(19) Crace Calvert some years ago drew attention to the fact that steel after immersion in caustic soda or caustic potash is preserved from liability to rust. This apparently valuable information does not seem to have been acted upon by chronometer makers aud others, to whom any method of securing immunity from rust would be of considerable service. Balance springs for chronometers have been occasionally coated with collodion, but the thickness and rapid decay of this film interferes with the timekeeping of the chronometer, and is therefore objectionable. On inquiring of one chronometer maker, we were told that he had treated some springs with caustic soda at the time Calvert published his work, and that these springs were still free from rust; but as they had not been exposed to damp with other springs not so treated, he was unable to deduce any opinion as to the advantage of the process. So it is generally. Practical men are continuously occupied with their work, and have unfortunately no time for experiments, though they suffer day after day from an evil the removal of which is probably within their grasp. (Mechanical World.)
(20) To preserve iron from rust in Ceylon it requires to be first well scraped and cleaned; and if for outside work, such as bridges, roofs, girders, etc, it should be well coated with tar paint. The following receipt will be found to answer well, viz. coal-tar 9 gal., slaked lime 13 lb., turpentine or naphtha 2-3 qt. The use of the lime is for neutralising the free acid in the tar. This paint is very fluid, and well adapted for roofing, galvanised or black corrugated iron, etc.
(21) At Port Chester, Pa., pipes have been treated by a new process, which is said to have advantages over the Bower-Bar if process. Haupt describes the plan as follows: After the pipes have been lowered into the retorts by means of a traveller, the retorts are closed for about 15 minutes until the contents are heated to the proper temperature. Steam from the boiler at 60 lb. pressure is then introduced into the superheater, which it traverses, and from which it escapes at the temperature of the iron, upon which it acts for about one hour. A measured quantity of some hydrocarbon is then admitted with a jet of steam, followed again by a fixing bath of superheated steam, which completes the process. The absence of pressure and of explosive gases is a proof that all the operations have been nicely regulated, and that a perfect absorption and union of the carbon, oxygen, and hydrogen with the iron has been effected. The protection is said to be a real conversion of the metal to a certain depth into a new material.
(22) Too much stress cannot be laid upon the condition of the surface of the iron at the time of coating; and it is perfectly essential either to have a dry surface or else a composition which is not affected by water. Prof. Leives remarks that when an old iron structure is broken up, on the backs of the plates may often be seen the numbers painted on them in white lead and linseed oil when the work was put together, and under the paint the iron is in a perfect state of preservation, the secret being that the paint was but on while the plates were hot and dry.
Compounds prepared with boiled linseed oil are open to objection, on account of the presence of lead. The drying of boiled linseed oil is due to the fact of its containing a certain quantity of an orgauic compound of lead; and the drying property is, moreover, imparted by boiling it with litharge (oxide of lead), so that lead compounds are present even when the oil in not mixed with red or white lead pigment. When boiled oil dries, it does so by absorbing oxygen from the air, and becomes con-serted into a kind of resin, the acid properties of which also have a bad effect upon iron. Protectives of the class of tar and its derivatives, such as pitch and black varnish, and also asphalt and mineral waxes, are regarded by Prof. Lewes as among the best. Certain piecautions, however, must be taken in the case of tar and tar products, both of which are liable to contain small quantities of acid and ammonia salts If care is taken to eliminate these, and if it could be contrived to always apply this class of protectives hot to warm iron, the question of protection would be practically solved; bituminous and asphaltic substances forming an enamel on the surface of iron which is free from the objections to be raised against all other protectives - that is, of being microscopically porous and therefore pervious to water.
Spirit or naphtha varnishes are condemned by Prof. Lewes as open to several objections. Varnishes to which a body has been given by some pigment, generally a metallic oxide, are preferable to the last class, if the solvent used is not too rapid in its evaporation, and if care has been taken to select substances which do not themselves act injuriously upon iron, or upon the gums or resins which are to bind them together, and are also free from any impurities which could do it.
At the present time, as the author truly remarks, the favourite substance for this purpose is the red oxide of iron; but care should be taken to exclude from it free sulphuric acid and soluble sulphates, which are common impurities and extremely injurious. The finest coloured oxides are, as a rule, the worst offenders in this respect, as they are made by heating green vitriol (sulphate of iron), and in most cases the whole of the sulphuric acid is not driven off, the heat required being injurious to the colour. The acid is often neutralised by washing the oxide with dilute soda solution; but very little trouble, as a rule, is taken to wash it free from the resulting sulphate of soda, which is left in the oxide. The best form of oxide of iron to use for paint making is obtained by calcining a good specimen of hematite iron ore at a high temperature. When prepared in this way, it contains no sulphates, but a proportion of clay which is harmless if it does not exceed 12-18 per cent. Paint makers can easily test their red oxide for soluble sulphates l>v warming a little of it with pure water, filtering, and adding to the clear solution a few drops of pure hydrochloric acid and a little chloride of barium solution.