Painting Historic Ironwork
weather vane, at St Mary's Church, Hendon, was repaired by
Architectural Metalwork Conservation, grit blasted and then
zinc sprayed before being painted with an epoxy sealer/primer
and build coats, and finished with an acrylic urethane. This
high level of intervention and the use of a difficult-to-reverse
system was justified in this case as all the original iron
surfaces had been lost to severe corrosion and regular maintenance
will be impossible at such a height. Gold leafing was carried
out by Howell & Bellion.
Lots of names
are thrown up when considering the most appropriate coating for
historic ironwork: oil paint, lead paint, alkyds, micacious iron
oxide, epoxies, two-pack, zinc phosphate, zinc-rich, iron oxide
and polyurethane, to name but a few. So it is little wonder when
faced with this barrage of choices that specifiers often retreat
to using a standard specification without considering the particular
requirements of the historic fabric. Over-specification may be
harmful to the historic fabric: inappropriate treatment may lead
to premature failure of the coating. The success of a coating
specification for ironwork can only be assessed once it has begun
to fail, and the optimism always felt when viewing new paintwork
can be a false portent of its true effectiveness.
The choice of
coating will be influenced by many different criteria, including
historic appropriateness or authenticity and decorative effect,
but perhaps the most important consideration is its effectiveness
in protecting iron from corrosion.
oxygen combined in contact with iron will initiate the electrochemical
process that is corrosion. This results in the metal being eaten
away as electrons are freed from iron atoms, characterised by
the formation of hydrated iron oxide (rust). The rate of corrosion
may be governed by a number of factors: the chemical and physical
condition of the metal; contact with metals of differing electrical
potential to release electrons; and environmental factors, including
concentrations of moisture and oxygen, air flows, chemical pollutants
COMPOSITION OF PAINT AND OTHER WET-APPLIED COATINGS
liquids which dry through solvent evaporation usually in conjunction
with oxidation or other chemical reactions. Other coatings found
on historic metal which behave in this way include varnishes,
bitumen and tar.
varnishes are generally made up from solvents, binders and pigments.
Solvents, for example white spirit in modern oil paints, dissolve
the binder (resins and oils) into a liquid. Initial drying takes
place as the solvent evaporates leaving the binder, which then
further hardens by polymerisation, a chemical reaction in which
smaller molecules link together to form longer molecular chains.
This is brought about by either the chemical reaction of two constituents
which are mixed together just prior to application, or by oxidation
(mainly contact with air) which leads to the formation of a hard
film across the surface of the coating. This gives rise to the
terms two-pack or film-forming (single-pack) coatings.
include natural oils, alkyd oils, resins, vinyl and chlorinated
rubber. The choice of binder has significant influence over the
character and performance of the coating. For example, surface
tolerant coatings, suitable for application over less scrupulously
clean surfaces, will contain binders with good wetting properties.
the small solid particles that provide corrosion protection or
inhibition in primers as well as opacity and colour in all coats.
Primers are usually named according to their pigments, the most
often encountered examples being aluminium, iron oxide, red lead,
zinc phosphate, and 'zinc rich', which contains metallic zinc
dust. Protective pigments are also sometimes found in other coats,
the most well known of these being micacious iron oxide, added
to intermediate/build coats to improve the dry film's moisture
barrier properties. Pigments can be carried in a range of binders.
For instance, zinc phosphate can be found in alkyd, chlorinated
rubber and epoxy resin binder paints.
Paints are usually applied
in systems which usually have two to four coats and sometimes
more, (preferably in distinct colours or shades) comprising: primers,
build coats, undercoats and finishes. A primer is designed to
provide a key for the subsequent coat and usually also provides
corrosion protection and inhibition. The build coat is also likely
to contain additives to protect the metal, but this layer is also
designed to add film thickness. Depending on the colour and texture
of the build coats, the undercoat is often omitted as its main
purpose is to provide a decorative base for the finishing coat.
The most common
type of original coating system found on historic ironwork is
traditional oil paint made up of a turpentine solvent, linseed
oil binder, white lead (lead carbonates and sulphates) pigment,
and other pigments and additives, principally dryers. These were
usually applied over a corrosion-inhibiting primer which, in the
17th and early 18th century, typically contained red ochre ('Spanish
brown') and a small proportion of white lead. Red lead, which
can be distinguished from red ochre by its lighter colour, was
sometimes also used in primers before the mid 18th century, and
it became the most common pigment in linseed oil primers in the
late 18th and 19th centuries. Other traditional coatings include
pitch, coal tars and bitumen, primarily used on engineering structures
such as bridges, piers and aqueducts.
coatings can have remarkable durability, with recent discoveries
on exterior ironwork dating to the Jacobean period. Traditional
oil-based paints tend to dry slowly and often need to be applied
in several thin coats. This constraint, along with issues of lead
content, the constant demand for labour-saving materials and the
development of 'high performance, low maintenance' coatings for
industry, has led to the almost complete disappearance of traditional
oil paints for coating ironwork. However, following the trend
towards the use of traditional materials in general, there is
a major re-evaluation of traditional coatings. In respect of ironwork,
the interest stems from the work of paint analysts researching
authentic colour schemes as alternatives to the ubiquitous black.
