The reduction of Ag 2S and treatment of paper in a magnetron glow discharge

Abstract

Ag 2S on silver antiquities can be reduced to metallic silver in hydrogen glow discharges but when heat-sensitive bases supporting corroded silver are exposed to such discharges there is risk of damage from bombardment of energetic electrons. The energetic electrons are secondary electrons released from the cathode by ion impact and accelerated in the adjacent positive ion sheath. Operation at large cathode work spacings or rough vacuum pressures (greater than 0.1 Torr) can reduce the electron energies by gas scattering but the work is then more remote from the active discharge region. Magnetron discharges using a closed magnetic tunnel field for electron entrapment do not cause electron heating and have been studied as a means of Ag 2S reduction. A high frequency (h.F.) crystal microbalance was used to follow the reaction process. Evaporated silver films deposited on the h.f. crystal were exposed to H 2S gas to form Ag 2S and the corroded layer was exposed to H 2 and Ar-H 2 discharges at a distance of 60 mm from an aluminium cathode with a tunnel magnetic field 52 mm in diameter. The frequency changes showed that Ag 2S films 400 Å thick could be reduced to metallic silver in 20 min at 10 −1 Torr in H 2 at an input of 250 V and 25 W. Reduction in an Ar-H 2 discharge ceased when only one half of the sulphide layer had been reduced. Experiments with gold-coated crystals showed that a sputtered film was deposited at the rate of about one monolayer per minute when argon was in the glow discharge. The sputtered layer was believed to be aluminium oxide which was impervious to atomic hydrogen. A low etching rate was observed when the sputter-coated crystal was exposed to a pure H 2 discharge. Ag 2S on evaporated silver films on paper could be exposed for reduction to the H 2 and Ar-H 2 magnetron discharges without apparent damage to the paper-cellulose base but uncoated paper became slightly discoloured, indicating that decomposition had occured. Uncoated paper was burnt rapidly in non-magnetron discharge in H 2 at 10 −1 Torr, 400 V and 25 W with a spacing of 60 mm. Less damage occured under the same conditions in non-magnetron Ar-H 2 discharges because the greater collision cross section of the argon atoms reduced the energy of the secondary electrons. It is believed that H 2 magnetron discharges could be used effectively for restoring corroded silver on cellulose supports using incontact masks to protect the uncoated base material from the discharge. Power input control would be necessary to avoid undue heating from hydrogen atom recombination and surface reactions.