THE RESTORATION OF ANTIQUE WROUGHT IRONWORK
TECHNIQUES AND TECHNOLOGY
It is worth remembering that the original maker of the ironwork was in business to earn a living and would have employed the most cost effective methods available at the time. Restoration work is likewise a commercial undertaking which must be economically viable for the blacksmith undertaking the project. There is no virtue in attempting to re-enact the original maker’s techniques if more modern methods will produce a result which is visually and functionally the same. The aim of the restorer is not to produce fake components which could be mistaken for originals. For this reason it is good practice to use a different material when carrying out restoration work so that any replacements are distinguishable from the originals and that any alterations are reversible should better techniques become available in the future.
A BRIEF SYNOPSIS OF VARIOUS TYPES OF IRON:-
Charcoal iron was the material used by wrought ironsmiths up until the mid Eighteenth Century and was produced by smelting iron ore in a furnace fuelled by charcoal. The charcoal furnace could only achieve a temperature of around 700ºC at which temperature the iron coalesces into a solid sponge-like mass (bloom) containing a large amount of slag but no carbon. Various carburizing processes were employed to introduce the 0.2% carbon typically found in wrought iron and the slag removed by repeated heating and hammering. This process left the charcoal iron with a slag content of around 2% distributed in fine layers or laminae.
By the end of the Eighteenth Century a new, much cheaper method of making wrought iron from pig iron had been developed. The pig iron was produced in coke furnaces which could reach 1200ºC, the so-called eutectic point where the iron, having become saturated with carbon (around 3.5%) becomes liquid and can be tapped from the furnace. The molten iron was stirred with rods (puddled) and passed between rollers enabling one ton of pig iron to be converted to puddled iron in just 45 minutes.
BRITISH STANDARD 51
It was not until 1910 that a British Standard (BS 51) for wrought iron was introduced providing a framework for the testing and certifying of the quality of new wrought iron for railway purposes. In 1939 this standard was extended to include wrought iron for general engineering purposes. The specification for Grade A wrought iron BS 51 : 1939 (Grades A, B and C), states that the manganese content should be no more than 0.10%. A further grade (Grade D) was added in 1959 for fencing which allowed a manganese content of 0.175%. The cold bend test for Grade A required that the material shall “without showing signs of fracture on the outside of the bend, withstand being bent cold through an angle of 180º round a former having a diameter equal to the diameter or thickness of the test piece until the limbs of the test piece are parallel”. The standard for Grade D required that the material be cold bent round a former twice the thickness of the test piece through an angle of only 90º.
Despite the huge variation in quality demonstrated by BS 51 and the different production methods employed over time, all of these materials are covered by the generic term “wrought iron”. The production of new wrought iron in the UK ceased in the 1970s due to the continuing decline in demand brought about by the introduction of mild steel.
The only source of wrought iron in the UK today is re-claimed scrap.
Pure iron is produced today by Corus in England in electric arc furnaces which are capable of reaching 1900ºC. At this temperature the pure iron becomes liquid and can be tapped from the furnace. Its primary use is in the manufacture of special steels and the pharmaceutical industry but it is also available for use by blacksmiths as an alternative to mild steel or wrought iron. Like mild steel it is a homogeneous material having no laminations. It is 99.8% pure iron which is chemically very similar to charcoal iron but without the 2% slag content. Perhaps a more familiar version of this new material is the ingot iron produced by Armco, a material well renowned for its corrosion resistance.
Mild steel gradually superseded wrought iron and is used for the majority of the new “wrought ironwork” created by artist blacksmiths today.
CHOICE OF MATERIAL FOR RESTORATION
By using a different material to the original it leaves no doubt as to which components are original and which are replacements should the written records of the restoration work ever be lost. Some eminent practitioners prefer to use mild steel which is cheaper than pure iron but less easy to forge. In practice, to distinguish between the three materials would require a detailed analysis because visually they are all the same. The choice of material is therefore based on practicality rather than aesthetics or ethics.
THE USE OF DISSIMILAR METALS IN CONTACT
It is often the case that where wrought iron fixings have been leaded into stonework, the wrought iron has corroded, de-laminated and the resultant expansion has caused damage to the stonework. One solution which is sometimes employed is to re-tip the wrought iron bar with either bronze or stainless steel to prevent the same problem from occurring in the future. However, extreme caution should be exercised in the use of dissimilar metals in contact.
All metals exhibit a tendency to be oxidized which is represented by their relative positions in the galvanic series. This series is called the Anodic Index and begins with gold at 0.00V through to beryllium at 1.85V. A difference of more than 0.15V between any two metals in direct contact will result in electrolytic action which causes one metal to corrode faster than it would if it were on its own and the other to corrode more slowly.
Iron and mild steel have an index of 0.85V and stainless steel 0.35V. The difference is 0.50V well above the acceptable figure of 0.15V. The result is that stainless steel tips welded to wrought iron bars will corrode more rapidly than stainless steel on its own. The same applies to bronze which has an index of 0.40V. The use of these materials is not ideal and will probably necessitate replacement in the future but it is preferable to using wrought iron which may risk perpetuating the original problem of damage to the stonework through de-lamination.
