Krefting’s Method. The electro-chemical method of Krefting was originally published in “Aarsberetning fra Foreningen till Norske Fortidsmindesmaerkers Bevaring,” 1892 (p. 51), but in “Finska Fornminnesföreningens Tidskrift[125]” there is a translation into German by H. Appelgren, and an additional series of observations and experiments by him. His remarks are equally applicable to Blell’s method, and the following extracts and quotations from this paper give Krefting’s method of procedure and the circumstances under which it should be applied.
Small fragile objects such as fibulae, thin clasps and bracelets or those which are much eaten away by rust, are not suitable for this mode of treatment, thus:
“A knife which is much corroded, and which when taken out of the earth shows a distinctive form (for example, that of the Early Iron Age), may lose so much by the application of the electric current that every distinct sign of its original character is destroyed. The characteristic edges of a spear-head or of an axe of the late Iron Age, or the equally characteristic point of an iron sword, may, if the rust has eaten deeply into them, be unrecognisable when removed from the electrolytic bath. A sword, the hilt of which is inlaid with copper wire or is plated with silver or gold, or the blade inlaid with inscriptions in gold, silver, or copper, may be totally destroyed by incautious treatment; for the ornamentation, if undermined by rust, may be detached with the rust from the underlying iron.”
On the other hand, objects of sound metallic iron covered with an incrustation of rust about 1⁄25 inch [1 millimètre] in thickness may be easily cleaned in this manner, but if on using a file the metal does not appear at all, or only at a depth of 1⁄8 inch [3 millimètres], great caution must be used. If there is reason to believe that there is gold or silver inlaid work undermined by rust, Appelgren recommends that the object should, as a preliminary, be laid in clean water, which should be renewed every day. After some time, three weeks at the most, sufficient rust will have been cleared away by carefully brushing with a steel brush to lay bare the ornamentation, at least in part, and it can then be ascertained whether there is any rust underneath which would, if Krefting’s method were used, cause the ornamentation to be detached.
The line of treatment is as follows: The metallic iron core is laid bare by filing in several places. The specimen is then wrapped with strips of zinc in such a way that the zinc is in actual contact with the bare metal (Fig. [28]). The whole is then placed into a 5% solution of caustic soda[126]. Appelgren uses a solution of 31⁄ 2-41⁄2 lbs. [11⁄2-2 kilogrammes] of caustic soda in 2 gallons [10 litres] of water. The rust is cleared away by voltaic action; the iron forms the negative pole, the zinc the positive of a voltaic cell, in which the water is resolved into its constituents, viz. oxygen and hydrogen. At the negative pole, i.e. the iron, the hydrogen rises up in small bubbles and acts in part by mechanically detaching the rust as in Blell’s method, in part also by the chemical conversion of the rust into metallic iron, or into a compound which contains a smaller quantity of oxygen than does ordinary rust. The oxygen combines with the zinc to form zinc oxide, which is dissolved in the soda solution. The process is usually completed in 24 hours[ [127]. The black powder which is loosely attached to the iron is best rubbed off with wet sand and fine wire brushes. Any hard pieces of black stable rust (Edelrost), magnetic oxide of iron, which have not yielded to the electric current should be removed by means of a small chisel. After rinsing the object thoroughly in water, it should be placed in melted paraffin at 240°F. [115°C.], which will expel every trace of moisture. On removal the melted paraffin should be allowed to drain off, and thus leave when cold a protective covering upon the iron[128].
The following points should be observed in the application of the method. Vessels of glass or glazed earthenware should be used for the reduction, while long swords can be put into tall glass cylinders or into wooden troughs, the interior of which must be coated over with paraffin. The soda solution must be kept in a closed glass bottle[129]. It should be diluted with water until the specific gravity, as shown by the hydrometer, is 1·06; the mixture will then contain about 5 per cent. of caustic soda. During the reduction process the mixture frequently assumes a brownish colour as the result of the presence of organic matter associated with the rust. On account of the dissolved zinc which it contains it cannot be used a second time, unless regenerated by boiling with quicklime. The solution is, however, so cheap that this is scarcely worth the trouble.
The objects should be handled with metal tongs, and should not be touched with the hand until they have at least been dipped or rinsed in water, for the soda solution has an injurious effect upon the skin. A basin containing vinegar, dilute hydrochloric or sulphuric acid should always be at hand into which the fingers should be quickly dipped if they have been in contact with the caustic soda. These materials will serve also for cleaning the vessels used in the reduction process.
The zinc strips should be 1⁄4 to 1⁄3 inch [1⁄2 cm. to 1 cm.] in breadth, and should be cut out of a piece of sheet zinc of moderate thickness, but of sufficient pliability.
Any firmly fixed rust may be removed by mechanical means, such as the graver, drill, etc., as has been previously mentioned. If in rinsing a slight layer of oxide appears, although this is rare, it should be brushed off with a steel-wire brush.
If one portion only of a specimen requires reduction (the other portion having, for example, remains of wood attached, and therefore being unsuitable for reduction), that portion only should be wrapped with the zinc and immersed in the solution.