Whilst an extremely interesting method with great potential, hydrogen plasma reduction remains a rarely-used method in artefact conservation. From the initial hypothesis in the 80s this was largely a limitation of size; the plasma reduction chambers accessible to conservators are few in number and generally quite small.
From 2002 the treatment has fallen out of favour somewhat for much the same reason as other heated reduction methods: it changes the metallography- the crystalline microstructure- of the treated object. In practice, this means that we lose information about the original type of steel, quenching, manufacturing methods, etc. which might otherwise have been accessible. Swiss National Museum revised their guidelines to fit in 2002, reducing operating temperature and total time. The revised guidelines prevent such damage, but make complete removal of chlorides and subsequent iron reduction all but impossible. It is still beneficial - it makes additional treatment easier and more effective in some cases - but it is not the easy cure which was hoped for initially, at least nor for archaeological objects.
Basically the theory remains sound, but there were unforeseen problems in implementation with iron. The Swiss and the Danes are still working on applications for iron, but I do not know if anyone else is. As far as I know it is still very effective - albeit rare due to cost/availability - for silver objects though.
A few good resources if you have interest (mostly English, one French):
Patscheider, J. and Veprek, S. (1986) Application of Low-Pressure Hydrogen Plasma to the Conservation of Ancient Iron Artifacts, Studies in Conservation Vol 31, No. 1 pp29-37. International Institute for Conservation of Historic and Artistic Works.
Wheatcroft, A. (ed) (1992) Science for Conservators Volume 2: Cleaning. 2nd Ed. New York, Routledge.
Dussère, F. (1997) Peut-on Concevoir le Plasma Comme un Traitement de Masse? In Macleod, I.D., Pennec, S.L. and Robbiola, L. (ed): Metal 95. London, James & James (Science Publishers) ltd, pp 138-146.
Schmidt-Ott, K. and Boissonnas, V. (2002) Low-Pressure Hydrogen Plasma: An Assessment of its Application on Archaeological Iron, Studies in Conservation Vol 47, No. 2 pp81-87. International Institute for Conservation of Historic and Artistic Works.
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