Guide: Architectural Lead
A guide to lead and its restoration. Explore metallurgy, production techniques, its architectural and historical context and best practice conservation.
Introduction
Lead (Pb from the Latin Plumbum) is a blue-white shiny metal that tarnishes into a dull grey. Lead is an exceptionally soft material and very malleable and ductile. It is highly resistant to corrosion, a poor electrical conductor and highly poisonous.
This combination of properties led to its use in a variety of exterior applications since antiquity including roofing, rainwater systems, lead cisterns, windows and later garden statutory.
Production Techniques
Primary Production
The earliest production methods of Lead aren’t well documented, however historians tend to agree it was originally produced through a process of dressing and bole smelting. Smelting was later achieved with the use of wind and water power and at the peak of its production smelted in a blast furnace or cupola.
Dressing
Prior to smelting, lead ore required dressing – a repetitive refining process of extracting ore from the rock and washing it. Waste minerals and dirt were removed by washing from the rocks and then crushed into shingle sized pieces either by hand or on a crushing circle using horsepower.
It was then washed a further time, normally in a sieve with fine enough holes to stop the lead ore from passing through, and plunged repetitively into a trough of water. The heavier lead rich particles would sink and the waste minerals would form a scum on the surface of the water which could by removed. These particles could then be taken to the smelter for final refining into useable lead.
Bole Smelting
The very earliest lead production required a bole smelter – a large fire built on a hillside to take advantage of prevailing winds to increase temperatures. A slow process, to function correctly, the bole needed two full days of high winds. It refined larger pieces of ore best, and as such was a fairly wasteful process resulting in accumulations of waste ore heaps.
Smelting Mills
In the 1500’s, mining techniques improved and bellows replaced unpredictable wind power, first powered by hand and later by watermills. The bole was replaced by a double smelting furnace. The first burnt dried branch wood to fuel the mills and the second fired by charcoal re-smelted the slag. All sizes of lead ore could be processed using this process reducing waste from production.
Despite their relative efficiency, the mills had a number of disadvantages. They often overheated resulting in the hearths burning out and needing rebuilding regularly and the toxic fumes they let off into the surrounding areas meant they had to close down at the end of each day. Reliant on branch wood for fuel, timber supplies were running low in the 1700’s and the introduction of coke as a replacement in the furnace resulted in a less pure form of lead.
As such, the smelting mills were superseded by the cupola furnace in the 18th century.
Cupola Furnace
An innovation first applied to smelting metals in the 1700’s, the cupola was a form of reverberatory furnace. With a series of chambers, this meant the lead could now be isolated from the burning fuel eliminating contamination from impurities formed when burning with coke or charcoal.
Furthermore, the air flow was less powerful than its predecessor and had much greater efficiency which meant the furnaces could operate continuously allowing production levels of many metals to ramp up.
Secondary Production
Lead, in sheet and shaped form, was traditionally formed through green sand casting. This method, involving pouring molten lead into a bed of wet sand prepared with a mould, and being left to cool, was used from antiquity up until the mid-1700’s. Sand cast lead sheets are still produced to this day in minimal quantities for specialised conservation works despite the process being replaced by milling and rolling methods.
In this technique, lead is first cast into a 2 tonne slab and left to cool. Following this, heavy rollers repeatedly pass over the slab to progressively thin the lead down to a sheet, which was then cut into lengths.
History
One of the earliest metals produced by man, the oldest known lead containing object is a small statue discovered in Turkey dating from approx. 6500BC. Evidence suggests metallic lead was already being produced circa 3000BC in China whilst the Egyptians used lead to glaze ceramics and the Babylonians are known to have produced and soldered lead sheet for use in building construction.
The most extensive and well known use of lead was by the Romans from 50BC – 500AD. Water pipes, aqueducts, cooking pots, cosmetics, paints and reservoir linings all utilised the material and it is estimated that the Roman Empire used 80,000 tons annually. Historians have since argued for and against cumulative poisoning from lead exposure as partially responsible for the fall of the Roman Empire.
Britain had a high amount of good quality lead deposits, and was mined extensively by the Romans during their occupation continuing well into the Anglo-Saxon period and past the Norman invasion.
