| Abstract: | SUMMARIES. Results are presented on the effectiveness of treating archaeological iron by immersion in an aqueous sodium hydroxide (NaOH) solution (2% w/v, pH 13.5, room temperature) followed by immersion in an aqueous 1,2-diaminoethane (ethylenediamine, EN) solution (5% v/v, pH 11.5, 50°C). This study was undertaken to determine the effectiveness of these solutions in removing dissolved chloride ions and to explain the occasional observation of rapid iron corrosion. Thirty-two archaeological wrought iron pieces were treated. Some were immersed in NaOH followed by EN, and, for comparison, others were treated first in EN, then NaOH. Each artifact was treated separately and solutions were changed on a regular basis. The chloride ion concentration was determined by potentiometric titration with a silver nitrate solution. For nine artifacts, solutions were analysed quantitatively for 26 different dissolved elements using inductively coupled plasma atomic emission spectrometry. The results demonstrate that chloride ions are more effectively removed from archaeological iron by immersion in NaOH than by immersion in EN. The results also demonstrate that heavily mineralized iron is more likely to remain unbroken if immersed in EN before immersing in NaOH. Unfortunately, the corrosion of iron can be stimulated by EN because it forms soluble complexes with iron(II) ions.
CONCLUSIONS. Results have been presented from a systematic assess¬ment of a treatment approach for archaeological iron developed at the Canadian Conservation Institute in the early 1980s. Thirty-two archaeological wrought iron objects from Ferryland and Renews, Newfoundland, were treated by immersion in individual treatment solutions containing an aqueous solution of either NaOH (2% w/v, pH 13.5) or EN (5% v/v, pH 11.5). Treatment solutions for all 32 objects were analysed quantitatively for dissolved Cl- ions. Treatment solutions for nine objects were analysed by ICP-AES for 26 additional dissolved elements. This systematic approach provided specific information about the quantity and time-dependence of Cl- ion and other dissolved ele¬ments as they diffused out of each object into a given treatment solution. The results demonstrate that immersion of archaeo¬logical iron in an aqueous sodium hydroxide solution is an effective way to treat archaeological iron with a substantial metal core where removal of chloride ions is important; the softening of the corrosion layers and the passivation of iron metal are contributing factors. The results also demonstrate that immersion of archaeological iron in an aqueous ethylenediamine solution is not particularly effective at removing Cl- ions although it is effective in preserving the corrosion layer on heavily mineralized iron; removal of dissolved iron(II) ions and minimal softening are contributing factors. The use of EN solutions in conjunction with NaOH solutions may provide an effective way to treat archaeological iron without a substantial metal core where maintaining the outer corrosion layers is important. Unfortunately, immersing archaeological iron with a substantial amount of remaining iron metal in EN solutions can be dangerous because of the ability of EN to form soluble complexes with iron(II) ions. If the iron metal is still corroding and forming new iron(II) ions at anodic sites, then the iron may continue to corrode (because of the reaction ot Fe2+ ions with EN to form soluble complexes) rather than passivate (through the reaction of Fe2+ ions with hydroxyl ions to precipitate iron(II) hydroxide). Finally, ICP-AES analysis of treatment solutions provided a better understanding of what other elements were being removed from the archaeological iron by treatment in alkaline solutions. |
|---|
