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 surveycalled ―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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