Previous  |  Next          

Watchmakers' Hand Book

Part II,
Page 1


      54. Iron is an elementary body, that is to say it cannot be decomposed. It is the most tenacious of the metals, having a breaking strain of about 75 kil. g per sq. mm. (or 106,000 pounds per sq. inch) of section. two pieces can be perfectly welded together when raised to a white heat.

      In the smaller horological appliances, the metal is not employed except after conversion into steel. In common clocks it is used from motives of economy, for forming pins, screws, etc. In turret clocks, however, considerable use is made of it, many of the parts after they are formed being cemented, that is to say, having their surface rendered hard in a manner subsequently indicated (65).

      Such a mode of manufacture is particularly applicable to pieces that are subject to a constant succession of impacts; their hardened, steelified surface resists wear, while the iron core affords security against rupture.

      It is important to carefully distinguish the cases in which iron is preferable from those in which its substitution for steel serves merely to augment the profits of manufacture.

      The fracture of a good piece of iron is characterized by long twisted fibres of a brilliant color.

      If heated frequently or carelessly, the quality of the metal is impaired-it ceases to befibrous and looses its tenacity; in this condition it is said to be burnt.

      It is better to work with a charcoal or gas fire, as coal acts more rapidly in rendering the metal brittle. Cold hammering, or "hammer hardening," also makes it brittle and diminishes its tenacity, but this is again restored by a suitable annealing.

      Iron dissolves slowly in dilute nitric acid; if not diluted, this acid rapidly oxidizes it. Dilute sulphuric acid dissolves the metal easily, but if concentrated, it has no action in the cold, whereas, on heating to ebullition, the iron is dissolved with the evolution of sulphurous acid gas. It is also dissolved by hydrochloric acid, or acqua regia.

      Iron is less magnetic than steel, especially hardened steel, which, owing to its great coercive force, is magnetized with greater difficulty, but retains its magnetism for a longer period. Indeed, soft iron, if properly prepared, can be magnetized and demagnetized instantaneously.

      Some workmen can distinguish iron from steel by the musical note emitted on striking. A more certain method, however, consists in using dilute nitric, or sulphuric acid. If the srface remains unaltered, or nearly so, when touched with a drop of either acid, the metal is iron, but, in the case of steel, a black mark will be left, owing to the liberation of carbon.

      55. To Remove Rust. The usual mode is to rub the object with a piece of oiled rag, or emery paper. It appears that more rapid and more satisfactory results are secured by using pure petroleum, and wiping with a hempen or woolen rag.

      56. To Prevent Rust. Dip iron or steel articles in a mixture of equal parts of carbolic acid and olive oil, rubbing the surface with a rag. Others rub the metal with a mercurial ointment, leaving a thin layer over the entire surface. It is stated that, if iron be dipped in a solution of carbonate of potash or soda in water, the surface will be protected against rust for a long time, and objects can be protected for any period by burying in quicklime. Rubbing the surface with plumbago has a similar effect, and Bariff has pointed out that, by exposing iron to the action of steam, heated above the boiling point of water, a coating of magnetic oxide of iron is formed, wich is equally serviceable.

      57. To restore iron and steel that has been burnt, or badly forged. When iron is burnt, or carelessly forged, it becomes crystalline and britttle; in order to restore it to its original condition, a fresh and very careful forging is generally needed. This can be avoided by having recourse to the following method, suggested by Caron: it consists in treating the metal somewhat after the manner adopted in hardening steel.

      He experiemented with a bar of good iron, which was easily beent when cold. without breaking or showing any cracks. It was then burnt and became brittle when cold, the fractured surface showing brilliant shining facets.

      Prepare a boiling saturated solution of sea salt, heat the piece of iron to a bright redness, and plunge it into the bath until it is of the same temperature (about 110° C. or 230° F.) After undergoing this operation, it is found that the metal can be easily doubled in the cold, exactly as it did before being burnt.

      Perret states that steel which has been deteriorated by frequent hardening can be restored as follows: Heat it short of dull redness and quench in melted tallow, repeating the operation, if necessary, when the steel may be again hardened in the ordinary manner, and will be nearly, if not quite, restored to its original condition.


      58. This is only used in the manufacture of tools and large clocks; the employment of cast iron wheels in the striking train of such clocks has materially reduced their price.

      Like steel, it is a compound body, consisting mainly of iron and carbon. Cast iron, however, differs from steel in the quantity of carbon present, for, whereas its proportion in cast iron varies from 2 per cent. upwards, there is never, in steel, an amount exceeding 1.5 per cent., and even .5 per cent. renders an iron hard, converting it into "mild" steel.

      Cheapness is not the only argument in favor of the use of cast iron. In virtue of its molecular structure, this material offers a considerable resistance to a crushing strain, so that the teeth of wheels, made of carefully selected cast iron, will work for a long time without sensible wear; moreover, the founder's art has made such important advances that there is no difficulty in casting, to a constant pattern, a wheel, together with the pinion that it carries, and any other projections, etc., that may be required; this economizes labor to a very great extent.

      The use of cast iron in the construction of certain classes of wheels, and parts of tools, presents advantages which we cannot afford to ignore; but it must be carefully observed that this material is not suitable in cases where great accuracy in the acting parts is required, as it cannot, like brass and steel, be conveniently worked by the turning tool or file. In recent years, however, this difficulty has been overcome by the introduction of what are termed 'malleable castings," produced as follows:

      59. Malleable Castings. The object is first made of ordinary cast iron, and the invention consists in rendering this malleable by the removal of the carbon that has served the very important purpose of rendering the metal fusible. In large cast-iron pots, the castings are laid with alternating layers of powdered red hæmatite, and the whole is kept at a temperature of about 900° C. (1,650° F.), or cherry-red heat, for 72 hours. on cooling, the castings are found to consist of nearly pure iron, and to be perfectly malleable, and, therefore, workable.

Submitted by: Samuel Kirk (##)

Previous  |  Next