Understanding & Conserving HistoricIronwork

A One-day Course for staff and volunteers looking after historic collections

Wednesday 9 March 2016

Wheal Martyn, Carthew, St Austell

Peter Meehan, Historic Metalwork Conservation Consultant

Understanding & Conserving HistoricIronwork

An Introduction to Wrought and Cast Iron

Peter Meehan, Historic Metalwork Conservation Consultant

1. Introduction To Iron (Fe)

Iron (Fe) is a metallic element, being lustrous and silver in colour. It is soft in its pure state and has magnetic properties.It is the sixth most abundant element in the Universe and makes up 5% of the earth's crust. As it readily oxidises in air it is not normally found in its pure state but as a compound. The most common of these are Haematite (Fe2O3), Magnetite (Fe3O4), Siderite (FeCO3) and Limonite (Fe2O3, H2O).

A sample piece of Haematite Ore

Iron has to be extracted from its ore by a process known as Chemical Reduction removing oxides and other impurities. Iron ore is the raw material used to make Pig Iron.

The process of reducing iron ore to Pig Iron takes place in a Blast Furnace.The key ingredients are iron ore, limestone and coke built up in the furnace in alternate layers. The limestone is added to aid liquid slag formation – to remove impurities. It is less dense than Fe so forms on top of the liquid metal.

Hot air is blown up through the raw materials to produce the very high temperatures needed for the Reduction reaction to take place.

In simple terms a chemical reaction takes place in the furnace as follows:

2Fe203 + 6C + 3O2 4Fe + 6CO2

Haematite + Coke Metallic Iron + Carbon Dioxide gas

This reaction relies on the fact that Iron (Fe) has a greater affinity for Carbon (C) than Oxygen (O).

The molten iron is drawn off at the bottom of the furnace, originally onto sand, flowing into a simple mould to form 'Pigs'.

Like several elements, Iron has the ability to exist in several different forms or Allotropes. This occurs where the atoms in the Element bond together in different forms. Another example is Carbon that can exist as Graphite or Diamond.

Iron can exist in three Allotropic forms:- Ferrite (alpha, α iron)- Austenite (gamma, γ iron)- Delta (delta, δ iron)

This is important as it allows us to manipulate its structure to produce irons and steels with different properties.

2. Principle types of Iron

As pure iron is very soft, it is almost always found as an Alloy, and historically we need to consider the three basic forms man has produced:

● Wrought iron● Cast iron● Steel

All are essentially alloys of Iron (Fe) with Carbon (C), but usually have some other trace elements present as well that alter the metal's properties.

Wrought Iron

Wrought iron is the earliest form of iron used. As pure iron melts at 1,535oC, it was not until the Industrial Revolution that man was able to readily produce it. Wrought iron was originally produced by mixing iron ore and charcoal together in a furnace known as a Bloomery. The charcoal was lit and air blown through it (via bellows) to raise the temperature to a point below the melting point. Impurities present melted and ran out. Carbon monoxide gas (CO) from the charcoal reduced the iron oxides to a spongy iron mass, known as a Bloom.

The Bloom was repeatedly hammered and heated in order to remove impurities and consolidate the metal – hence the term Wrought Iron.

Wrought Iron

The iron bloom is consolidated by repeated hammering.........

…...... before repeated rolling to produce the shapes required.

Wrought IronWrought iron typically has less than 0.25% carbon present, most of it having been driven out during its forging. It has a characteristic fibrous structure that formed whilst being worked. It also has slag inclusions present that can be seen under a microscope.

As wrought iron is a malleable material it can be (and was) worked into many shapes and forms.

Wrought IronWrought iron production was only small scale until the 14th Century when the blast furnace was developed allowing larger quantities to be produced.

The iron was used both in structural applications and decoratively, with church buildings having some of the earliest known wrought iron present. This includes 14th Century wrought iron straps that support the tower of Salisbury cathedral.

Many churches also had decorative ironwork screens inside, such as the one illustrated from St John's church in Frome.

Wrought IronIron was traditionally worked by blacksmiths using a forge and anvil. Their skills at working the material led to the creation of some fantastic decorative ironwork.

Wrought IronPerhaps one of the greatest exponents of the blacksmith's craft in England was Jean Tijou. He created the famous Hampton Court screens in 1690's. Today he is seen as one of the father's of the craft.

An example of Tijou's work at Burghley House: the 'Golden Gates'.

Cast Iron

Cast iron was the 'new material' of the Industrial Revolution. It had been made in small quantities prior to this, the earliest in China. It was also used in Europe for items such as cannon balls, but had limited uses as it was found to be brittle.It's champion was Abraham Derby. He patented a method of producing thin cast iron cooking pots in 1707. He produced re-usable patterns so that uniform multiple pots could be cast.

Cast IronTraditionally, charcoal was used in the manufacture of iron. It was made from timber and the felling of trees meant its supply was becoming increasingly difficult. Other materials like coal, had impurities present that spoiled the iron.The key discovery was that Coke could be used in the process, rather than charcoal. Coke is made by heating coal to a high temperature in an oxygen-free atmosphere. This process removes all impurities. Coke revolutionised the production of cast iron.

Cast iron was produced in a blast furnace with the molten iron running out of the bottom of the furnace to produce 'pig iron'

Cast IronCast iron typically contains 2 – 4% Carbon and between 1 – 6% of Silicon. It can be contaminated with Sulphur and Phosphorus. It has a characteristic crystalline structure.

Traditional cast iron has graphite flakes present throughout its structure. This is what causes it to be brittle.

Cast IronThere are several 'types' of cast iron:

● Traditional cast iron or 'grey iron' - Contains graphite flakes. Has good wear resistance and can be easily machined.

● White cast iron – characterised by the presence of iron carbides, forming due to the quick cooling process. It is a very hard material and has good wear resistance

● Ductile iron – has nodules of graphite present formed as the result of the addition of magnesium. Produces an iron of high strength and ductility

● Malleable iron – formed when white cast iron is heat treated to produce a more ductile iron

Structure of Ductile iron; also known as Spheroidal Graphite Iron (SG Iron).

Cast IronCast iron objects are made by pouring the molten metal into suitable moulds. In order to make a mould a pattern is required: an exact copy of the item to be cast is made in another material. This would traditionally be of wood. Today other materials are also used – resin, wax, plaster.Patterns need to be made slightly larger to allow for shrinkage of the metal on cooling.

Cast IronPatterns need to be made carefully so they can be removed from the moulding material – normally sand – to leave a void into which the metal will flow. Moulds are normally made in two halves.

Preparing a mould for casting

Cast Iron

Complicated shapes can be cast but the foundry man needs to incorporate 'cores' into the mould. The simplest cores allow for casting hollow items like the rainwater pipe opposite.

Traditionally 'Green Sand' was used as the mould material. Today foundries use a sand/resin mix.

Cast Iron

Pouring cast iron at Barr & Grosvenor Ltd, Wolverhampton.

Cast IronA raw casting removed from the sand mould.

Cast Iron

Raw castings need to be finished by removing the' flash lines' formed at the joins in moulds. These are carefully ground out in a process known as 'fettling'.

Steel

Steel was known in Antiquity but was only made in small amounts in Asia and the Middle East.Modern steel was developed as part of the Industrial Revolution in the 17th Century. It was made from either 'bar iron' or pig iron. Placing wrought iron in a crucible and melting it created crucible steel. This was cast into ingots.

Modern steel making came about as a result of the development of the Bessemer Process, named after Henry Bessemer, who patented it in 1855. Pig iron was converted into steel by a process of oxidation with air being blown through the molten metal.

Steel

Modern steel comes in many forms as its properties can be manipulated during manufacture to produce a material for a specific purpose.

SteelMild steel typically contains 0.2 – 2.0% Carbon with some other alloying elements added including manganese, chromium, vanadium and tungsten. Varying these alters the steel alloy's hardness, ductility and tensile strength. Stainless steel has at least 11% Chromium added.

With the development of the Bessemer Process, the cost of steel manufacture fell significantly. It made the continued production of wrought iron, which was much more labour-intensive, uneconomic. As a consequence wrought iron went into a rapid decline.

Early Bessemer steel.

