Defect Atlas — Stained Glass

Chemical alteration of the glass surface producing visible degradation of the original finish.

Dulled, frosted or iridescent surface; sometimes with localised pitting; loss of original gloss.

Original glass composition, atmospheric moisture, condensation and pollutants may contribute.

• extent and location
• comparison between interior and exterior faces
• any loss of paint or decoration over affected areas
• association with condensation pathways

“Surface corrosion was recorded on the external face of the medieval glass, with a generally dulled appearance and localised iridescence.”

A fine network of cracks within the glass surface associated with instability of the original composition.

Fine, often interconnected surface cracks; dull, sometimes sweating surface; in advanced stages, flaking of the surface layer.

Unstable original glass composition combined with humid environments may contribute to crizzling.

• extent of affected glass
• presence of associated surface flaking
• any visible moisture on the surface
• environmental conditions around the panel

“Crizzling was observed across several pieces of pale glass in the lower panel, with a fine network of surface cracks visible in raking light.”

Small discrete cavities in the glass surface produced by localised corrosion.

Pinpoint to small-diameter pits, isolated or clustered; sometimes filled with corrosion product or weathering crust.

Glass composition, prolonged surface wetting and pollutant deposition may contribute.

• distribution and density of pits
• association with run-off patterns
• any loss of paint over pitted areas
• comparison between exposed and sheltered glass

“Surface pitting was recorded on the external face of several pieces in the exposed lower lights, with clustered pits in run-off zones.”

An adherent layer of degradation product on the surface of the glass.

Whitish, greyish or yellow-brown crust on the surface, sometimes obscuring painted detail.

Long-term reaction between glass surface and atmospheric moisture and pollutants may contribute.

• extent and thickness of crust
• whether crust obscures original decoration
• association with external pollution sources
• comparison with sheltered glass

“A weathering crust was recorded across the exterior of the medieval panels, partly obscuring painted detail when viewed from outside.”

A rainbow-like surface appearance produced by interference within weathered glass layers.

Patchy or pervasive iridescent sheen, often with peeling or flaking of the affected layer.

Long-term surface weathering producing thin layered alteration of the original glass surface may contribute.

• extent and location
• any associated flaking of the surface
• stability of paint over affected glass
• comparison between faces

“Iridescence was recorded on the exterior of several pieces, with associated flaking of the surface layer in places.”

Fine cracks through the glass thickness without offset.

Thread-like cracks within individual quarries; sometimes only visible in raking or transmitted light.

Thermal stress, original glass condition, fixing stress or impact may contribute.

• location and orientation
• number of pieces affected
• whether cracks are confined to one quarry
• any associated movement of the panel

“Hairline cracking was recorded in three quarries of the upper light, visible most clearly in raking light from inside.”

Cracks where the two sides have moved out of plane relative to one another.

Clearly stepped cracks; sometimes admitting light through the offset; possible loose fragments.

Significant impact, panel movement, frame distortion or sustained loading may contribute.

• extent and orientation
• size of offset
• stability of fragments
• implications for water ingress

“A through-crack with approximately 1 mm offset was recorded in the lower border, with the affected piece appearing stable at the time of survey.”

Cracked pieces of glass whose stability within the leaded matrix appears compromised.

Visibly displaced or movable fragments; sometimes only retained by paint, cement or proximity.

Cracking, loss of cement, lead deterioration or vibration may contribute.

• location and number of fragments
• extent of movement under light handling
• evidence of past stabilising repair
• implications for water ingress and falling material

“A loose fragment was recorded in the upper border, retained only by adjacent leadwork at the time of survey.”

Pieces of glass absent from the leaded matrix.

Empty calmes, voids in the design; sometimes temporarily infilled with non-original glazing.

Past damage, theft, vandalism or earlier removal without replacement may contribute.

• location and extent
• any documentation of original appearance
• current weathering of empty calmes
• presence of temporary infill

“A lost piece was recorded at the head of the right-hand light, with the calmes weathered and no temporary infill present.”

Alteration of the original glass colour by long-term exposure or composition effects.

Yellowed, purpled or otherwise altered colour, sometimes affecting only certain pieces or batches.

UV exposure, glass composition and long-term weathering may contribute.

• pieces or batches affected
• comparison with sheltered or replacement glass
• whether discolouration is consistent across the design
• association with corrosion or weathering

“Mild purpling was recorded in a batch of pale glass within the central light, consistent with manganese-bearing glass exposed to long-term UV.”

Loss of vitreous painted detail from the surface of the glass.

