Usuwanie części metalowych z drewnianych obiektów zabytkowych

A problem non infrequently encountered in conservation practice is that o f removing metal elements from historical objects and monuments. The mechanical method, usually employed for the purpose, is not always effective. In some cases the whole metal elements, or their fragments, are left therein because of the impossibility of their being removed without destroying the object concerned. None of the applied mechanical methods has given the expected results but made a threat to the object as such and the condition o f its preservation. Looking for some other solution to the problem, the conservators have turned to the Institute of Electronic Machines and Steering Systems, at the Academy o f Mining and Metalurgy, Cracow. The proposed method, assuming employement o f magnetic forces, has been rejected by the Institute’s specialists, no account o f the character o f the objects concerned, i.e. two Gothic wooden statues, provided with a polychromy coating and representing angels holding candelabriums. In that situation two novel methods were applied. The first of them consisted in the use o f electrodes, the second ■— o f induction reheater. The nails with two ends visible were removed by way o f both the electrode method and the induction one. However, the metal parts o f which but one ending was be seen were taken out solely with the use o f induction reheater. The proceedings o f the two methods are ultimately reduced to incadescence o f the metal element and its rapid removal. A red heated nail grows somewhat larger, the degree o f thermal expansibility o f the mass o f the nail and that of the stratum o f oxidized mass being different, o f course. As a result the rust on its surface breaks, especially when the metal element is cooling. The heated nail causes partial scorching o f the wood along it what, in turn, makes it get loose therein and possible to be driven out, and the rust covering it crumbled away together with wood from the hole left by the nail. The time of heating the metal parts ranges from a few, to less than twenty seconds. Burning of the wood around the metal parts has some advantages as well, to mention but the fact that the wood is thus cleared o f rust, since its charred fragments are tipped out together with the latter. Moreover, this proceeding also eliminates — from the wood tissue — the acidic and alkaline media rising therein in the course o f rusting of the nail and also the bacteria developing in the said tissues. Nails were also removed from the parts of the sculptures coated with silver and gold-foil, without any damage to those layers. That operation had been preceded by tests made on objects specially prepared for the purpose. The iron nails having a much greater mass density get hot very quickly. Nevertheless, that brief span o f time — though sufficient for the metal element to become red- hot — is not long enough to effect thermal changes in the silverand gold-foil. The danger of thermal damage to the foil is imminent at the time of warming the copper nails. The scupltures moved under the inductor o f the reheater were then subject to final inspection aimed at detecting the still present, deeply set, nails. An indication o f their presence was the smoke appearing in the outlet. In such a case the clearing procedure was repeated. The method calls for a more detailed scientific elaboration which would furnish a theoretical basis for the problem discussed.