Why Use Wrought Iron?
To the naked eye there is no visible difference between forged bars of Wrought Iron & Mild Steel (its modern day equivalent) however Wrought Iron is not distinguished by its looks but by its working properties and its resistance to corrosion. Wrought Iron is softer to forge, as it is workable at a higher heat than Mild Steel making it more ‘malleable’ under the hammer. However it is Wrought Iron’s superior weathering properties that are most notable.
While Wrought Iron does, in common with all ferrous metals corrode in certain circumstances, (see maintenance), evidence of its durability and long life expectancy is commonplace in our towns and countryside. Victorian town house railings – Wrought Iron two hundred years old. Vast ornamental gateways to eighteenth century great houses – Wrought Iron three hundred years old. Stone cramps on Tudor bridges – Wrought Iron five hundred years old. The simple fact that so much Ornamental Wrought Ironwork survives, often with little or no maintenance, speaks volumes for the material.
Problems with the rapid corrosion of Mild Steel in comparison with Wrought Iron were well known by the beginning of the twentieth century and this prompted Matthew Verity, in USA to investigate. He concluded that the carbon in Steel is responsible for its corrodibility; leading to the theory that removing the carbon removes the problem of corrosion. His efforts to produce pure iron resulted, not in a metal which was proven to have any resistance to corrosion, but which was very tough and malleable and found favour in the emerging markets for mass-produced cold pressings; ARMCO iron. No convincing evidence was ever produced to back up claims of the non-corrodibility of pure iron and in fact Pure Iron manufacturers themselves make no claims to its corrosion resistance. In fact these same manufacturers offer other materials known as ‘Weathering Steel’ in Britain, or ‘Corten Steel’ in the USA, for which they do make such claims. One of the chief uses for Pure Iron in today’s industry is as sacrificial anodes to protect steelwork tanks and ships as it has been found to corrode preferentially to mild steel.
Verity’s examination of puddled wrought iron and mild steel was a chemical analysis, which appears to have overlooked the presence of the slags. During the manufacturing process, wrought iron does not become molten, as do more highly refined metals, so that impurities are included in the matrix of the iron rather than being separated and disposed of. At these high temperatures the impurities are turned to glass, and are commonly known as ‘slag’ consisting of carbides and silicates which give Wrought Iron its fibrous structure; approximately 250,000 siliceous fibres appear in each cross-sectional square inch of good quality wrought iron. A specific test is required to discover slag of which Wrought Iron contains up to 5% however it is this slag which provides wrought iron its corrosion resistant properties.
The slag present in the structure of wrought iron inhibits corrosion in a number of ways:
- Slags themselves are non-corrodible and serve as an effective mechanical barrier against the progress of corrosion.
- The structure of the iron gives rise to a very rough (microscopically speaking) surface texture, which interlocks with the oxide layer, be it rust or mill scale, preventing it from flaking off the surface. The oxides therefore act as a protective coating preventing further corrosion.
- Electrically speaking, where the slag appears on the surface it acts an insulator between the areas of reactive iron, retarding electrolytic action.
The important corrosion properties of wrought iron are therefore due to its impurities in the form of slag. It follows that iron and steel without the slag will not exhibit the same corrosion resistance, and this is what is found in practise. Furthermore the slag has additional advantageous in relation to traditional forging techniques, especially fire welding.
When Wrought Iron is heated in the fire to a high temperature the slag melts and covers the surface of the iron in a rather similar manner to flux. This glassy layer retards oxidation to the extent that the iron can be heated rather more than purer metal without burning and for this reason Wrought Iron is more malleable to forge than any other metal but more importantly this is why it is so beautiful to fire weld, the slag acting as a flux. Smiths often comment on a property of wrought iron, which renders it softer to forge than even pure iron. This is brought about by the slags, which melt within the iron at forging temperatures and act as an internal lubricant, reducing internal friction and hence resistance to distortion under the hammer.