A practical guide to recognising and describing common architectural metalwork conditions and defects during conservation surveys.
The Defect Atlas is a recognition and description aid for surveyors recording architectural metalwork. It is designed to support consistent terminology and cautious observation, not to provide diagnosis, treatment or specification.
The atlas helps users:
The atlas does not:
Entries in this volume are grouped informally under the broad themes below. Many conditions span more than one theme; record what is observed rather than forcing a single category.
20 entries. Use cautious language throughout: describe what is visible, and reserve interpretation for sections explicitly identified as such.
Visible corrosion products on exposed metal surfaces.
Orange-brown, red-brown or dark corrosion products depending on material and exposure.
Moisture retention, coating failure, environmental exposure.
“Corrosion products were recorded on the lower rail adjacent to the fixing point.”
Corrosion concentrated in discrete areas rather than spread across the surface.
Patches of corrosion product surrounded by relatively sound metal or coating.
Water pathways, coating defects, contact with dissimilar materials, retained debris.
“Localised corrosion was recorded around the base of the upright, consistent with water retention at the junction.”
Corrosion that has progressed sufficiently to suggest possible loss of material.
Thick, layered or flaking corrosion products; visible reduction or distortion of the original profile.
Prolonged exposure, long-term coating failure, sustained moisture contact.
“Advanced corrosion was observed along the lower 200 mm of the post, with visible thickening of the corrosion layer.”
Loss of integrity of a protective paint or coating system.
Dull, chalky, cracked or thinning coating; partial loss of finish exposing the substrate.
Weathering, UV exposure, age, inadequate preparation of previous systems.
“Coating breakdown was recorded across the upper face of the rail, with localised exposure of the substrate.”
Raised bubbles within a coating, indicating loss of adhesion to the substrate.
Domed or raised areas in the coating, intact or broken, sometimes containing moisture or corrosion product.
Moisture beneath the coating, contamination prior to application, poor adhesion.
“Coating blistering was recorded across the sheltered face, with several blisters broken to expose corrosion product beneath.”
Detachment of coating in flakes or sheets, exposing the substrate.
Lifted, curled or detached coating fragments; exposed metal areas of varying size.
Coating failure, underlying corrosion, repeated thermal cycling, inadequate preparation.
“Coating flaking was observed on the upper surface of the gate frame, with exposure of corrosion-stained substrate beneath.”
Small, discrete cavities in the metal surface visible after corrosion product is disturbed or lost.
Pinpoint to small-diameter pits, isolated or clustered, sometimes obscured by corrosion product.
Chloride contamination, sustained moisture contact, localised electrochemical activity.
“Surface pitting was visible on the cleaned area of the bracket, with clustered pits across an area of approximately 50 mm².”
Discolouration on adjacent surfaces caused by corrosion products migrating from a metal element.
Brown, orange, green or dark streaks on adjacent stone, render, timber or paint.
Active corrosion upstream of the staining, rainwater wash, runoff pathways.
“Corrosion staining was recorded on the limestone plinth below the iron fixing, consistent with rainwater runoff.”
Conditions in which water is retained against or within a metal element.
Pooling, residual moisture, debris-filled junctions, blocked drainage paths or saturated assemblies.
Design detailing, blocked drainage, debris build-up, distortion of components.
“Water trapping was observed at the junction between the base plate and the masonry, with associated staining and corrosion product.”
A fixing that no longer performs its intended function, whether through corrosion, fracture or loss.
Missing, fractured, severely corroded or displaced fixings; visible movement of fixed elements.
Corrosion, fatigue, overload, incompatible materials, prior damage.
“A failed fixing was recorded at the upper bracket, with the bolt sheared and the bracket displaced from its seating.”
A fixing that is still in place but no longer fully restrained.
Movement under light pressure, visible gap between fixing and host material, surrounding corrosion or staining.
Corrosion at the fixing or substrate, mortar loss, vibration, distortion.
“A loose fixing was recorded at the lower rail, with visible movement when light hand pressure was applied.”
Departure of a metal element from its original geometry.
Bowing, twisting, bending, sagging or out-of-plane movement.
Impact, overload, thermal movement, loss of adjacent support, long-term creep.
“Deformation was recorded in the central bay of the railing, with a visible bow of approximately 20 mm out of plane.”
Physical damage from impact, abrasion, tooling or external loading.
Dents, gouges, scrapes, tool marks, fractured edges, missing fragments.
Accidental impact, vehicle contact, maintenance activity, vandalism.
“Mechanical damage was recorded on the lower section of the gatepost, consistent with vehicle contact.”
Linear discontinuities through coating, surface or substrate.
Fine to wide cracks, straight or branching, sometimes following weld lines or section transitions.
Fatigue, overload, casting defects, weld failure, thermal movement, corrosion-induced stress.
“Cracking was observed across the upper face of the cast bracket, extending approximately 60 mm from the casting edge.”
Loss of integrity at welded joints.
Corrosion concentrated along weld lines, cracking through or alongside welds, visible loss at weld toes.
Galvanic effects, original weld quality, sustained moisture exposure, stress concentration.
“Weld deterioration was recorded along the underside of the rail-to-post weld, with corrosion product following the weld line.”
Visible separation between components that were originally fitted closely.
Gaps at junctions, exposed substrate, ingress of debris or vegetation, daylight visible through assemblies.
Movement, corrosion-induced expansion, fixing failure, mortar loss, thermal cycling.
“Joint opening was recorded between the base plate and the masonry, with a gap of approximately 5 mm visible at the front edge.”
Living organisms colonising metal surfaces or adjacent fabric.
Algae, lichen, moss, plant growth in joints or on horizontal surfaces.
Moisture retention, sheltered orientation, debris accumulation, lack of maintenance.
“Biological growth was recorded across the horizontal top rail, with moss colonising debris retained in the channel.”
Non-biological material accumulated on metal surfaces.
Soiling, soot, dust crusts, salt deposits, runoff residues.
Atmospheric pollution, rainwater runoff, proximity to ground or vegetation, lack of cleaning.
“Surface deposits were recorded on the sheltered face, with a soiling layer obscuring assessment of the underlying coating.”
Loss of surface material from repeated contact or use.
Polished, thinned or grooved surfaces; rounded edges; coating loss in handling zones.
Use, maintenance activity, cleaning regimes, contact with moving parts.
“Abrasion and wear were recorded on the handle of the gate, with the coating lost across a worn area approximately 80 mm long.”
Evidence of earlier intervention to the metal element.
Patches, welded inserts, replacement sections, applied coatings, fillers, mismatched components.
Past damage, historic maintenance, programme of phased repair.
“A previous repair was recorded at the lower rail, comprising a welded plate of differing profile to the original section.”
Describe what is visible. Record the observed condition, its location, extent and any associated evidence before attempting interpretation.
Avoid:
Prefer cautious language such as:
Other Defect Atlas volumes — open a sibling area within Metals or browse across the wider atlas programme.
This atlas supports recognition and description only. It does not confirm causes, prescribe treatment, replace testing or replace specialist conservation advice. Where evidence is uncertain or risk is unclear, refer to qualified specialists.