Lead mining flourished from the 1400’s with the invention of the printing press and increased use of glazed windows. By the 1600’s lead was the second most important export second only to wool. By this time, it was being used extensively in construction but also for water storage and piping, in ammunition and for decorative applications including garden sculptures and ornaments.
Lead continued to be used in architectural applications throughout the 17th and 18th centuries until the Industrial Revolution and the introduction and mass production of cast iron lead to its use in construction diminishing.
Previous Architectural Applications
After the development of smelting furnaces (and later the cupola) British lead mining, production and its use in architecture thrived. Pre regency, most lead use in construction had been reserved for ecclesiastical buildings and the very rich.
Roofing
Used in roofing since antiquity, Leads resistance to corrosion and the ease at which it could be folded over and sealed, made it a perfect material for roofs and flashings.
Lead roofing was of particular popularity in the Georgian period as it was well suited to low-pitched roofs. Flat and concealed roofs had become fashionable as they were deemed neo-classical in design and allowed for more intricate and detailed parapet designs.
Lead Rainwater and Plumbing Systems
Over the years, rainwater goods have been made from a variety of materials; lead, zinc, copper and cast iron. The first documented gutter installation dates back to 1241, when the Tower of London had a gutter fitted to protect its newly white washed walls. In the 16th century, lead was most popular – arguably due to the increased availability of recycled lead due to the dissolution and destruction of monasteries. The use of lead, a malleable material, allowed scope for ornamentation. Lead piping, formed from sheet lead wrapped around a rod, was used to carry water supplies to the kitchen and the then new water closets and for rainwater systems. Hooper heads, guttering, pipes and rainwater sumps were all made from cast lead.
Until the late 1700’s, the high production cost of rainwater systems meant the goods were predominantly reserved for high status individuals and their residences.
Leaded Lights
Leaded lights are one of the oldest technique for glazing still in use today. Leaded lights, are of fairly simple construction. Small panes of glass (quarries) are joined together using lead cames which are shaped like an H-Section to hold the pane. Lead’s material properties are soft, flexible and affordable. Which made it a perfect choice for early glazing.
Copper, brass or lead ties are soldered into the rear of the cames which are then fastened to the main window bars to provide structural strength to the panels. Glazing cement (traditionally consisting of linseed oil, whiting, lamp black and white spirit) is installed under the flanges of the lead came to secure the quarries in place. The leaded window unit was then installed into stone, timber or metal surrounds using traditional putty.
This glasswork technique came about due to limitations in early glass production. Only small pieces of glass could be produced, and as such a technique needed to be developed to create larger glazed spaces.
Lead in Paint
Frequently added to paint, lead based paints were one of the most durable materials used as a protective exterior coating. Red lead was used as a galvanic protective layer on iron and white lead was used to paint timber houses. Its use now has been restricted due to health and safety issues concerning lead poisoning.
Typical Repair Techniques
Cleaning
Not susceptible to corrosion, lead rarely needs intensive cleaning. Organic dirt can normally be removed by hand. In particularly polluted areas or when dealing with stubborn dirt, low pressure water sprays may be used.
Reshaping
Generally, dents and distortions are better left as they are unless functionality is being impeded. In such cases, the lead can be manipulated back into shape whilst heated gently.
Lead Burning
A repair technique involving a form of welding using a filler of pure lead. It is important that the surface area is cleaned to bare metal prior to lead burning to ensure a clean bond.
Epoxy Repairs
Two-pack epoxy resins can be used where lead burning is not feasible for non-structural repairs such as re profiling small gaps, cracks or faults in a casting.
Replacement
Where leadwork needs to be replaced entirely, existing components will often be used to provide a mould. Unless the original leadwork was produced using the milling and rolling technique, sand cast lead will most likely be used in replacement.
Repatination
Artificial patinas can be applied to replacement sections of lead using a chalk or oil. Although, this is rarely recommended as the patina may not develop at the same rate or to the same colour as the existing one.
Click below for further guides on typical fabric specific conservation and restoration techniques employed:
