The Repair of Wrought and Cast Iron Work - Dublin City Council
The Repair of Wrought and Cast Iron Work - Dublin City Council
The Repair of Wrought and Cast Iron Work - Dublin City Council
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IRON THE REPAIR OF WROUGHT AND CAST IRONWORK<br />
7. <strong>Iron</strong> <strong>and</strong> Common Substitute Materials<br />
<strong>The</strong>re are many types <strong>of</strong> iron <strong>and</strong> steel, <strong>and</strong> while they<br />
may <strong>of</strong>ten appear to be similar superficially, each has a<br />
distinct set <strong>of</strong> characteristics. This is an important<br />
consideration when planning repairs to traditional<br />
ironwork. This chapter outlines how the most common<br />
types <strong>of</strong> iron, steel <strong>and</strong> their substitutes differ from<br />
one another so that an informed decision can be<br />
made when planning repairs.<br />
Distinguishing wrought iron<br />
from cast iron<br />
Before embarking on repairs, it is important to<br />
establish whether ironwork is made <strong>of</strong> cast iron,<br />
wrought iron or a mixture <strong>of</strong> materials as this will<br />
determine what course <strong>of</strong> action is required. One <strong>of</strong><br />
the most frequently-encountered problems when<br />
looking at historic ironwork is determining whether it<br />
is made <strong>of</strong> wrought iron or cast iron. Issues also arise<br />
when distinguishing wrought iron from mild steel,<br />
which is commonly used to repair wrought iron. While<br />
it is usually possible for the trained eye to distinguish<br />
between wrought iron, cast iron <strong>and</strong> steel, scientific<br />
analysis may occasionally be required to confirm the<br />
material where visual identification is difficult.<br />
<strong>Wrought</strong> iron<br />
slag, <strong>and</strong> rolled into bars or sheets by passing the<br />
bloom through a rolling mill. <strong>The</strong> bloom was then<br />
chopped up, tied into a stack using wire, re-heated,<br />
hammered <strong>and</strong> rolled again. This process was repeated<br />
numerous times. <strong>The</strong> more <strong>of</strong>ten the process was<br />
repeated the better the quality <strong>of</strong> iron. Traditionally,<br />
wrought iron was classified as ‘best’, ‘best best’ or ‘best<br />
best best’ quality.<br />
<strong>The</strong> last puddling furnace for producing wrought iron<br />
in Engl<strong>and</strong> closed in 1974, <strong>and</strong> since then no wrought<br />
iron has been produced in Irel<strong>and</strong>, the UK, or the rest<br />
<strong>of</strong> Europe. <strong>Wrought</strong> iron is currently only available as<br />
recycled material <strong>and</strong> can be obtained from a single<br />
supplier in the UK. As a result, the cost <strong>of</strong> wrought iron<br />
can be up to ten times that <strong>of</strong> mild steel (its most<br />
common substitute), <strong>and</strong> sourcing wrought iron can<br />
take much longer. Because new wrought iron is no<br />
longer produced, it is a virtually irreplaceable material.<br />
Every effort should therefore be made to retain as<br />
much <strong>of</strong> it as possible. Despite the difficulties in<br />
obtaining the material, there are very real benefits to<br />
using wrought iron for repairs. First, using the same<br />
material eliminates the risk <strong>of</strong> galvanic corrosion<br />
which is caused by two dissimilar materials being<br />
placed beside each other. This is a common problem<br />
with mild steel repairs. In addition, wrought iron is<br />
generally considered to have excellent corrosion<br />
resistance – a repair using wrought iron will be a long<br />
lasting one.<br />
<strong>Wrought</strong> iron contains very little carbon <strong>and</strong> is fibrous<br />
in composition (similar to wood), due to the presence<br />
<strong>of</strong> long str<strong>and</strong>s <strong>of</strong> slag. It is malleable (it is easily<br />
shaped by hammering <strong>and</strong> rolling) <strong>and</strong> ductile (it can<br />
be shaped by extrusion through dies to form wires). It<br />
is strong in tension <strong>and</strong> generally has good corrosion<br />
resistance. Because wrought iron contains relatively<br />
little carbon it is mechanically weaker than steel.<br />
<strong>Wrought</strong> iron was formed by ‘puddling’ pig iron in a<br />
reverberatory furnace. This process involved re-melting<br />
the pig iron in a furnace which kept the iron <strong>and</strong> fuel<br />
separate. When the iron was molten it was constantly<br />
stirred, or ‘rabbled’, to expose as much <strong>of</strong> it as possible<br />
to the air so that carbon would be given <strong>of</strong>f as gas,<br />
turning it into a relatively pure iron. Eventually the iron<br />
became thicker <strong>and</strong> spongy <strong>and</strong> could be rolled into a<br />
ball known as a ‘bloom’. This was then hammered, or<br />
‘shingled’, using a steam hammer to drive out molten<br />
Note the pin just below the top rail <strong>of</strong> these wrought<br />
iron railings which would have been used to hold the<br />
rail in place during the assembly <strong>of</strong> the railings in situ<br />
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