Small articles can be freed from rust by immersion in strong fuming nitric acid[137], for strong acid dissolves the rust only, while it induces in the iron the so-called “passive[138]” condition in which it is not acted upon even by dilute acids, and can be safely washed in water. When thoroughly cleaned, the most suitable protective is some neutral substance such as paraffin wax, vaseline, or paraffin dissolved in benzine, but any of the numerous forms of oil or fat may be used.
(i) Bronze and Copper[139].
Well-preserved bronzes with a stable patina, such as the highly esteemed glossy stable or “edel” patina, or that which, although not glossy, covers the bronze with a rough and often crystalline coating, should not be interfered with. Such bronzes as need treatment should be subjected either to simple cleaning or to some appropriate method of preservation.
The Cleaning of Bronzes. Bronzes, the metallic substance of which is more or less intact, while the surface is hidden under earthy or sandy material cemented together by copper compounds, may be cleaned either by mechanical or chemical means. When the materials forming the incrustation are more firmly cemented together than they are to the material beneath (which often still retains a polished surface), a small hammer may be used, but more adherent portions require the use of small chisels, which can be made to order in different shapes or sizes. I have used with advantage hammers with striking surfaces like those shown in Fig. [36]. The two on the right are rounded so that they touch the object at one point or on a line only. The process may be facilitated by the use of Springer’s method. A warm thick solution of glue should be spread upon the incrustation covering the bronze. As the glue dries and becomes cool it scales off, carrying with it some portion at least of the crust, thus leaving the metal clean. That part of the glue which remains can then be readily detached by gentle strokes with a hammer. The eyes should be protected when using the hammer, whether on the incrustation or on the glue.
Fig. 36. Hammer heads, natural size.
Other Methods. Since metallic oxides are scarcely, if at all, soluble in water, washing with water, even when a brush is used, will remove only earth or soil which is loosely attached. Compounds containing oxygen or oxygen and chlorine are, however, more or less soluble in ammonia, and, if they are thin and not too compact, after immersion for some time can be removed with a brush. Thick compact layers are loosened with difficulty.
Immersion in 2-5% hydrochloric acid acts more effectively, while sulphuric acid, nitric acid, and concentrated acetic acid have the same action. The frequent use of these reagents is, however, strongly to be deprecated, for it is impossible to remove the acid by simple washing with water after the incrustation has been removed. The bronze should be washed and placed in a very dilute soda solution or in dilute ammonia, after which it should be again well washed with distilled water. As has been explained in [Part I.], it is to chlorine compounds that the destruction of bronzes is chiefly due, and these are actually produced by the hydrochloric acid treatment. If the bronzes are not thoroughly washed, and this is no easy matter, sooner or later efflorescences will make their appearance, and the process of preservation must be repeated if the destructive action is to be arrested.
Various attempts have been made to remove the incrustation by raising the bronze to a red heat. This process is not recommended; for not only does it give to the bronze an unpleasant appearance, but it detaches any inlaid metal (gold or silver) or enamel which may be present.