There are also strong technical reasons for considering traditional
coatings. Firstly, they are likely to be the most compatible materials
for over-coating original materials. Furthermore, their tolerance
to application over iron with forge and foundry finishes and less
than perfectly cleaned rusted surfaces, coincides with resolving
the conundrum of minimum intervention versus the vigorous surface
preparation methods often specified for modern paints.
The most common
singular replacement for traditional oil paint is paint with
alkyd oil binders. These single-pack materials offer a higher
build (film thickness) from one application and faster drying
than traditional oils. Premium products may contain additives:
silicone, for example, is added to gloss finishes to improve long-term performance against weathering.
As a general rule, alkyd
oil coatings and traditional oil paints fall into the category
of coatings which have the shortest service life, requiring the
most frequent maintenance. Yet when compared to the more industry-oriented higher performance coatings, alkyd oils do have the advantages
of being both widely available from ordinary paint stockists and
relatively easy to use.
Other single-pack coatings offering a
longer service life, particularly in adverse environments, include
solvent drying vinyls and chlorinated rubbers, and moisture-cured
urethanes in which a rapid chemical cure is brought about by airborne
moisture. Meanwhile, two-pack epoxies, usually decoratively over-coated
by two-pack polyurethane coatings, are considered to offer the
longest life between major maintenance.
AND OIL TREATMENT
to paints there are a number of other coatings which are sometimes
used on ironwork, including oils and waxes.
The principles of
minimum intervention and reversibility are best served by wax
and oil treatments. Prior preparation of the metal can be limited
to washing down to remove dirt, and the coatings are usually simple
to remove. However, they have the disadvantage of requiring very regular
maintenance and, while sometimes used on interior ironwork, they
are usually only applied externally to stainless steels and nonferrous
metals such as bronze.
AND SPRAYED METAL FINISHES
At the other extreme, there are a number of long lasting treatments
which cannot be applied or removed without considerable intervention,
inevitably resulting in the removal of the original substrates.
Hot dip galvanising, first developed 160 years ago, is resistant
to abrasion damage and provides cathodic protection due to its
sacrificial corrosion at small breaks in the cover. The coating
is both the most effective method of protecting ironwork against
corrosion and the most difficult coating to remove. It is ideal
for new functional steelwork but has no place as a replacement
for paints on historic pieces, as the process leaves a layer of
zinc alloy chemically bonded with the iron.
at Countess of Huntingdon's Chapel, Bath. Samples taken from
the earliest railings (below left) suggest that when the chapel
was originally built in 1765 they were painted grey. (Lisa
Oestreicher Architectural Paint Analysis)
grit blasted surfaces, thermal sprayed ('metal sprayed') zinc
and aluminium coatings are sometimes used in restoration work
as well as in new work as they offer a physically tough barrier
and cathodic protection. They are less likely than galvanising
to mask fine decorative detail and their slightly roughened surface
finish offers a key to paints. However, the spray method of application
limits the operator's ability to reach hidden detail and penetrate
joints. This disadvantage is sometimes reduced by flooding the
unreachable areas with zinc rich paints, but the process remains
most beneficial on components which can be fully exposed for treatment.
Galvanised ironwork is ready for service, while thermal sprayed
coatings are usually further treated with a low viscosity sealer.
Both processes can be over-coated for increased protection or
more usually for decoration. Subsequent primers must be suitable
for non-ferrous metals; mordant solutions (known as 'T wash')
and etch primers are often used to prepare the surface. The full
range of wet and powder coatings is used for overcoating, yet
it should be noted that oil binder paints can prematurely fail
due to saponification (alkaline attack).
protection is required for structural ironwork, it may be possible
to use an intumescent paint. This will swell when exposed to
heat, providing insulation which will delay the time before the
iron buckles or fractures. Intumescent paints will change the
appearance of the structure due to the high film thickness required
and they are available only in a limited range of colours.
PREPARATION AND PAINTING
||Micrograph showing a section through a paint layer revealing
a total of 37 schemes, 35 of which are traditional
lead paints. Orange fragments of the priming layer can be
seen at the bottom of the sample followed by two thin layers
of a grey scheme. Subsequent schemes include cream and stone,
sky blue and many brightly coloured layers. Only the most
recent scheme is black. (Lisa Oestreicher Architectural Paint
several publications designed to help in the selection of coating
systems and which give advice on their application. The obsolescent
British Standard BS 5493:1977 and its successor BS EN ISO 12944
Parts 1-8: 1998 are particularly useful. They set out a range
of systems from surface preparation through to finish coat and
project their performance in a variety of environments.
out major repairs to ironwork it is current usual practice to
remove all corrosion and existing coatings. Although this approach
may not be appropriate, particularly where the paint coating is
itself historic, it has a number of clear advantages, including
facilitating repairs and removing materials which are potentially
hazardous to health, while also allowing for archaeological investigation
of the underlying metalwork and revealing hidden defects.
