A guide to architectural metalwork, wrought iron and its restoration

Our next issue will feature a fascinating insight, exploring metallurgy, production techniques, its architectural and historical context and best practice conservation.

Wrought iron, normally produced from pig iron, is a ferrous carbon alloy with a significantly lower carbon content (less than 0.1%) than cast iron or steel. Wrought iron is produced with slag inclusions (the top layer of melt formed during smelting containing oxides and other impurities) which gives wrought iron a characteristic fibrous nature. The slag provides a range of material properties including, toughness, ease of welding, resistance to corrosion, malleability when hot and tensile strength.

There are two main types of wrought iron. Pre the 18th century, Charcoal Iron which was produced in a bloomery using charcoal was primarily used. Its production process resulted in signification variations in its composition. After the 18th century puddled iron, considered the first large scale process in iron production, was becoming widespread in use.

Bloomeries

The earliest production method of wrought iron was in a bloomery - A stone or clay wall furnace with a pit and chimney. A pipe near the base of the pit allowed for airflow either from a natural source or by bellows. The bloomery required charcoal both as a fuel and as a source of carbon monoxide in the refinement process.

Once the furnace was heated, iron ore would be mixed in a 1:1 ratio with charcoal, and fed into the bloomery. The air forced through the furnace would fuel the fire to temperatures just below iron’s melting point.

The carbon monoxide would force the naturally occurring oxides out of the ore leaving impure metallic iron to fall to the bottom of the furnace, where naturally occurring molten slag ( a compound of silicon, oxygen and other impurities from the ore) which welded together to form a spongy mass known as bloom.

The final refinement stage came from the beating and forging process (blacksmithing) once the iron was removed from the furnace. The hammering and working of the product would drive out some of the slag and turn it into the useable “wrought” iron.

Production was both small scale and expensive. Early bloomeries could smelt no more than a kilo of iron with each firing and had limitations to the temperatures they could reach. It wasn’t until the medieval period and the development of mechanised smelting, that wrought iron production increased.

Blast furnaces

Blast furnaces increased production from around 15kg to 300kg per firing. Bellows powered by waterwheels fed air into the furnace reaching temperatures over 1400˚c which when combined with charcoal was sufficient to melt iron.

The molten iron (now cast iron and of a high carbon content) was then fed out of the furnace into ingot moulds called pigs to cool down. Aptly named pig iron after the billets, it was useless to the blacksmith at this stage, and needed converting back into wrought iron in a finery.

Heated in a furnace, the carbon and other impurities were removed and the iron was worked by repeated hammering and folding until it was sufficiently malleable for the black smith to use.

The quality and output of iron increased and production became cheap enough in the mid 1600’s for it to be used in a variety of applications. With the increased demand, a charcoal shortage ensued in the late 17th century and alternative smelting methods were developed.

Abraham Darby, famous for his Coalbrookdale furnaces, started to use a purified form of charcoal called coke to fuel production. Carbon no longer contaminated the iron with impurities formed when burning with charcoal and furnace efficiency improved greatly.

Puddled Iron

In the early 1780’s, ironmaster Henry Cort, started to develop alternative refining processes for converting pig iron to wrought. In 1784, he patented the reverbatory furnace (separate chambers for fuel burning and refining) and the puddling technique, which eliminated charcoal from wrought iron production.

The puddling technique involved the molten cast iron being manually stirred in a hearth to agitate and remove the carbon via the oxidised gases within the furnace. The resulting mix of low carbon content molten slag and iron was then collected in large lumps and removed from the furnace to be rolled to form iron bar which could later be worked.

This large scale production technique enabled the significant expansion of iron use throughout Great Britain, responsible in part for some of the momentum behind the Industrial Revolution. At the same time, steam power developments meant furnaces could now produce great quantities if quality wrought iron for use on construction.

History and architectural applications

The earliest documented iron items are a series of small beads dating back to around 3000BC. Discovered in Northern Egypt they were formed from meteoritic iron and shaped through hammering. The Iron Age in Northern Europe however, is dated to have begun somewhere between 1100 and 800BC.

Commonly considered to have occurred directly after the Bronze Age, historians now argue ironworking techniques were perfected alongside bronze manufacture. Weapons and tools were now being made predominantly from iron.

The same extraction and refining techniques, honed in Britain between 800BC and 43AD remained practically unchanged until the development of the blast furnace in the 15th century. Small scale and expensive, wrought iron use was confined to coins, ornaments jewellery and some fixtures and fittings in ecclesiastical and noble dwellings. Some of the most elaborate examples of locks and keys were worked from iron.

Architectural and decorative wrought ironwork really took off in the Middle Ages. In the 13th century, a new technique called stamping was developed, which allowed patterns to be produced by hammering wrought iron into a detailed die. Rosettes, leaves and faces were commonplace designs. Later iron could be cut into shape.

