A modification of Krefting's method (p. 108) has proved eminently successful in the treatment of iron objects inlaid with silver. Krause[121] recommends that the article be placed, with the inlaid surface downwards, for 24 hours in a mixture of
10 grammes of 40% acetic acid, 10 grammes of ammonium chloride, 70 grammes of distilled water, 10 grammes of aluminium powder.
It is then removed from the bath, carefully brushed and washed, and, if the inlaid work is not yet cleaned, is replaced in the bath. This is repeated until the inlaid work is completely exposed. Spots of ferroso-ferric oxide which are difficult to remove may be ground away by an emery wheel, care being taken that the inlaid surface is held against the lower side of the wheel (which must be rotated in the reverse direction) so that it is always in sight.
All the methods of this group, which have been applied to many articles in various Museums, exhibit one inherent defect, for any rust which remains after treatment may cause the continued oxidation of the iron. The effects of this action of rust are, I believe, extremely small, and it must at the same time be admitted that iron antiquities, even if they have been well steeped and afterwards impregnated, do not always remain in a permanent and sound state of preservation. If in such a case the well-known small watery bubbles should make their appearance, the steeping has undoubtedly been insufficient. This evil can be remedied by gradually heating the object to redness to destroy the impregnating material, and by a careful repetition of the steeping and impregnation.
(3) Preservation of Iron Antiquities by Removal of the Rust.
STEFFENSEN'S METHOD (COPENHAGEN). The objects are carefully heated over a flame and are then laid in dilute sulphuric acid. The sulphuric acid dissolves a certain amount of the iron, and it is found by experience that the chemical action is strongest at those spots where any rust remains, and that this is detached by the hydrogen which is produced. When the cleaning is sufficient, the iron is laid in a dilute soda solution to neutralise the acid, and is afterwards well washed with water and dried in an oven. When dry the iron is brushed over with a solution of bees'-wax (or better of paraffin) in benzine, the evaporation of which leaves a protective coating of bees'-wax or paraffin.