HISTORIC IRONWORK REPAIRS IN TIMBER-FRAMED BUILDINGS.

Chapter 7.

7. The origins of nails, staples, screw fixings and bolts. 6/6.

Bolts

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 The form continued in use right up until the early C20th by which time screw-threaded bolts completely superseded them.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.

Screw-threaded bolts 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).

Fig.7.17. 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. (tilthammer.com, 2001).

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.

8. Modern Availability of Wrought Iron and Compatible Fixings