The Repair of Wrought and Cast Iron Work - Dublin City Council

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