In addition to these methods, there are others which are of an intermediate kind, either special or a combination of methods from both these groups.
(1) Methods of preserving Objects of Iron without removal of the Rust.
IMPREGNATION. The earliest processes, which are to some extent still in use in some collections, are simple impregnation methods, in which the object is either painted once or more with the impregnating medium by means of a brush, or is placed directly in the medium itself. In either case the penetrating power of the solution used is directly proportional to its fluidity.
The following media may be used for the purpose:
(5) Rubber solution in carbon bisulphide. The mass after swelling is dissolved in benzine[110].
(6) Copal varnish diluted with turpentine.
(7) Copal varnish mixed with linseed oil[111].
(10) Linseed varnish mixed with an equal quantity of petroleum.
(16) Oleate of lead: 100 grammes of olive oil, 100 gr. of lead oxide, and 100 gr. of water are boiled until all the water has evaporated and the mass has become grey. The mass is extracted by shaking it with alcohol, and the residue is dissolved in absolute ether, in the proportion of 100 gr. of ether to 5 gr. of the substance. Before use it should be diluted with a little ether[112].
(17) Speerschneider's mixture. This consists of 8 parts of rape oil, 1 part of bees'-wax, 1 part of pine resin, and 2 parts of benzene[113].
(18) Collodion, or the mixture used in the Museum at Donaueschingen, which consists of 30 grammes of collodion, 2 gr. of camphor, and 1 gr. of oxalic ether.
In addition to these materials, there are other mixtures of resin, varnishes, and bees'-wax, with their appropriate solvents, but they do not possess any special advantages as impregnating solutions.
After treatment with size or isinglass, iron objects may be given when dry a coating of linseed oil, linseed varnish, solution of shellac, etc.
The materials numbered 7 to 10 in the above list should be applied warm to enable the viscid fluids to penetrate the rust, for the more readily the solution enters the object the better is the result obtained. Apart from the fact that they are easily ignited at a high temperature, they must not be heated beyond 230°F. [110°C.], otherwise objects which consist largely of rust will fall to pieces[114].