Historic Ironwork for Repainting
for salt contamination carried out by English Heritage on stay bars of Garrison
Church, Portsmouth immediately after blast cleaning: staining on the potassium
hexacyanoferrate paper indicates the presence of salts.
ironwork, including wrought iron, cast iron and steel, has almost always been
coated wherever it was used externally. In addition to providing a decorative
finish, the paint coating is essential to prevent corrosion.
carrying out major repairs to ironwork it is current usual practice to remove
all corrosion products (rust) and existing coatings. This has a number of clear
advantages, including facilitating repairs and removing materials which are potentially
hazardous to health, while also allowing for archaeological research and revealing
Thorough stripping may also be required to ensure the effectiveness
of the new coatings and provide the aesthetic rejuvenation which is usually desired.
However, it may be unnecessary or undesirable to strip back to bare metal across
the whole structure. Some corrosion products can themselves make a stable crust
protecting the underlying metal. Furthermore, the paint layer itself may also
contain important historical information, providing an invaluable insight into
past coating technology as well as the decorative history of the metalwork itself.
benefits of cleaning must always be weighed against the risk of accelerated decay
and loss of historic material. Cleaning should not be carried out as a matter
of course. In the recent restoration of Bolsover Castle, Derbyshire, for example,
English Heritage discovered traces of the original painted decoration on two 17th
century balcony railings. As a result it was decided to reinstate the original
colour scheme and to retain as much of the existing paintwork as possible. To
achieve this, the metalwork repairs targeted the areas which had corroded, avoiding
collateral damage to adjacent areas of sound historic paintwork.
stripping important metalwork of its historic coating cannot be avoided, a specialist
should be brought in to take samples first, so that all the historical and archaeological
information can be salvaged and recorded before it is lost forever.
COATING REMOVAL AND SURFACE PREPARATION
corrosion on historic ironwork is typically localised around difficult-to-paint
areas which are liable to retain moisture. Adjacent plain bars will frequently
remain in perfect condition beneath a lifetime’s accumulation of paints. A range
of targeted cleaning methods may provide the best way to prevent further deterioration,
while avoiding unnecessary disturbance of sound paintwork and historic surface
finishes. 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.
surface preparation is the key to new coatings reaching their 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 processes.
HAND AND POWER TOOL CLEANING
by hand and with the aid of power tools is labour intensive and so is generally
only carried out commercially as a pre-treatment to reduce the overall work cost
when combined with other processes, for example to remove heavy corrosion scales
before air abrasive cleaning. Yet it is surprising the speed at which some existing
coatings can be removed with hammers or needle guns. However, great care must
be taken to avoid bruising, distorting or, in the worst case, fracturing the metal.
careful selection and application of materials, chemicals offer an effective,
controllable method of cleaning. A strongly alkaline sodium hydroxide solution
(caustic soda) is sufficient to remove drying oil coatings. A range of proprietary
products usually based around dichloromethane (methylene chloride) is used to
break down oil and resin paints as well as re-liquefy vinyl and bitumen. Chemical
cleaning of large areas usually involves dipping the ironwork in a solution bath
while smaller areas can be treated with gels.
solutions are occasionally applied to remove or stabilise corrosion but care must
be exercised when using acids to avoid any potentially damaging contact with sound
material. On completion of chemical treatments the metal must be thoroughly cleaned
of residues which would otherwise subvert the new coating.
cleaning, involving wire brushing while burning with an oxy-acetylene or oxy-propane
torch, is sometimes used (either alone or as a pre-treatment to air abrasive and
chemical cleaning) to break down thick coatings and detach heavy corrosion. However,
the crystal structure of iron can be significantly affected by heat treatment,
and cast iron is particularly vulnerable to fracturing. This process should only
be carried out by a highly skilled operator.
DRY AIR ABRASIVE CLEANING
abrasive blast cleaning is now the most widely used method of surface preparation
for volume work. Dry blast cleaning methods are divided into shot blasting, an
automated factory process, and grit blasting, which is manually carried out in
both the workshop and on site. Both methods can be adjusted to produce a range
of cleanliness levels and surface profiles, which are often specified by the coatings
manufacturers to achieve optimum performance of their products.
back to the silvery grey colour of the base metal may suggest that all the contaminants
have been removed. However, if left to stand uncoated, corroded areas sometimes
develop a dark brown to black staining caused by environmentally-borne chemical
contaminants which can remain on the metal even after cleaning. These can include
chlorides introduced in the salt used on roads or in sea air, and sulphates from
atmospheric pollution for example. These contaminants are hygroscopic (that is
to say that they attract moisture from the atmosphere), chemically affecting the
performance of the new coating and reinitiating corrosion. If not treated, these
will reduce the effectiveness of even the most high performance paint systems.
HYDRO-BLASTING AND WET ABRASIVE CLEANING
methods incorporating water, such as wet grit blasting, pressurised slurry cleaning
and in particular hydro-blasting (using water alone at pressures greater than
30,000psi), are effective practical methods of cleaning salt contaminated iron.
The addition of water raises the possibility of rapid re-rusting (gingering).
This is sometimes dealt with by adding rust inhibitors to the water. However,
residual inhibitors can affect adhesion of the new coating. An alternative solution
to this problem is to use paints which are tolerant to gingered surfaces. Some
manufacturers even produce ‘wet blast primers’ that are designed to accept damp
surfaces post wet blasting.
applied to historic ironwork, abrasive blast cleaning may remove stable, well-adhered
and often protective surface skins created during the original casting or forging,
as well as removing sound coatings. Consequently, there has been a move, similar
to that in masonry conservation, toward selective cleaning using equipment that
can be finely adjusted, working at lower pressures often using fine blast media.
method 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.
- English Heritage Research Transactions - Vol 1: Metals, James and James Ltd, London, 1998
article is reproduced from The Building Conservation Directory, 2002
KEITH 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
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