Iron Corrosion in Archeology
By Jacob Selmer
May 5, 2004
MSE 4164
Introduction:
There are a number of methods used to remove corrosion from artifacts and to prevent further corrosion. Iron is often the most frequently discovered metal at archeological sites. Unfortunately, due to the number of environments which cause iron to corrode and the complexity of its corrosion products, iron is the most difficult of the old world metals to conserve (Hamilton, File 9). “The treatment and preservation of metal artifacts occupy most of a conservator’s time, for literally hundreds of iron and brass objects (or parts of objects) are likely to be found even on a small domestic site.” (Hume, 274) This paper will introduce the methods used to remove corrosion and how to resist further corrosion.
Mechanical cleaning:
A number of fast methods of cleaning have been promoted in the past, but those require giving up fine control over the artifact and potentially destroying it or ending up with a lump of metal with no historical interest. These methods range from sandblasting and tumbling to ultrasonics. One starting point is to go over the object with a magnet. If a large section has a weak pull, it is likely that the base metal is in poor shape. Particular caution should be taken when cleaning these areas. Manual cleaning tools include dissecting needles, electric engraving tools, and rotary brushes. These tools will remove almost all of the rust on an object. If the condition of the object is poor enough not to continue, it can be coated in a 50/50 mix of microcrystalline and paraffin wax. This is not an ideal shield against corrosion because heated display cases can cause wax to melt, and improper handling can scratch through the wax layer. (Hume, 275-6)
Electrochemical Cleaning:
If an object is in good enough condition to be fully cleaned, the most common method used to stop, stabilize, and reverse corrosion is electrochemical.
Galvanic Cleaning:
Galvanic cleaning uses two metals from different locations on the galvanic chart. This method is generally considered out of date, but still an option. For cleaning an iron object, the object is placed in a vat of electrolyte and surrounded with a more reactive metal. Zinc and Aluminum anodes both work for this role. The process can be speeded up by heating, but the electrical potential cannot be controlled. Once the surrounding metal has completely oxidized, it has to be removed and cleaned or replaced. This method is impractical for large artifacts due to the amount of the sacrificial anode required. This method also obscures the object from view, making it hard for the conservator to observe the progress. (Hamilton, File 10a)
Electrolytic Reduction Cleaning:
Electrolytic reduction cleaning is the most common method of iron conservation. It requires a simple setup and allows the conservator a lot of control. This method also uses the artifact as the cathode in a vat of electrolyte. The anode material can also be iron based. Mild steel or 316 stainless is recommended. Mild steel is more cost effective and easier to work with. If copper wires are used, the copper must be insulated from the electrolyte in order to keep from corroding and plating onto the artifact. Bare steel clips should be used for the same reason. A regulated DC power supply or battery charger is used to drive the system. A power supply with voltage and current controls is recommended because the resistance of the electrolyte decreases as the process continues. Adjustments are necessary to keep the system at a stable current and voltage (Hamilton, File 10A)
The current and voltage settings can be decided based on what type of cleaning needs to be done. At high current densities, around 0.1 amp/cm2, vigorous cleaning causes hydrogen evolution and mechanical cleaning. At medium current densities, around 0.05 amp/cm2, chloride removal is optimized without excess hydrogen evolution. At low current densities, from 0.001 to 0.005 amp/cm2, ferrous corrosion compounds can be reduced, generally to magnetite. (Hamilton, 10B)
The ideal anode would be formed around the artifact to allow uniform current densities. Having a single anode on one side or using supports or labels on the artifact will cause uneven cleaning unless it is regularly rotated. (Hamilton, File 10B)
Alkaline Sulfite Treatment:
For artifacts with a solid core, alkaline sulfite treatment is another option after mechanical cleaning. This method is typically used for marine cast iron finds or wrought iron. The artifact is placed into a solution of 0.5 M sodium hydroxide and 0.5 M sodium sulfate. The container must be air tight and heated to 60 C. The object must go through several cycles with fresh solutions for up to several months until all of the chlorides are removed. Once stabilized, the artifact should be rinsed in de-ionized water and bathed in 0.1 M barium hydroxide.
Chemical Cleaning:
There are a number of chemical treatments that can be used to create a corrosion resistant film. Acids such as oxalic acid, citric acid, and phosphoric acid can be used to make an attractive surface, but these acids do not remove chlorides or prevent future corrosion caused by these chlorides. (Hamilton, File 10B)
Annealing:
In Oxygen:
Annealing in an oxygen environment at 700 C or higher causes hydrogen to be driven off as water and ferrous and ferric chlorides to sublime. Unfortunately, the corrosion products crumble off, leaving no trace of decoration and a disfigured surface and the remaining surface is covered with a red iron oxide layer. The high temperatures also destroy any metallurgical data. (Hamilton, File 10B)
In Hydrogen:
Annealing in a reducing hydrogen environment can be done at 1000 C. Chlorides are again driven off and oxygen from the oxides combines with the hydrogen to form water as it is driven off. The oxide is then returned back to its metallic state. There is apparently very little damage to the surface as a result of this treatment. Unfortunately, the hydrogen kilns required to do this are few and far between and the hydrochloric acid gas produced eats away at any metal components inside the kiln. This method also destroys any metallurgical data of interest to the archaeologist.
Hydrogen Plasma Reduction:
Hydrogen plasma reduction is a fairly recent technique used in metal cleaning. The artifact is placed into a quartz discharge tube in a low pressure hydrogen environment. High frequency radio waves are then used to ionize the gas into plasma. The magnetite and ferric oxide on the surface of the iron object are reduced back to a metallic state. The process is done under 400 C, so the crystalline structure of the object is not altered. While initial results are promising, there are prohibitive high costs associated with the equipment required. (Hamilton, File 10B)
Final Rinsing and Sealing:
After any of the above-listed cleanings, the artifact must be thoroughly rinsed to remove any unwanted residues. Several alternating rinses of boiling and cold de-ionized water remove these residues and put a flat black iron oxide layer on the surface. Many coatings have been tried as a sealant including paints, oils, and plastics. However, the most successful sealant from a conservator’s standpoint is microcrystalline wax.
Summary:
There are many methods for cleaning iron archaeological finds. Each method has its disadvantages, but the most commonly used method is electrolytic reduction. This process allows the conservator a great deal of control at a low expense. For highly corroded, fragile objects, alkaline sulfite treatments or extended rinsing can be used. Other highly effective methods include hydrogen reduction and hydrogen plasma reduction, but their cost often makes these prohibitive.
Figure 2: Late 15th century helmet still showing part of the original finish before cleaning.
Works Cited
Hamilton, Donny L. “Metal Conservation: Preliminary Steps.” http://nautarch.tamu.edu/class/ANTH605/File9.htm Conservation Research Laboratory, Texas A&M University. 2000
Hamilton, Donny L. “Iron Conservation Part I: Introduction and Equipment.” http://nautarch.tamu.edu/class/ANTH605/File10a.htm Conservation Research Laboratory, Texas A&M University. 2000
Hamilton, Donny L. “Iron Conservation Part II: Experimental Variables and Final Steps.” http://nautarch.tamu.edu/class/ANTH605/File10special.htm Conservation Research Laboratory, Texas A&M University. 2000
Hamilton, Donny L. “Final Conservation Steps.” http://nautarch.tamu.edu/class/ANTH605/File10bpart2.htm Conservation Research Laboratory, Texas A&M University. 2000
Hume, Ivor Noel. Historical Archaeology. Alfred A. Knoph: New York, 1969.