Gaps in painted lines or shading; reduced legibility of features; sometimes outlines retained only as ghost images.

Original firing temperature, surface deterioration of the glass and abrasive cleaning may contribute.

• extent of loss
• which pieces or figures are affected
• association with surface corrosion or crizzling
• any evidence of past cleaning

“Paint loss was recorded across the figure of the central saint, with significant loss of facial detail.”

Painted detail that releases material to light touch without yet showing developed loss.

Powdery or soft paint surface; material lifts to gentle brushing for diagnostic purposes; sometimes visible only in raking light.

Underfired paint, surface weathering and condensation may contribute.

• extent of friable areas
• association with environmental conditions
• stability under handling
• comparison between figures and backgrounds

“Friable paint was recorded on the interior face of the central figure, with material released to light brushing for diagnostic purposes only.”

Detachment of painted detail as small flakes from the glass surface.

Lifted or curled paint fragments; loss of legibility; sometimes loose flakes captured between glass and protective glazing.

Underfired paint, glass corrosion beneath the paint or moisture cycling may contribute.

• extent and location of flaking
• size of detached areas
• condition of underlying glass
• evidence of active loss

“Flaking paint was recorded on the inscription panel, with several flakes detached and resting in the lower calme.”

Alteration of fired silver stain decoration in colour, intensity or stability.

Faded, patchy or discoloured yellow staining; sometimes associated surface corrosion of the glass beneath.

Long-term weathering, surface corrosion and original firing may contribute.

• extent and location
• comparison with sheltered or interior areas
• association with general glass deterioration
• any current loss

“Alteration of silver stain was recorded on the haloes in the upper figures, with reduced intensity in exposed areas.”

Loss, crazing or alteration of fired enamel decoration on the glass surface.

Crazed, lifting or partly lost enamel; sometimes with associated discolouration.

Original firing, thermal expansion mismatch with the glass and moisture cycling may contribute.

• type of enamel where identifiable
• extent and location
• evidence of active loss
• association with glass condition

“Enamel deterioration was recorded on the post-medieval panel, with crazing visible across the blue drapery.”

Surface soiling that obscures painted detail but may also conceal underlying paint instability.

Dark surface deposits over painted areas; reduced legibility of decoration.

Atmospheric soiling, condensation and proximity to internal heating may contribute.

• extent of soiling
• association with internal or external sources
• whether soiling is stable or actively accumulating
• any evidence of paint instability beneath

“Soiling over paint was recorded on the interior face of the chancel windows, with painted detail less legible than on comparable panels nearby.”

Loss of structural performance of lead cames associated with long-term flexing and oxidation.

Cracked, opened or visibly weakened calmes; sagging panels; lead that crumbles under light handling.

Age, wind loading, thermal cycling, vibration and original lead composition may contribute.

• extent of affected cames
• evidence of panel deflection
• associated loose glass
• approximate age of leadwork where known

“Lead fatigue was recorded across the lower light, with several calmes cracked at the heart and the panel bowing into the interior.”

Visible fracture through the heart or flange of a lead came.

Open cracks along the came; sometimes admitting daylight; reduced restraint of adjacent glass.

Fatigue, movement, fixing stress or earlier mechanical damage may contribute.

• location and extent of cracking
• associated loose glass
• evidence of repair attempts
• implications for water ingress

“A cracked came was recorded across the upper border, with associated movement of two adjacent pieces of glass.”

Loss of integrity at a soldered junction between cames.

Open or partially open joints; sometimes with corrosion product around the solder; loose cames at the junction.

Original solder, oxidation, movement and stress concentration may contribute.

• location and number of failed joints
• evidence of associated movement
• presence of past resoldering
• implications for the wider leaded matrix

“A failed solder joint was recorded at the intersection of the central horizontal saddle, with the joint visibly open.”

Loss of the waterproofing cement between the lead flanges and the glass.

Open gap between glass and calme; loose glass under light pressure; sometimes light visible through the joint.

Age, weathering, vibration and original cement formulation may contribute.

• extent of affected calmes
• associated loose glass
• implications for water ingress
• evidence of internal condensation

“Loss of cement was recorded across the lower border, with several pieces visibly loose under light handling.”

White or grey corrosion products on the surface of lead cames.

White, powdery or crusty deposit on the calmes; sometimes localised to internal faces; can stain adjacent glass.

Organic acid emissions, condensation and proximity to certain timber species may contribute to lead corrosion.