In a corrosion test carried out by Keighley Laboratories the wrought iron (wi) sample showed a 2% improvement in corrosion resistance over the mild steel sample (ms) and the pure iron sample (pi) a 22% improvement. In practice it is often the way in which the metal corrodes which has the greatest impact rather than the rate of corrosion. The grain structure of wrought iron can be likened to a multi-layered plywood which gradually de-laminates as water ingress enters the crevices between the laminae causing rust to form and expand the material to many times its original size.
Corrosion is a complex subject but it is known that crevice corrosion is a particularly aggressive form which happens when moisture enters a crevice between two metal surfaces and an anodic reaction occurs. This is likely to be the cause of the accelerated corrosion that can be routinely observed in unprotected wrought iron where the lack of a protective coat of paint has allowed moisture to enter between the laminations in the iron.
Mild steel and pure iron are both homogenous materials which, when exposed to the atmosphere will form oxides on their exposed surfaces but will not de-laminate. In the case of pure iron this oxide layer is a closely bonded microscopic layer which serves to protect the underlying iron from further corrosion.
Notwithstanding these variations a study of those structures which have survived the test of time reveals that their longevity is primarily attributable to the continuous care and attention they have received rather than the attributes of the raw materials from which they are made. The Eiffel Tower and the Forth Railway Bridge are both approximately the same age and both are subject to rigorous re-painting programmes involving prodigious amounts of paint. The Forth Bridge is in a harsh salt-water environment with adverse weather for most of the year. The Eiffel Tower is in a milder but more polluted atmosphere and yet both continue to give good service. The Forth Bridge was constructed from mild steel and the Eiffel Tower from wrought iron. For restoration work the Eiffel Tower authorities have chosen a structural steel, EN10025 because it has a “limite élastique plus importante que le fer puddle”. (A greater elastic limit than puddled iron.)
Wrought ironwork was traditionally fixed into stonework using molten lead. There is a theory that a leaded-in iron bar can be removed by heating it until the lead plug melts. In practice the heat created would dissipate into the surrounding stone (probably causing irreparable damage) whilst the lead is unlikely ever to reach a temperature sufficient to melt it. The generally accepted practice is to drill a series of small diameter holes around the bar taking care not to damage the bar itself until it is loosened sufficiently to be withdrawn. If the stonework is to be replaced it can be gently chipped away from the ironwork.
Careful records should always be kept during the entire restoration procedure. This is crucially important where dismantling is necessary. Each separate item should be numbered and labelled, preferably with a metal tag wired to it referencing its position on a clearly annotated detailed plan. Great care should be taken not to cause any further damage to components and those which are to be replaced because they are too fragile or too damaged should be carefully labelled, packaged and returned to the client.
Antique wrought ironwork typically has many layers of paint which require removal before detailed examination can be undertaken. It is more than likely that the paint will contain lead and require workshop procedures which meet current Health and Safety legislation. Various methods can be employed including blast cleaning which, when carried out by a skilled operator, will effectively remove all the paint layers down to bare metal without causing any erosion of the metal itself. For ornate ironwork which is in a particularly fragile condition a gentler option is flame cleaning. An oxy/acetylene torch set to an oxidizing flame is used to soften the paint which is then removed with a scraper or wire brush. This will leave a clean, bare metal finish. Any blackening or sooty deposits are a sign of poor technique i.e. too much acetylene creating a carburizing flame. After cleaning a coat of good quality metal primer should be applied immediately before any oxides form on the surface. This will ensure that the final paint finish has good adhesion with the metal thus providing effective protection in the future.
OUR POLICY ON THE RESTORATION OF WROUGHT IRONWORK
The restoration of antique ironwork raises many contentious issues, the answers to which are necessarily subjective. The first and most important issue is to ascertain from the client and any funding body which may be contributing to the cost of the project what they expect the finished result to be. They may want to return the ironwork as near as possible to its original appearance or they may wish only to arrest deterioration and repair damage with a minimum of intervention. It may be that poor restoration work has been carried out at some stage in the item’s history. Is this work to be undone and reworked or should it be left alone on the grounds that it is part of the history of the item? Has the item been altered at some stage in its history and if so should it be changed back to its original design as intended by its creator? If components are to be repaired or replaced what materials should be used? How should old paint layers be removed and should the item be stripped back to bare metal? Should traditional techniques be used or can modern methods be used if the result will look the same?
There may well be further issues for consideration and many different variables and criteria to be taken into consideration before any decisions can be made. We would not presume to have definitive answers to any of these questions but we do offer qualified advice based on our forty years of experience in restoration work.
To read more on this subject the following link is to a published article which includes contributions from three of Britain’s leading experts in the conservation of antique ironwork:- Master Blacksmith Don Barker FWCB, Russel Turner ACR, Director of Eura Conservation and Richard Quinnell MBE CWCB. www.buildingconservation.com/articles/antiquewrought/antiquewrought.htm