Defect Atlas — Stone

Progressive loss of surface material reducing the original profile of the stone.

Softened arrises, rounded mouldings, loss of crisp tooling, generally weathered face.

Long-term weathering, exposure, rainwater wash and atmospheric action may contribute.

• extent and location
• loss of carved or moulded detail
• comparison with sheltered areas of the same stone
• whether loss appears active or historic

“Surface erosion was recorded across the south-facing string course, with loss of moulded profile observed when compared with sheltered returns.”

Loss of cohesion between mineral grains causing the surface to release sand-sized particles.

Friable, sugary surface; loose grains accumulating at the base of the element; loss of tool marks.

Salt crystallisation, freeze-thaw cycling, loss of binder and atmospheric acids may contribute.

• extent across the element
• presence of detached grains at the base
• associated salt deposits
• relationship to moisture zones

“Granular disintegration was observed on the lower courses, with loose grains visible on the plinth below.”

A specific form of granular disintegration producing a fine, sugar-like surface, most often recorded on marbles and fine-grained calcareous stones.

Crystalline, sparkling surface; loss of polish or carved finish; fine grains released to the touch.

Thermal cycling, acid deposition and intergranular weakening may be associated with this condition.

• stone type where known
• extent and location
• loss of original finish or polish
• any associated cracking

“Sugaring was recorded across the exposed face of the marble panel, with loss of original polish noted.”

Detachment of a thin surface layer that follows the worked face rather than the bedding.

Curling or lifting flakes following the face of the stone; sound-hollow response on light tapping.

Salt subflorescence, gypsum crust formation and thermal cycling may contribute.

• extent of affected area
• thickness of scale where measurable
• areas sounding hollow on light tapping
• loss of carved detail beneath

“Scaling was recorded on the upper face of the cornice, with several scales sounding hollow on light tapping.”

Detachment of small, thin platelets from the stone surface.

Small plate-like fragments lifting from the surface; loose flakes accumulating below.

Salt action, freeze-thaw cycling or surface treatments may be associated with flaking.

• approximate size of flakes
• extent across the element
• evidence of active loss
• condition of stone beneath flakes

“Flaking was recorded on the sheltered face of the buttress, with loose platelets observed on the offset below.”

Separation of stone into successive layers, often parallel to bedding planes.

Layered separation visible at arrises; sheet-like loss aligned with bedding; sound-hollow zones.

Face-bedding, freeze-thaw cycling and salt action within bedding planes may be associated with delamination.

• bedding orientation where visible
• thickness and extent of detached layers
• evidence of active loss
• structural significance of affected element

“Delamination was observed on the parapet coping, with separation visible along the bedding plane on the weather face.”

Localised raised domes on the stone surface caused by detachment of a surface layer.

Hollow, dome-shaped lifting of the surface; sound-hollow response; sometimes broken to expose substrate.

Salt subflorescence, trapped moisture beneath surface treatments or applied coatings may be associated.

• size and distribution of blisters
• whether blisters are intact or broken
• evidence of salt or moisture activity
• presence of previous surface treatment

“Blistering was recorded across the rendered patch, with two blisters broken to expose granular material beneath.”

A network of interconnected cavities resembling honeycomb produced by differential weathering.

Honeycomb pattern of small cavities; loss preferentially following weaker laminae or zones.

Salt crystallisation, wind action and selective dissolution may contribute.

• extent and depth of cavities
• orientation relative to exposure
• associated salt deposits
• any active loss of fragments

“Alveolar erosion was observed on the seaward face, with cavities up to 20 mm deep recorded across the lower course.”

Small, discrete cavities in the stone surface.

Pinpoint to small-diameter pits, isolated or clustered, sometimes following original surface defects.

Localised dissolution, salt activity or original mineralogical weakness may be associated.

• distribution and density of pits
• approximate depth where measurable
• relationship to bedding or jointing
• evidence of ongoing activity

“Pitting was recorded on the limestone ashlar, with clustered pits across the lower 300 mm of the face.”

Progressive loss of definition in carved, moulded or tooled surfaces.

