Method of Restoration for Ancient Bronzes and other Alloys

Cannone nel castello di Haut-Koenigsbourg, photo by Gita Colmar

Without any preliminary cleaning the bronze object to be treated is hung as cathode into the 2 per cent. caustic soda solution and a low amperage direct current is applied.  The object is suspended with soft copper wires and is completely immersed into the solution.  In case the object is very soft and fragile or completely mineralized, fine annealed copper wire is wrapped around the object, one to two turns per inch, and electrical connections are made with several turns of this wire.  Where there is danger that object might not hold together upon the removal of the hard supporting shell, we have found it advisable to to pack the whole object in clean white sand, after making proper electrical connections, and then filling the containers with the caustic soda solution.

The anodes are hung on either side of the object.  Iron, duriron, and platinum anodes have been used with success.  A rectangular glass battery jar of one litre capacity serves well as a container for the treatment of small bronzes.  For large objects we have used stoneware tanks and there is no objection to the use of large tanks made of heavy sheet iron welded at the joints.

For a small object of about two to ten square inches of exposed surface, the cell is connected in series with a rheostat and the 110 volts direct current circuit, so as to send from 0.1 to 0.5 amperes through the circuit.  A slight gassing at the anode will occur.  Sometimes the crust resitance at the point of contact of copper wire is so high that an appreciable current will not pass through the cell at first.  Rather than file a clean contact, which is hardly ever to be recommended, it is best to start the cell at night, allow the solution to slowly penetrate the crust and usually be the next morning, the 110 volts potential will be sufficient to have broken through the submerged crust without injury to the surface.  Often wetting the copper wire contact with electrolyte will break down the dry resistance of the crust without injury to the surface.  Often wetting the copper wire contact with electrolyte will break down the dry resistance of the crust at that point.  The object being treated is always made cathode.

The action of the electrolysis is to evolve hydrogen at the cathode and to so reduce the crust to finely divided or spongy copper.  This is effectively accomplished in the caustic soda solution.  At times reguline copper metal will be deposited in place.  Very low current densities are preferred.

The reduction of a thin crust 1/16 to 1/8 inch thick usually requires three or four days.  In the case of clayey crusts it is a good plan to change the electrolyte once every twenty-four hours.  The use of too strong an electrolyte, or too high a current density will cause excessive gassing at the cathode (the object being treated) and may give rise to warping and falling apart of the object unless it is underlaid by a strong metal core.

Complete reduction is indicated by a free evolution of gas at the cathode with comparatively low current values.  The object is then removed from the solution and carefully washed by soaking or steeping in several changes of warm water.  This will remove all but traces of caustic soda.

We now arrive at a stage in the process where we must use judgement enough to modify subsequent treatment to fit the general appearance and physical strength of the specimen.  If the object treated originally had a hard metal surface under a thin sandy layer of crust, we my remove the reduced copper film or layer by a gentle brushing with a stiff bristle brush or soft brass-wire brush. If, however, examination preliminary to electrolytic treatment indicates that no true metallic core is present, we do not dare subject the treated surfaces to pressure or friction of any kine.  In this case a weak acid dip may be necessary and is used as follows:

After thorough washing to remove all but traces of caustic soda, carefully dip the reduce object in to dilute nitric acid (one part acid to four parts of water).  There will result a rapid action and evolution of gases, as the reduced outer layer of copper dissolves away, we bring into view the hard, brownish-colored copper oxide surface which preserves the detail of design. This oxide layer does not dissolve readily in the weak acid used, but takes on a greyish tint (due probably to intermixed tin compounds).  Inspection with a magnifying glass will show when the treatment has proceeded far enough.  It is an exciting moment when we see the the copper dissolve, showing a smooth surface, or design, beneath.  The oxide surfaces now exposed are almost as smooth to the touch as metallic surfaces and they will take on a greenish tint or patina when dipped into dilute ammonium acetate and dried in an oven at 40 to 60 Centigrade (104 to 140 Fahrenheit).

Sometimes, especially if the original surface shows cracks in the patina or conspicuously high lumps or “boils” of crust, we would find the underlying oxide layer pitted and roughened in spots if we had used the weak acid dip.  Such regions have been highly corroded by localized action and the treatment in our electrolytic bath causes brown copper to fill the pits.  Therefore in these cases the weak acid dip is omitted.

Source: The Restoration of Ancient Bronzes and Other Alloys by Colin G. Fink, Ph. D. Professor of Electro-Chemistry at Columbia University and Charles H. Eldridge, B.S. First Report, 1925, The Metropolitan Museum of Art

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— Leonardo Da Vinci