The application of subcritical fluids for the stabilization of marine archaeological iron

The use of subcritical fluids for the stabilisation of archaeological iron: an overview

“…20.4, Experiment D). Once the system reached subcritical conditions, there was a dramatic increase in chloride extraction, supporting the hypothesis that it is heat and not pressure that is the main contributing factor to high chloride release rates [18]. It must be noted that while the minimum pressure required to achieve subcritical conditions at 180°C is 10 bar, the working pressure of approximately 40 bar was adopted for technical reasons as a sudden decrease of pressure in the system if working at 10 bar would cause a catastrophic vaporisation of the treatment solution.…”

“…The same experiment was carried out in a 0.5% w/w solution (0.125 M) of sodium hydroxide with a pH of 13, using the remaining sections of the same rivets. The result was a much lower residual chloride level in the metal of about 200 ppm (0.022% w/w) [18]. The importance of the pH was clearly demonstrated during this experiment and confi rmed the critical role played by the diffusion of hydroxide ions deep into the corrosion layers that act as counter-ions to replace and release the chloride from anodic sites.…”

“…However, analysis of the residual chloride content suggested equal effi cacy in the removal of chloride for the three temperatures considered [18]. Based on these results, 180°C was selected as the best compromise between the amount of time needed for a treatment run, the technical considerations for heating a pressurised vessel (particularly if a very large ar tefact were to be treated in the future), treatment effectiveness and any potential effects on the materials undergoing treatment [14,17,18].…”

“…A wide variety of samples and archaeological artefacts from marine and terrestrial contexts have been treated, including freshly excavated artefacts and those that have been allowed to dry out. Under subcritical conditions, the transport properties of the treatment fl uid are considerably modifi ed, resulting in a very signifi cant increase in the chloride extraction rate compared to traditional techniques [12,14,18,24]. No treatments using subcritical have exceeded three weeks, compared to over 6 months of treatment using traditional methods on some of the cast iron specimens.…”

“…As part of a collaboration with Clemson University during the development of a long-term conservation plan for the stabilisation treatment of the H.L. Hunley (1864) submarine at the Warren Lasch Conservation Center (WLCC) in Charleston, South Carolina, a new technique for the treatment of chloride-ridden iron was evaluated and compared to the traditional techniques of immersion in sodium hydroxide with and without electrolysis [14][15][16][17][18]. This new technique, known as subcritical water extraction (SWE) or pressurised hot water extraction, was based on prior research carried out with sub-and supercritical fl uids for waste water oxidation and the analysis of extractable material from textile yarns and fabrics [19][20][21].…”