Softened arrises, blurred carving, reduced depth of mouldings, indistinct tool marks.

Long-term weathering, atmospheric action and prior cleaning may contribute.

• comparison with sheltered or replacement sections
• extent and location of loss
• any evidence of prior aggressive cleaning
• significance of the affected detail

“Rounding of carved detail was recorded on the exposed capital, with shallow definition of the foliate carving when compared with the sheltered return.”

Erosion or removal of original surface tooling that contributed to the appearance of the stone.

Loss of chisel, pick, droved or boasted finishes; smoothed or weathered face.

Weathering, abrasive cleaning or surface erosion may be associated.

• type of tooling where identifiable
• extent of loss
• comparison with adjacent areas
• evidence of past cleaning interventions

“Loss of tooling marks was recorded across the exposed face of the quoin, with the original droved finish only visible on the sheltered return.”

Surface that releases material to light touch without yet showing developed disintegration.

Powdery or soft surface; material lifts under finger pressure; loose grains in shadow areas.

Early-stage salt or moisture action and binder loss may be associated.

• extent of friability
• associated salt or moisture indicators
• location relative to weathering pathways
• whether all faces are affected or only specific exposures

“A friable surface was recorded on the lower course, with grains released to light brushing for diagnostic purposes only.”

Fine surface cracks visible on close inspection.

Thread-like cracks, often following surface features; minimal width; sometimes only visible when damp.

Drying, thermal cycling, early-stage stress or surface treatments may be associated with hairline cracking.

• orientation and length
• whether the crack passes through bedding
• any associated movement
• comparison with previous records if available

“Hairline cracking was observed on the upper face of the cill, running approximately 80 mm parallel to the front edge.”

Cracks with visible width sufficient to admit moisture or debris.

Clearly open cracks; sometimes with debris, vegetation or staining within.

Movement, sustained stress, freeze-thaw cycling or corrosion of embedded fixings may contribute.

• orientation, length and approximate width
• evidence of active movement
• presence of debris or staining within the crack
• implications for water shedding

“An open crack was recorded across the keystone, approximately 1 mm wide and 220 mm long, with associated staining on the soffit below.”

A through-section break in the stone.

Fully separated stone; visible offset; in some cases held in place only by surrounding fabric.

Impact, overload, long-term stress or corrosion-induced expansion of embedded metal may contribute.

• location and orientation
• any displacement across the fracture
• relationship to fixings or embedded metalwork
• implications for stability of the element

“A fracture was recorded across the lower section of the pinnacle, with no measurable displacement at the time of survey.”

Detachment of a fragment of stone, often associated with expansion of embedded materials or freeze-thaw action.

Missing fragments leaving fresh fracture surfaces; sometimes associated with visible embedded metalwork or salts.

Corrosion of embedded metal, freeze-thaw cycling and salt action may contribute to spalling.

• size and location of spalled area
• evidence of embedded metalwork or salts
• whether the spall is recent or weathered
• associated cracking nearby

“Spalling was recorded adjacent to the iron cramp, exposing a corroded fixing within the fracture surface.”

Loss or fracture preferentially aligned with sedimentary bedding within the stone.

Layered loss or cracking along bedding; sometimes a stepped profile on weathered faces.

Face-bedding, freeze-thaw cycling and salt action within bedding may contribute.

• bedding orientation where visible
• extent of loss along the bedding plane
• comparison with correctly bedded stones nearby
• implications for ongoing loss

“Bedding-plane weakness was observed on the face-bedded sill, with a layered loss approximately 5 mm deep.”

Visible white deposit of soluble salts on the stone surface.

White, crystalline or powdery deposits; sometimes seasonal; may bloom after wet weather.

Salt-bearing water reaching the surface and evaporating may produce efflorescence. Sources can include rising damp, run-off, de-icing salts or marine spray.

• location and extent
• relationship to moisture pathways
• seasonality where known
• association with surface loss

“Salt efflorescence was recorded on the lower courses, more prominent following recent rainfall.”

Crystallisation of salts within the pore network just beneath the stone surface.

