode CO3
                                   Cover to keep
crustation can
                                  fumes contained
 be a problem

                    Artifact is
    build up in
       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

• 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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