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IRONWORK REPAIRS IN TIMBER-FRAMED BUILDINGS.
Techniques used to
form wrought iron components. 2/5.
One of the most distinctive
features of wrought iron ties are the upsets. The thickening of a metal section
by heating it and compressing it is called upsetting (Fig. 6.3). A bar is
upset to thicken it locally. 'Upsetting can be used to bring metal to the
end of a bar to create a knob or handle, make the head of a rivet or provide
a mass of material as a starting point for forging some other form. It can
similarly be used to increase thickness in the middle of a bar, to provide
more metal for a punched hole (Fig. 6.4), forged corner or some other detail'
(Parkinson P. 2001).
A cramped L-tie with an upset crook. The metal has been thickened locally
to strengthen the curve.
A local upset to provide a shoulder for a staple and more metal for a punched
hole. Upsetting achieves this without thinning the section.
An early technique, it
was first noted by Moxon in 1677: "You may Up-set it, that is, take a
Flame Heat, and set the heated end upright upon the Anvil, and hammer upon
the cold end till the Heated end be beat or up-set into the Body of your Work."
There are a number of
ways to upset:
The work is heated locally to red heat and the cold end placed on the anvil
while the other end is hit with the blacksmiths' hammer. Hammering gives a
short upset the length of which can be controlled by the weight of the hammer.
A light hammer gives a short, abrupt upset whereas a heavier hammer gives
a longer tapering upset.
2. Pounding. The
hot work is trapped in a vice or under the steam hammer and hit using a ram
suspended on a chain. Otherwise for smaller work the bar is beaten directly
on the anvil by hand. Pounding gives a gentler flaring along a greater length.
3. Jumping. This
is a process that requires a jig to hold the work in. The area to be upset
is heated red and the bar slightly bent and lodged into the jig and hammered
down onto the anvil. This swells the work sideways and shortens the work whilst
maintaining its thickness. This is a method used to created upsets to pass
other bars through to create grates and railings.
4. Swaging. The
work is hammered hot into chilled steel swaging blocks to create purely decorative
Upsetting is used on
the larger, cruder L-ties to form a mass where the crook of the L is formed
(Fig. 6.5). The tail is then hammered in a plane at right-angles to that of
the arm. It is thought that this is an early form of wrought iron technique
used when the quality of the iron was more questionable and the understanding
of its strength very limited.
A side view of an upset cramped L-tie. The metal has been locally thickened
and the plane of the tail changed without twisting. However, the holes have
been punched without upsetting, causing a slight entasis around the holes
and a ragged finish.
Upsetting is also used
to form shoulders against which staples are housed (Fig. 6.6) and to form
eyes to take holes for nails These are both later developments, probably from
the end of the 18th century onwards.
A double-shouldered L- tie. The tie has been formed by hot-cutting the bar
along its length, forming two upsets and then welding the halves back together
in the fire. A complicated L-tie it has been used to reinforce a Victorian
partition in the Granary at Cressing Temple. There is another in the Barley
Barn. Both are likely to have been reused.
6.3 Spreading, 6.4 Fullering