Typically not visible directly; inferred from associated blistering, scaling or granular loss without surface salt deposit.

Cycles of dissolution and crystallisation of salts within stone pores may be associated.

• associated surface conditions
• moisture sources nearby
• any history of salt exposure
• extent of associated loss

“Subflorescence is suspected behind the active scaling on the lower course, though confirmation would require sampling.”

Discolouration consistent with persistent or repeated wetting.

Darker zones on the stone surface, often following drainage or capillary pathways.

Rising damp, run-off, leaking services or blocked drainage may contribute.

• extent and orientation
• relationship to drainage or services
• evidence of associated salts or biological growth
• any seasonal variation where known

“Damp staining was recorded on the lower 600 mm of the plinth, consistent with a persistent moisture source.”

Linear or fan-shaped discolouration following the path of rainwater across the stone surface.

Streaks, drips or fans of discolouration extending downward from features such as cills or projections.

Rainwater wash from upper features, blocked or absent drips and accumulated soiling above may contribute.

• source feature where identifiable
• extent and orientation
• associated soiling or biological growth
• drip detail condition above

“Water run-off staining was recorded below the cill, with a fan-shaped stain extending approximately 400 mm downward.”

Living organisms colonising the stone surface.

Algae, lichen, moss or higher plants on or within the stone.

Moisture retention, sheltered orientation, lack of maintenance and nutrient availability may contribute.

• type of growth where identifiable
• extent and location
• association with water retention
• any apparent physical effect on the substrate

“Biological growth was recorded across the north-facing buttress, with moss colonising the recessed bed joints.”

Establishment of lichen species on the stone surface.

Crust-like, foliose or fruticose growth; often patchy and variable in colour.

Exposure, low pollution levels and suitable substrate chemistry may support lichen colonisation.

• extent and distribution
• any visible physical effect on the surface
• ecological or heritage significance where known
• differentiation from staining or crust

“Lichen colonisation was recorded across the upper surface of the chest tomb, with several species present.”

Green discolouration associated with algal growth on persistently damp surfaces.

Soft green, dark green or black-green films, often following moisture pathways.

Persistent moisture, sheltered orientation and water run-off may support algal growth.

• extent and location
• association with run-off or drainage
• any active source of moisture
• any associated softening of the substrate

“Algal staining was recorded below the cill, consistent with the run-off pattern observed above.”

Hardened black deposit found on sheltered surfaces of limestones and lime-rich stones.

Dark, hard, often blistered crust; sharp boundary at run-off lines; may obscure underlying carving.

Reaction of atmospheric sulphur compounds with calcareous stone and deposition of particulates may contribute.

• extent and thickness
• relationship to sheltered and washed areas
• any associated scaling beneath the crust
• obscuring of carved detail

“Black crust was recorded across the sheltered face of the cornice, with sharp boundary at the rain-washed line.”

Surface accumulation of particulate matter from the atmosphere.

General grey or dark soiling layer; uneven distribution between sheltered and washed areas.

Urban pollution, traffic, proximity to combustion sources and time since cleaning may contribute.

• extent and uniformity
• any obscuring of condition assessment
• relationship to sheltered surfaces
• comparison with adjacent buildings

“Atmospheric soiling was recorded across the elevation, with heavier deposits in sheltered zones.”

Crust composed largely of gypsum produced by reaction between calcareous stone and atmospheric sulphur compounds.

Hardened, often blistered crust; can incorporate dark soiling; may show fine cracking.

Atmospheric SO₂ exposure, particulate deposition and sheltered exposure may contribute.

• extent and condition of crust
• any scaling or loss beneath
• relationship to weathering pattern
• evidence of active detachment

“A gypsum crust was recorded on the sheltered soffit, with localised lifting noted at the lower edge.”

A section of stone or carved element showing signs of separation from its support.

Visible gap, hollow sound on light tapping, or visible movement at the interface.

Corroding fixings, mortar loss, frost action behind the element or original construction may contribute.

• location and extent
• evidence of associated fixings
• implications for falling fragments
• evidence of recent movement

“Detachment was suspected on the upper section of the panel, with a hollow response on light tapping.”