Doors and windows were soon covered with intricate scrolls, curves and crescent primarily for security and reinforcement against invasion. It was used for moving parts such as hinges, handles and machinery, strap joints in timberwork and for tie rods in masonry.

After the blast furnace, craftsmanship reached new heights and ironwork became increasingly sophisticated and decorative. Baraoque styled, it began to be used for stately gates and railings, balconies and patios.

Following Colt and Darby’s innovations in the late 1700’s a mix of cast and wrought ironwork, hit an all-time boom. Decorative architectural wrought iron was now commonplace across many British cities.

The advent of steam power and mass production saw railways and canals spring up across Britain with great momentum in the 1800’s and demand for wrought iron further increased. 25,000 bridges were built between 1830 and 1855 and a mixture of cast and wrought iron were used widely to support the new steam trains.

Wrought iron continued to be used in many prominent buildings and structures across Britain during the 1800’s but eventually cast iron, which was cheaper to produce en masse, overtook wrought iron in decorative and structural applications.

The turn of the century, a few catastrophic structural cast iron failures and innovations in the mass production of cheap mild steel saw the age of both cast and wrought iron dissipate. The last wrought ironworks ceased production in the early 1970’s and all subsequent wrought iron available is now recycled, only obtainable from a single producer in the UK.

Restoration Techniques

As evidenced, architectural ironwork had a huge range of applications from ornamental to structural. This, paired with historical significance will determine any conservation plan and have obvious implications on which restoration methods, if any, may be employed.

Since wrought iron is now only available as a recycled material, its retention and sympathetic restoration is even more important. Generally, the aim is to stabilise, halt or slow further deterioration through cleaning or recoating and where necessary repair.

Mechanical Cleaning

Needle guns and descaling chisels can quickly and efficiently remove thick layers of paint and rust. Nominal dust is produced making it a suitable method when dealing with ironwork covered in hazardous lead based paints. Care does need to be taken however as there is a significant risk of surface damage.

Water-based Cleaning

An effective way to remove oil, rust, loose paint and dirt warm/cold and high pressure water jetting are all techniques employed with wrought ironwork. Detergents can also be added when necessary to help remove any build-up of salts and oils.

Blast Cleaning

Dependent on application, a variety of abrasive mediums are used in blast cleaning, carried in a stream of high pressure air and propelled towards the ironwork. Whilst fast and effective it is also particularly aggressive.

Dry blast cleaning is primarily only used where wrought iron is serving a structural purpose and where there is no fine detailing that could be damaged. Water is often added to provide a cushioning effect particularly useful when dealing with fine ironwork, though it does slower and less effective.

Flame Cleaning

Flame cleaning is particularly suitable for wrought iron. The treatment involves heating the iron to soften and remove the paint layer and any loose rust or mill scale - useful when dealing with ornamental and delicate wrought ironwork.

Chemical Based Cleaning

Phosphoric acid based chemicals can be applied to the surface of wrought iron to remove a build-up of corrosive material and old paint. Great care needs to be taken to avoid accumulation of chemicals in the micro structure of the iron.

Chemical baths involving a mix of acid and corrosion inhibitors can also be employed in some circumstance but require dismantling the ironwork into individual components.

There is an argument against chemical cleaning methods where possible. The process can remove the mill scale layer from the iron, which acts as a natural protective layer against corrosion.

Forge welding

Forge welding is the traditional method for forming and repairing wrought iron whereby two sections are brought to a high temperature in a forge and hammered together, normally by hand, to form a joint. This type of repair normally recovers about 80% of the original component strength.

Whilst the most unobtrusive and preferred method, it isn’t always possible to dismantle and move larger pieces of wrought ironwork to a hearth. In such instances, other in situ repair techniques would be employed instead.

Hot set riveting

Another traditional joining technique, conical or round head rivets are heated until white hot and placed through pre drilled holes. Still hot, the rivet expands to fill the hole and the protruding end is hammered, either by hand or using a hydraulic tool on larger sections, until it forms a head over the other side. Left to cool, the rivet contracts pulling the two sections together forming a tight joint.

Arc Welding (MMA, MIG and TIG)

Modern welding techniques are often used instead of forge welding for in situ repairs and in the case of structural components. Care needs to be taken to choose a suitable alloy.

Manual Brazing

Suitable for small scale non-structural repairs, manual brazing uses a bronze or brass filler rod coated in flux. The iron is flame heated to around 600˚c and the filler then melted and applied on or near the joint in need of repair using a gas flame. To braze metals together a very tight joint must form so as the filler can be drawn in successfully.

Article published with the kind permission of Dorothea Restorations. With thanks to Geoff Wallis of G Wallis Conservation for the use of his material in the research and production of this article.

For further information visit www.dorothearestorations.com

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