The chelation of silicone-treated wood artifacts to address sulfate precipitation

• Iron corrosion causes sulfate generation in artifacts treated via siloxane polymers. • Chemical chelation effectively penetrates siloxane-treated artifacts. • EDTA and ammonium citrate are effective in chelating siloxane-treated artifacts. • Ammonium citrate causes leaching of siloxane polymers. While the generation of sulfuric acid and iron sulfates has been known for at least 25 years in water-logged artifacts conserved with a mostly-reversible polyethylene glycol treatment, it has not previously been reported or treated in artifacts conserved using other methods. The issue is at least partially related to the presence of iron and iron corrosion products in the wrecks, and is therefore often treated through chelation to remove chemically iron from the artifacts. Now, sulfate salts and sulfuric acid have precipitated in artifacts from the 1686 La Belle wreck, which were conserved using silicone oil polymerization, a currently irreversible conservation technique. To determine whether the silicone would allow direct chelation of the iron, archaeological samples from the Heroine wreck were conserved using silicone polymerization and chelated for three months using ethylenediaminetetraacetic acid (EDTA) and ammonium citrate. The chelated solutions were tested weekly with inductively coupled plasma-mass spectroscopy (ICP-MS) to determine the quantity of iron removed. Both EDTA and ammonium citrate were able to remove iron in similar quantities, with ammonium citrate initially removing it at a slightly faster rate. However, the ammonium citrate disrupted the silicone used to preserve the samples, making it an unviable method for treatment.