SteelMild steel typically contains 0.2 – 2.0% Carbon with some other alloying elements added including manganese, chromium, vanadium and tungsten. Varying these alters the steel alloy's hardness, ductility and tensile strength. Stainless steel has at least 11% Chromium added.

With the development of the Bessemer Process, the cost of steel manufacture fell significantly. It made the continued production of wrought iron, which was much more labour-intensive, uneconomic. As a consequence wrought iron went into a rapid decline.

Early Bessemer steel.

SteelWrought iron production/use shrank rapidly after 1870:

Date: Wrought Iron Steel1870 3,000,000 tonnes 250,000 tonnes1900 250,000 tonnes 5,000,000 tonnes

Eiffel Tower built 1887-1889. Forth Rail Bridge built 1883 – 1890.

Wrought Iron Steel

3. IdentificationVisual clues to help identify the iron:- shape, design: recognisable cast iron designs – columns, panels. Cast iron is used in compression, wrought iron in tension. Cast iron sections will be heavier.- construction: if riveted will be of wrought iron (occasionally steel). If forge or fire welded it will be wrought iron.- surface: if beaten or tooled it will be wrought iron. Presence of porosity, blow holes or inclusions it will be cast iron.- mould lines: flash lines or mismatch halves reveal cast iron.

Tests to identify different types of Iron:

- partially cut then bend a sample:Cast iron will break with crystalline surfaceWrought iron will break with fibrous surfaceSteel will break with smooth fracture plane- grind surface and observe sparks:

Cast iron produces ragged red sparksWrought iron produces narrow straw sparksSteel produces many white sparks

- cut a metal sliver with a cold chisel:Cast iron will chip with small fragmentsWrought iron produces continuous curled shavingsSteel produces continuous curled shavings

Tests to identify different types of Iron:

- partially cut then bend a sample:Cast iron will break with crystalline surfaceWrought iron will break with fibrous surfaceSteel will break with smooth fracture plane

- grind surface and observe sparks:Cast iron produces ragged red sparksWrought iron produces narrow straw sparksSteel produces many white sparks

- cut a metal sliver with a cold chisel:Cast iron will chip with small fragmentsWrought iron produces continuous curled shavingsSteel produces continuous curled shavings

4. Corrosion of IronIron (Fe) is naturally an unstable material and readily oxidises in the presence of Oxygen (O2) and Water (H20). This process is known as corrosion - the metal's interaction with its environment.When dealing with iron and steel the specific term of Rusting is used, and refers to the red-brown coating of oxides formed on the metal surface. The word originates from the German for Red (Rost).

The chemical process that occurs can be written as follows:

2Fe + O2 + 2H20 2Fe(OH)2Iron Hydroxide

With more Oxygen the process continues:

2Fe(OH)2 + O2 2FeO(OH) Fe2O3 + H2OIron (II) Hydroxide Akageneite Ferric Oxide

Red Rust

4. Corrosion of IronThe process of rusting is very dependent on the availability of both oxygen (from the air) and water. In essence electrons are transferred from the Iron (Fe) to the Oxygen (O2). With limited Oxygen available different corrosion processes occur producing Magnetite (Fe3O4), a black oxide layer that is more stable.

The corrosion process is affected by the presence of other materials, and in particular the presence of what are known as Electrolytes (ions in the water). The most significant is Salt that contains Chloride ions (Cl-).

Rust on anIron surface

4. Corrosion of IronThe corrosion of iron can be represented graphically:

4. Corrosion of IronAnodes and Cathodes can form in a variety of different ways, even within the same piece of metal:

4. Corrosion of IronThe likelihood of a metal forming as an Anode (- ive) or a Cathode (+ ive) relates to the ease with which it will give up its electrons and is known as its Electrode Potential. This Potential can be used to assess the reactivity of one metal against another. When listed together they produce the Galvanic Series.

GOLDSILVER

COPPERLEADTIN

NICKELBRASS

CADMIUMIRON

CHROMIUMZINC

ALUMINIUMMAGNESIUM

+ Positive

- Negative

More Noble than Fe

More Base than Fe

For Iron (Fe) it can be seen that it will corrode when associated with other historic metals.

