Wrought Iron Architectural Metalwork
Designed & manufactured in the U.K
By: Chris Topp & Co Ltd, North Yorkshire
Tel: 01845 501415
In its simplest definition 'Wrought Iron' is a specific type of iron, and the traditional material of the blacksmith, deriving its name from the word 'wrought', which is the medieval past tense of the verb 'to work'. Wrought Iron literally means 'worked iron', which refers to the method of manufacturing the metal by working repeatedly under a hammer. In the past the work of the blacksmith therefore became known as 'Wrought Ironwork', a name that has persisted for the art form even though the metal in use may not be wrought iron. Today the common material of the blacksmith is Mild Steel which is a cheap industrial product lacking many of the virtues of its ancestor.
Wrought iron is best described as a two-component metal consisting of iron and a glass-like slag. The slags are in effect an impurity, the iron and the slag being in physical association, as contrasted to the chemical alloy relationship that generally exists between the constituents of other metals. Wrought iron is the only ferrous metal that contains siliceous slag and it is to this slag that wrought iron owes the properties, which are of interest to the conservator and the blacksmith.
There are essentially two types of wrought iron:
Historically Wrought Iron has been worked by blacksmiths, using traditional techniques in both forging and construction, to make high end 'Decorative Wrought Ironwork'. Today however, the term Wrought Iron is becoming debased and misinterpreted, as demonstrated by any Internet search, to cover all ornamental ironwork, including cast iron and mild steel as well as incorporating modern construction techniques. The difference in quality and value is enormous. Whereas it would be unthinkable to repair historic stonework with concrete or cast stone and Portland cement, it is common for historic Wrought Iron to be repaired using mild steel and electric welding.
For those involved in conservation work it is therefore necessary to ensure the correct use of the term Wrought Iron and that both the material and the working methods are properly specified in order to ensure appropriate repairs and like for like quotations. A specification for such work is available free of charge for use in tender documents, please email us for a 'word document' - enquiry@christopp.co.uk
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:
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.
Wrought Iron Bottom Stile & Rail from 1858. Material sections still intact.
Arc welded insert - mild steel bottom stile & rail from 1950's. Material sections heavily corroded
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.
Given its limited availability and high price compared to Mild Steel, Wrought Iron is not always the first or right choice for all metalwork projects. Wrought Iron is ideally suited for external and traditionally forged work and although a number of blacksmiths and commissioners utilise these properties in new work, its prime use today is in the restoration and conservation of historic ironwork.
Modern conservation practice insists on the replacement of materials with like materials (BS7913: 1998 7.3.2.4). As Wrought Iron is available for the repair and replication of wrought ironwork, it is not appropriate to use Mild Steel or Pure Iron. It would, for example, be considered wrong to repair historic stonework with concrete or cast stone and a similar principle applies to Wrought Iron. Furthermore it is generally accepted that mild steel be used on external work should be zinc coating by galvanising or hot spraying. The intricate forms and water traps of traditional decorative 'wrought ironwork' are notorious hotspots for corrosion and as neither of these zinc treatments is permissible nor effective with ancient work, the use of Mild Steel is effectively ruled out.
Also please see TECHNICAL INFORMATION - click here