stripping may also improve the effectiveness of the new coatings
and provide the desired aesthetic rejuvenation. However, some
original surface finishes (oxides for example) as well as some
corrosion products can themselves make a stable crust to reduce
the speed of attack on the underlying metal. The need for cleaning
must be weighed against the risk of accelerated decay and loss
of historic material rather than be carried out as a matter of
of the existing coating should always be considered before removing
it, whatever the material or component. Historic layers of paint
on iron and other architectural elements offer an insight into
the appearance of a building (internally and externally) and its
surroundings at different points in its history. Paint layers
may also provide invaluable information on the age of a component,
and the date alterations were made, and will also provide an insight
into past coating technology (see The
Archaeology of Decoration by Lisa Oestreicher in The Building Conservation
If the surface of important historic
metalwork is to be stripped of coatings, a specialist should first
be brought in to take samples, so that the historic/archaeological
information can be recorded.
addition to cast, wrought iron and steel gates and railings,
other iron features found on churches and in churchyards include
door furniture, rainwater goods, ridge crestings and weather
vanes, light fittings and many other features
on historic ironwork is typically localised around difficult to
paint areas which are liable to retain moisture, for example where
leaves spring from bars. Adjacent plain bars will frequently remain
in perfect condition beneath a lifetime's accumulation of paints.
A range of targeted cleaning methods may be required in order
to prevent further deterioration in the difficult to paint areas,
while avoiding unnecessary disturbance of sound material. Aesthetic
considerations aside, it may only be necessary to remove loose
paint and corrosion in addition to any grease and dirt which will
compromise the all important adhesion and unbroken coverage of
the new coating.
Appropriate surface preparation is vital if the
new coating is to reach its full service potential. The main methods
of removing corrosion and old coatings include: hand and power
tool cleaning using scrapers, wire brushes and chipping tools
such as needle guns; chemical stripping; flame cleaning; air abrasive
methods commonly described as shot blasting and grit blasting;
high pressure water blasting. All these methods can damage ironwork
and their success depends on the skill, experience and judgement
of those carrying out the process. These methods are discussed
in detail in The Building Conservation Directory 2002.
of cleaning ironwork is chosen it is important to bear in mind
that the removed material may be highly toxic. In particular,
historic paint may contain leads and other heavy metals, with
implications for both personal health and the environment. When
planning the work, consideration must be given to the control
and disposal of all waste material.
||The use of lead carbonate or lead sulphate in paint is banned in the UK except for 'the restoration and maintenance of an historic building, or fine decorative work of art, where it is required to maintain historic textures and finishes' (see the Environmental Protection (Controls on Injurious Substances) Regulations 1992: Statutory Instrument No 31). Those containing red lead continue to be available unlicensed from specialist manufacturers.
coatings within suitable temperature and atmospheric parameters
is crucial to achieve the desired protection and decoration. Coatings
manufacturers provide advice on these and other technical aspects
together with product data and health and safety sheets that should
be studied before final commitment to a system. Wet paints can
be applied by brush, roller or a number of different spray methods.
Again reference should be made to the manufacturer's literature
to ensure the most suitable techniques are used for a particular
product, and to be aware of any specific health and safety issues
which may arise, as well as any environmental concerns. Roller and spray
methods have clear advantages of speed, but skilled brush application
is the method most likely to reproduce historic finishes. Brush
application also encourages thorough application to joints, crevices
and difficult to reach areas on highly decorated pieces. Whatever
the method of application it is vital to ensure that the coating
is within specification thickness throughout, and that it is free
of any breaks which might expose the iron to the agents of decay.
during the metalwork repairs, areas liable to trap moisture should
be modified to allow drainage. Where this is not possible or ethically
acceptable, particular attention should be given to ensure good
film coverage. This can be achieved by flooding with paint, or
applying paint-compatible fillers. Red lead putty and compounds
primarily consisting of pitch are encountered on existing ironwork
and have proven to be effective. Meanwhile, acetic acid free silicones
and polysulphide mastics are used in conjunction with modern high
performance coatings. Imperfections in the metal's surface, in
particular blow holes (casting flaws) in cast iron may be exposed
by thorough cleaning. These also need to be filled. Traditional
fillers are generally red and white lead putties, and 'Beaumont's
egg', a mixture of iron filings, grease and ammonium chloride.
Modern fillers are usually based on epoxy or polyester resins.
Finally to maintenance: unfortunately, it is generally the case
that this crucial area of preserving fabric rarely receives the
financial support and attention lavished on major restorations;
themselves often carried out in response to long-term neglect.
Little and often should be the mantra of those charged with caring
for historic ironwork.
article is reproduced from Historic
BLACKNEY specialises in the conservation of ferrous metalwork.
Prior to setting up his own company, Architectural Metalwork
Conservation in 1997, he worked as a conservator for English
Heritage. He now regularly leads craft training workshops
at the British Waterways Heritage Skills Centre, Warwick.
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