HISTORIC IRONWORK REPAIRS IN TIMBER-FRAMED BUILDINGS.
Chapter 6
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.
Fig 6.1. 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).
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.
