IRONWORK REPAIRS IN TIMBER-FRAMED BUILDINGS.
6. Wrought Iron Manufacturing
Techniques used on Iron Ties. 1/5.
"Wrought iron is
best described as a two-component metal consisting of high purity iron and
iron silicate- an inert, non-rusting, glass-like slag. These two materials
are in physical association (Fig. 6.1), as contrasted to the chemical or alloy
relationship that generally exists between the constituents of other metals.
A photomicrograph illustrating the fibrous nature of wrought iron. The iron
and slag are intertwined in close physical association but do not constitute
an alloy. (Aston & Story, 1936).
In finished wrought iron
the iron silicate or slag content varies from about 1% to 3% depending upon
the type of product
This constituent is distributed throughout the iron
base metal in the form of threads or fibres" (Aston & Story, 1936).
Wrought iron is the only
ferrous metal that contains these non-rusting slag fibres and they are the
reason for its tough, fibrous nature, making it ideal for working under the
hammer (Fig. 6.2).
A fractured wrought iron bar showing its fibrous structure making it ideal
for working under the hammer. (Aston & Story, 1936).
The design of iron ties
is dependent upon the manufacturing techniques used in forming them. An understanding
of the processes involved in the manufacture of wrought-iron ties focuses
attention on a range of diagnostic features that can provide a basis, in some
cases, for distinguishing ties of different historical periods. Hand wrought
iron is likely to be earlier than machine hammered iron and certainly earlier
than milled stripwork.