4. Corrosion of Iron

The presence of Chloride Ions (Cl-) significantly increases the rate of iron corrosion. These migrate to the iron surface and produce a much more acidic environment. They are absorbed by the FeO(OH). The FeO(OH) is also hygroscopic, attracting moisture to the metal surface and encouraging corrosion to continue.

Many other metals corrode and form an oxide layer on the metal surface (e.g. aluminium, zinc, lead). The significant difference with iron is that Iron Oxide (Fe2O3) occupies 9 x the volume of the corroded iron, so does not act to protect the iron surface beneath from further corrosion as the oxide layer does on many other metals.

4. Corrosion of IronIn order to stop/prevent corrosion of Iron it is necessary to control its access to both Oxygen (O2) and Water (H2O):

- The anode and cathode need to be separated from the Electrolyte (water + negative ions) and dried out- access to oxygen needs to be excluded- electrical contact between the anode and cathode needs to be broken

5. Conservation of IronworkIntroduction: Conservation is the overall term used to describe all the measures and actions needed to safeguard any cultural heritage for the future to prevent further change, damage, loss or neglect. In its broadest sense it encompasses Preventive Conservation, Remedial Conservation and Restoration.

As part of the conservation process it is necessary to gain a thorough understanding of an object through examination and research. Keeping a record of any actions taken is also important.

5. Conservation of IronworkPreventive Conservation: Preventive Conservation refers to any measures or actions taken to avoid or minimise future deterioration. In essence these are indirect activities undertaken that alter the overall environment an object is exposed to. This could include:● Registration● Suitable storage● Correct handling● Appropriate packing & transportation● Environmental management -

➔ Light levels➔ Humidity➔ Temperature➔ Pollution (dust & dirt)➔ Pest control

● Planning for an emergency● Public awareness

5. Conservation of IronworkPreventive Conservation:

5. Conservation of IronworkRemedial Conservation: Remedial Conservation are all actions applied to an object aimed at stopping present deterioration processes and providing support or reinforcement if required. These are solely designed to stabilise an object in its present condition but may alter its the appearance.This could include:● Cleaning of dust and dirt from an object's surface● Stabilisation of corroded metals● Desalination of ceramics● De-infestation of textiles● De-acidification of paper● Consolidation of decayed timber

For Iron this will normally involve the removal of rust layers, stabilisation of corrosion, and the application of a protective coating to the object surface.

5. Conservation of Ironwork

Cast iron overthrow above a gateway exposed to sea air for many years before and after cleaning and protection.

5. Conservation of IronworkRestoration: Restoration forms all actions directly to an object that changes the nature of that object to help facilitate its understanding, appreciation and use. These are carried out when the object has lost part of its form or its significance or function. It may also be to rectify deterioration or to undo earlier alterations. It should always respect the original form of the object.This could include:● Reassembling a broken ceramic vessel● Filling losses to a glass vessel● Replacing a corroded section of steel to a vehicle● Retouching a canvas painting

Often Conservation actions will incorporate more than one of these actions. Generally, any works to an object would also include looking at what preventive conservation measures can be taken so as to prevent any interventive treatments from being 'undone' in the future.

5. Conservation of Ironwork

Cast iron and Bronze plaques commemorating an earlier bridge across the river Tawe, Swansea.

5. Conservation of IronworkConservation Ethics:

The Conservator-Restorer is a professional who has the training, knowledge, skills, experience and understanding to act with the aim of preserving cultural heritage for the future, and according to underlying principles that govern their actions. These are known as Conservation Ethics.

Every object needs to considered individually and a whole series of factors can influence the final decision on the level of intervention and what treatments are best carried out.

5. Conservation of IronworkConservation Ethics:

There will always be many factors that have to be considered, including:● To understand a range of treatment options and their effects on the object● Conflicting requirements of 'stakeholders' – owners, curators, designers● What point in an object's history should be selected for preservation?● Do you keep previous repairs as they are part of the object's history?● Need to consider 'reversibility' of treatments● The influence of the skill of the conservator/restorer on the level of treatment

considered● Risk to an object from the treatment to be used● The final destination of the object: inside, outside, on display, in storage● The documentation/recording of changes made

5. Conservation of IronworkWhat should be done with the corrugated iron roof of this Crib Hut?

Ethics: