Blell’s Method. The method proposed by Blell and applied by him to many of the objects in his collection is distinct from that described above, although in its earlier stages the principle is the same. The following quotation is taken from the description of his method which the author read before the Antiquarian Society[122] at Königsberg:
“If a specimen is found to have a sufficiently strong core of iron it should be heated in the furnace to bright redness and then dipped into water. The expansion of the iron caused by the heat and the subsequent contraction caused by the sudden cooling thoroughly loosens the layer of rust. Large iron objects with a strong and firmly attached incrustation of rust will require a repetition of the process. By this means not only is the rust converted into a red powder which is easily rubbed off, but the object itself is rendered more suitable for the subsequent treatment. At the same time the heating process removes any coating of oil, fat, etc., which may have remained from previous attempts at preservation, and which would interfere with the further stages of the process. Smaller or delicate specimens should be treated in the flame of a spirit-lamp, but special care must be taken that there is sufficient iron present. Sword blades and other tools and weapons with sharp edges should be heated only, for the sudden cooling may cause cracks in the cutting edges.”
To complete the removal of the incrustation of rust which has been loosened by the heating process, or by the heating and sudden cooling, the object should be placed
“in a well-stirred mixture composed of one part by weight of sulphuric acid in nine parts of water. Bubbles of hydrogen will immediately rise and the rust will begin to separate. In freshly prepared acid objects which are not very rusty will be freed from rust after four to six hours, those covered with a deeper layer of rust in about twelve hours, but several days, or even weeks, may be necessary. The duration of the process depends upon the strength of the acid and the character of the rust, viz. whether it is thick and solid, or thin and porous, and whether the iron is of a soft, or of a hard character.
When first making use of this method it is advisable to use dilute acid and to take out the objects several times in the course of the day and examine them, while during the night they should be taken out of the acid and placed in soft water[123].
For the acid bath and for rinsing it will be found convenient to have two pairs of wooden troughs having the following internal measurements:
(1) An internal length of 10 inches [25 cm.] by 71⁄2 inches [19 cm.] in breadth and 43⁄4 inches [12 cm.] in depth, which will be useful for the larger number of objects.
(2) For long narrow objects, e.g. sword-blades, and long spear-heads, the internal measurements should be 40 inches [100 cm.] long by 4 inches [10 cm.] broad and 3 inches [8 cm.] deep.
Small fragile objects are most satisfactorily treated in glass vessels or glazed earthen pots or vases.
The acid must have free access to all parts of the object; if a sword, for example, lies flat upon the bottom, the under-surface apparently remains unacted upon by the acid. This should be remedied by the use of a couple of small wooden supports.