(13) Petroleum.
(14) Vaseline.
(15) Melted paraffin.
(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].
In the process of impregnation a twofold result is aimed at, viz. to prevent the rust from crumbling, and to exclude air from the specimen. The application of heated linseed oil or linseed varnish is founded upon the supposition that these substances enter into a chemical combination with ferric oxide to form a stable compound; this is, however, disputed by some modern authorities[115]. Neutral substances offer a safer method for the exclusion of air, and of these melted paraffin is undoubtedly the best. The paraffin must be quite pure and free from stearine, as can be ascertained from the melting point; thus pure paraffin melts at 130°-150°F. [55°-65°C.], stearine at 160°F. [70°C.]. Paraffin with a melting-point higher than 65°C. should be looked upon with suspicion.