They are then rubbed dry or are laid in hot sawdust, after which they are brushed over with melted lard and placed in this condition for at least half an hour in a moderately hot cupboard until the fat has penetrated into the finest pores of the iron. That this has really taken place may be proved by the use of a file.
When by this means all trace of sulphuric acid has been removed the fourth stage of the process is reached, viz. the removal of the grease from the surface and the subsequent application of some preparation to prevent the access of air and moisture. Most of the grease is removed by placing the objects in a warm place on blotting-paper. Any grease still remaining on the surface can be entirely removed with a cloth or paint-brush by means of benzine. If no restoration or repair is required nothing more is necessary than to apply the protecting solution."
A white varnish has much to recommend it from its protective power, but as it gives to iron an unsatisfactory gloss, it is preferable to use a solution of bees'-wax in benzine.
Having made use of Blell's method in a number of cases I have a few suggestions and modifications to offer. The heating should be carried out carefully and gradually, lest the sudden conversion of the moisture in the rust into steam should cause small explosions which would scatter pieces of rust. There is no danger of this if the objects are heated in an oven; they should not therefore be heated in an open flame. For smaller objects I use a box six inches [15 centimetres] square, of strong tin-plate loosely covered with an iron lid, or with a piece of asbestos sheet; but if the objects are large, e.g. swords, spearheads, etc., I heat them on a strong piece of tin-plate bent round to form a channel, and covered with a long piece of asbestos sheet, the edges of which are bent over the edges of the channel, to retain the heat as much as possible.
It is advisable, in my experience, to use the sulphuric acid well diluted, e.g. in the proportion of 1 to 20, and to renew it several times if necessary. In mixing concentrated sulphuric acid with water great caution is required on account of the evolution of heat. The acid should be poured in a thin stream into the water, but not vice versâ, and the mixture should be constantly stirred with a glass rod. If a glass vessel is used for the mixing, it must not be too thick lest the heat should cause it to break, but the larger the proportion of water to the sulphuric acid, the less considerable will be the rise of temperature.