For boring out rust spots which have eaten deeply into the iron a dental drill can be used with success, and a great variety of drills and milling cutters can be obtained. The rinsing, which Blell carries out by moving the object to and fro close under the surface in a vessel full of water, may be sufficient for thin iron objects, such as swords, knives, spear-heads, and similar objects. Larger specimens should be freed from the acid by putting them into a still more dilute solution, and, when necessary, by steeping for a short time in water. It may also be advisable to put the objects into dilute soda solution to neutralize the sulphuric acid, but this does not do away with the necessity for steeping in water. The brown coating of rust which may possibly follow the steeping can be removed by the use of steel-wire brushes, which can now be made of such fine wire that their softness almost equals that of a moderately soft tooth-brush. Brass-wire brushes should not be used, on account of the yellow colour which they give to the iron. I always put the objects directly after steeping into clean fat heated to 250°F. [120°C.], for brushing over with fat and warming in a stove often caused a slight tarnish to cover the surface. I have also used paraffin wax instead of fat.
For the method of restoring iron antiquities and of filling up large gaps, the reader should refer to Blell's detailed account; it will here suffice to quote his statement that a mixture of iron filings with tin filings can be used for this purpose. These are melted and applied by the aid of a blowpipe.
The accompanying illustrations represent iron antiquities which have been treated by Blell's method: the sword (Fig. 25) proved, after reduction, from its two ridges to be a scramasax; on the spear-head (Fig. 26) treatment revealed a small copper ring at the most constricted part, while the fibula, which previously had been a mass of rust, now shows the spiral which had been totally disguised.
[Illustration: Fig. 25. Iron sword treated by Blell's method.]
[Illustration: Fig. 26. Iron spear-head treated by Blell's method.]
[Illustration: Fig. 27. Iron fibula treated by Blell's method.]
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.