Fig. 34. Piece of iron sword-blade showing inscription, after treatment by Krefting’s method.
Hartwich’s Reduction Method[131]. This method is only applicable to small objects, because it necessitates the subjection of the objects to red-heat in a glass tube in a current of hydrogen. By these means the hydrogen combines with the oxygen of the oxides, which are thus reduced to metallic iron. Owing to the explosive nature of a mixture of hydrogen and air, this process should only be carried out by one who is conversant with chemical methods, for results which are equally good can be obtained at less expense by Krefting’s method. For Hartwich’s method a strong core of metal is essential, for although objects which are entirely oxidized may be thus reduced, the result will be the formation of a more or less loose iron powder which is frequently in such a fine state of division that by union with the oxygen of the air, in consequence of the great amount of surface presented, it becomes red-hot with the formation of ferric oxide as a combustion product.
It is advisable to apply a combination of Blell’s or Krefting’s method with one of the first group (under certain conditions) to such iron objects as are found, during the process of preservation, to be penetrated by black stable rust to such a degree that the complete removal would only leave a kind of iron skeleton. Fig. [35] represents such an iron dagger-sheath[ [132], the dark spots upon it being rust. After heating and cooling down and a short treatment with acid the removal of the rust was proceeded with mechanically, but was not completed. The object was then well steeped, and when dry was warmed in the varnish-petroleum mixture[133].
Fig. 35. Iron dagger-sheath after treatment by a combination of Blell’s and Krefting’s methods.
Iron objects, the size of which is inconsiderable, such as arrow heads, small rings, etc., can be very quickly reduced, if they still have a well-preserved core, by heating them for a short time in molten potassium cyanide[134]. The cyanide may be melted in a porcelain crucible supported by wire gauze on a tripod over a good-sized Bunsen burner, and the object introduced by the aid of tongs. The reaction is accompanied by vigorous effervescence and is soon complete. It is then taken out and dropped into cold water. By repeatedly boiling in fresh quantities of water it is thoroughly cleansed, then treated with paraffin wax, or the water may be expelled by alcohol. It is then dried, and finally impregnated with zapon. If the cyanide treatment is insufficient, any remaining rust may be removed by drills or other suitable tools. Hitherto this method has only been applied to a small number of objects, but there is no doubt that its use may be largely extended. Owing to the poisonous nature of the cyanide this method should be left to those who possess chemical knowledge. The disadvantage of the process lies in the difficulty of fusing large quantities of the potassium cyanide[135].
(4) Preservation of Medieval Iron Objects.
A complete treatise on this subject would be beyond the limits of a handbook, the following observations, therefore, will be sufficient for our purpose. The rust spots on objects of this kind are frequently only superficial and can be removed either mechanically by rubbing with pumice or emery, etc., or chemically by a concentrated solution of sodium sulphide[136]. To prepare this, sodium sulphide is dissolved in water, or flowers of sulphur are boiled in a solution of caustic soda. If the object is too large for immersion, the solution may be applied with a brush, and if the layer of rust is thick, the application must be repeated. After treatment the object must be rinsed in water and dried.