IRONWORK REPAIRS IN TIMBER-FRAMED BUILDINGS.
7. The origins of
nails, staples, screw fixings and bolts. 6/6.
A bolt is a metal pin
with an enlarged head at one end, a plain shank and a detachable fitment at
the other end. Its purpose is to bind two or more objects together by passing
completely through them and attaching the fitment. Unlike a screw it cannot
cut its own path and therefore the hole has to be pre-drilled.
Early bolts rely on keys
passing through a slot forged into the tip to create the clamping force. The
keys are triangular wedges called 'forelocks' and the bolts are called 'forelock
bolts'. Again these were prevalent from Roman times being used in carpentry
and especially in shipbuilding.
The term forelock was
definitely known in the 16th century. In 1532 'One great capsteye, and xv
boults with their forelockis and keys
.' were ordered for a crane at
Westminster. (Salzman LF. 1952 p326).
In 1300 the first iron-framed
public clock was installed in Paris. Early forelocks appear in the construction
of clock frames of the C14th, a surviving example from c.1360 being in Wells
Cathedral (Lucie-Smith,E. 1981). In this country forelocks appear in the late16th
century in greater numbers to be used in a number of repairs to timber-framed
buildings. The form continued in use right up until the early C20th by which
time screw-threaded bolts completely superseded them.
They are used to prevent
upstands from splitting out, to marry a new frame to an older one but mostly
they appear in clamps. Clamps are timber repair plates that are paired with
those that are failing and then pinned together with the bolts. The heads
are invariably round mushroomed shapes. The forelocks are bent over after
insertion and metal and hide washers are used to maintain the tension.
By the 17th century forelock
bolts were being routinely designed into the structure of great timber roofs
such as Lichfield, Worcester and Winchester cathedrals (Hewett CA, 1980) by
which time screw-threaded bolts were becoming more commonplace.
are believed to have been used by the Romans on the basis of one threaded
nut displayed in the Provincial Museum of Bonn and dated to AD180-260 (Rybczynski,W.
2000). Stronger evidence comes from Josephus of the first century AD, who
described iron tie rods reinforcing supporting columns, "The head of
each rod passed into the next by means of a cleverly made socket crafted in
the form of a screw
. These were held by these sockets, the male
fitting into the female". (Humphrey et al).
Vitruvius, in describing
a trispast ( a type of sheer legs), noted that the two legs of the A-frame
were fastened with a bolt. If it was to be demountable it would have to be
a screw-threaded bolt rather than a forelock bolt.
However, the screw-thread
disappeared until the fifteenth century as previously detailed when machines
and armour demanding adjustments began to be manufactured. Again in the Mediaeval
Housebook of Wolfegg Castle there is illustrated probably the earliest screw-cutting
lathe dating from between 1470 and 1490. Other screw cutting or swaging devices
must have existed and led the way somewhat slowly to mass production of nuts
and bolts. Otherwise they were hand-forged and lacking in conformity.
In the Westminster account
books of 1532 there is a reference to ' a newe platte varnysshid, with staples,
skrewis, and vices to the same, provided for a dore'. (Salzman, 1952 p309).
Salzman himself notes that "I have not found any similar entry and I
am inclined to think that the device of bolt and nut for such purposes was
not introduced much before this date."
Identifying nuts and
bolts specifically brought in for timber frame repairs may be possible from
the historic accounts associated with the building. For example at Ashes Farm,
Cressing, Essex the accounts note the purchase and fitting of a nut and bolt
to repair the barn in 1714. This is probably one of the few cases where the
actual ironwork can be definitely identified and the owner, Mr Peter Ratcliffe
has retained it as an heirloom. (Fig7.17).
A bolt dated to 1714 compared with a 19th century bolts used by the Great
Eastern Railway in the late 1800's.
The bolt has a very thick
shank, deformed with use, and a heavy square head. The nut is very deep, not
far off being a cube, with a deep thread and no chamfers.
Dating early bolts may
be problematic. The first successful screw-cutting lathe was developed in
1770 by Jesse Ramsden but it would have been used exclusively to create precision
engineering. Low quality work would still have been swaged in the fire or
possibly cut with hand taps and dies.
Real advances were made
by Henry Maudslay who 'In his system of screw-cutting machinery, his taps
and dies, and screw-tackle generally, he laid the foundations of all that
has since been done in this essential branch of machine-construction.' In
1800 he produced a large screw-cutting lathe for industrial applications.
Thread forms were not
standardised at this date and each smith or engineer would use his own particular
form. It was Joseph Clement who introduced the concept of a standard thread
around 1828 when he started manufacturing fluted taps and dies. His aim was
to obviate the problems associated with repairing machines where it was always
necessary to drill out the threads and replace them with his own. Clement
promulgated the idea that every screw of a particular length ought to have
a determined number of threads of a settled pitch.
This simple revolutionary
idea, was very shortly adopted everywhere. It was one of Clement's workmen,
Joseph Whitworth who later refined on this practice and gave to it his own
name, although the original idea was certainly Clement's. Whitworth's achievement
was to lay down the mathematical formula by which the thread was to be designed.
It was universally accepted as the British Standard Whitworth thread by the
1860's and remained in use until the 1970's when metrication was introduced.
In America, William Sellers proposed the American Standard Coarse and Fine
series of thread form in 1864. (boltscience.com).
Of historical interest
here is the fact that the burgeoning railway industries did not adopt Whitworth's
standard thread but developed their own. The reason, anecdotally given, is
that it was to prevent the theft of the tools as they could not be put to
other use. (Peter Ratcliffe, pers comm). In a serious investigation it may
be possible to determine the thread form as Whitworth and therefore give an
earliest date possible.
Modern Availability of Wrought Iron and Compatible Fixings