A stone or assembly that has moved measurably from its original position.

Offset between adjacent stones, visible projection or recess, opening of joints.

Settlement, overload, lateral pressure, freeze-thaw action or fixing failure may contribute.

• direction and magnitude of displacement
• associated joint opening
• comparison with reference points
• evidence of recent movement

“Displacement was recorded at the upper voussoir, with an offset of approximately 8 mm relative to the adjacent stone.”

Joints between stones that have opened beyond the original construction profile.

Visible gaps between stones; ingress of debris or vegetation; daylight visible through assemblies.

Mortar loss, movement, thermal cycling and corrosion-induced expansion may contribute.

• location and width
• associated displacement
• presence of debris or vegetation
• implications for water shedding

“Open joints were recorded along the upper course of the parapet, with widths up to 6 mm visible at the front face.”

Loss or breakdown of mortar within joints between stones.

Recessed, missing, cracked or detached mortar; voids in joints; sometimes accompanied by vegetation.

Age, weathering, original specification, vegetation growth or movement may contribute.

• extent and depth of loss
• any evidence of recent loss
• presence of vegetation within joints
• associated stone condition

“Failed pointing was recorded across the upper courses, with joints recessed by up to 20 mm.”

Use of hard, cement-rich mortars or repairs adjacent to softer historic stone.

Hard mortar lines standing proud of weathered stone; localised stone loss adjacent to the repair.

Use of Portland cement-based materials with low vapour permeability adjacent to historic stone may contribute to localised damage.

• extent and location of cementitious material
• condition of adjacent stone
• any associated salt or moisture activity
• differentiation from lime-based repairs

“Cementitious repair incompatibility was suspected at the lower course, with localised granular loss observed in the stone adjacent to a cement-rich mortar.”

Mortar that appears unsuitable for the stone type, exposure or original construction.

Visually mismatched mortar; standing proud, recessed or differently weathered to original.

Choice of binder, aggregate, mix proportions or application may not match the original specification.

• location and extent of inappropriate mortar
• comparison with surviving historic mortar
• associated stone condition
• any evidence of moisture or salt activity

“Inappropriate mortar was recorded on the repointed section, standing proud of the adjacent weathered stone.”

An earlier repair that is no longer performing its intended function.

Cracked, detached, eroded or differentially weathered repairs; sometimes accompanied by renewed loss in the original stone.

Original specification, age, compatibility and exposure may contribute to failure of earlier repairs.

• type of repair where identifiable
• extent and nature of failure
• condition of surrounding original fabric
• any documentary record of earlier work

“A previous repair failure was recorded on the cill, with a stone repair cracked and partially detached from the original substrate.”

Brown, orange or red discolouration associated with iron-bearing materials.

Brown to red-brown streaks or patches, often radiating from fixings, cramps or natural inclusions.

Corrosion of embedded ferrous fixings, run-off from adjacent metalwork or iron-bearing inclusions within the stone may contribute.

• source of staining where identifiable
• extent and orientation
• evidence of associated spalling or cracking
• presence of embedded metalwork

“Iron staining was recorded radiating from the upper cramp position, consistent with corrosion of the embedded fixing.”

Green or blue-green discolouration associated with copper or copper-alloy elements.

Green to blue-green streaks running below copper-bearing flashings, gutters or fixings.

Run-off from copper or copper-alloy elements may deposit corrosion products on the stone below.

• source element where identifiable
• extent and orientation
• any associated softening or loss of the stone
• condition of the metal element above

“Copper staining was recorded below the copper flashing, with a green streak extending approximately 600 mm down the face.”

Non-biological accumulations on the stone surface from external sources.

Soot, dust, salt films, paint splashes, droppings or other adhered materials.

Atmospheric pollution, run-off, adjacent activity or lack of cleaning may contribute.

• type of deposit where identifiable
• extent and thickness
• whether deposits obscure condition assessment
• any associated staining or biological growth

“Surface deposits were recorded on the lower courses, with a dark soiling layer obscuring assessment of underlying condition in places.”