• location and extent of corrosion product
• association with internal conditions
• any associated staining of adjacent glass
• environmental context of the window

“Lead corrosion product was recorded on the internal face of several cames, with associated white deposits on the adjacent stonework.”

Lead cames that have departed from their original section or profile.

Flattened, twisted or out-of-plane calmes; sometimes with associated loose glass.

Past handling, impact, panel movement or attempts at repair may contribute.

• location and extent
• associated glass condition
• evidence of past intervention
• implications for restraint of glass

“Distorted came profiles were recorded along the lower edge of the panel, with several flanges flattened and loose against the rebate.”

Corrosion of horizontal support bars within the window opening.

Orange-brown corrosion product on bars; sometimes with staining of adjacent stone or lead; reduced effective section.

Atmospheric moisture, coating failure and contact with damp masonry may contribute.

• extent and location
• any associated staining
• evidence of section loss
• condition of ties between bar and panel

“Corroded saddle bars were recorded across the south-facing window, with visible corrosion product and brown staining on the adjacent stonework.”

Loss of restraint at the connections between leaded panel and saddle bars.

Broken, missing or unsoldered copper ties; gaps between panel and bar; panel movement away from the supporting bar.

Fatigue, corrosion, original soldering and movement of the panel may contribute.

• number and location of failed ties
• evidence of panel movement
• condition of remaining ties
• implications for restraint at height

“Failed copper ties were recorded along the upper saddle bar, with the panel standing approximately 4 mm clear of the bar in places.”

Departure of ferramenta from its original geometry.

Bowed, bent or twisted bars; sometimes with associated panel deflection.

Impact, sustained loading, panel movement or earlier intervention may contribute.

• type and extent of distortion
• associated panel movement
• evidence of past intervention
• implications for restraint

“A distorted saddle bar was recorded at the central horizontal of the upper light, with approximately 10 mm of bowing into the interior.”

Discolouration of glass or adjacent stone caused by corrosion products migrating from ferramenta.

Orange-brown, dark or rust-coloured streaks on glass and stone below or alongside bars.

Active corrosion of ferramenta, rainwater run-off and condensation may contribute.

• source of staining if identifiable
• extent and direction
• substrate affected
• association with weathering pathways

“Corrosion staining was recorded on the stone tracery below the saddle bar, in a pattern consistent with run-off from the bar.”

Loss of integrity at the perimeter seal between leaded panel and frame or rebate.

Open joints at the rebate, missing pointing or mastic, visible water ingress paths.

Age of pointing, movement of the surround, original detailing and weather exposure may contribute.

• location and extent of failure
• evidence of water ingress
• condition of adjacent masonry or timber rebate
• any associated staining inside

“A failed glazing seal was recorded along the lower edge of the eastern light, with visible open joints and damp staining inside.”

Gap between the leaded panel and its supporting rebate.

Visible gap; sometimes admitting daylight; debris or biological growth within the gap.

Movement, pointing failure, panel deflection or original construction may contribute.

• location and approximate width
• any associated debris or growth
• implications for water shedding
• evidence of associated panel movement

“An open rebate was recorded along the lower edge of the central light, with daylight visible through the gap.”

Evidence of water passing through the glazing system into the interior.

Damp staining inside the rebate, water trails on the interior face of the glass, deposits on internal cames.

Failed seals, open rebates, defective rainwater goods above or condensation may contribute.

• location and pattern of ingress
• evidence of run-off pathways
• association with rainfall events where reported
• condition of adjacent fabric

“Evidence of water ingress was recorded at the lower rebate of the south-facing window, with damp staining extending onto the internal stonework.”

Visible condensation between the historic glazing and any protective glazing or on the internal face of the window.

Misting, droplets or run-marks on glass surfaces; sometimes with associated salt or biological deposits.

Differences between internal and external temperatures, ventilation arrangements and protective glazing design may contribute.

• location and frequency of condensation where reported
• ventilation arrangement of any protective glazing
• associated paint or glass deterioration
• internal heating regime where known

“Condensation was recorded on the internal face of the protective glazing in cold weather, with associated run-marks on the historic glass beneath.”

Out-of-plane deflection of a leaded panel within its frame.

Visible curvature of the panel when viewed obliquely; gap or excessive pressure at saddle bars.

Lead fatigue, wind loading, failed ties, panel weight and movement of the surround may contribute.

• approximate magnitude of bow
• direction (internal or external)
• evidence of progression
• association with failed ties or fatigue

“Panel bowing of approximately 25 mm was recorded in the central light, with the panel deflected into the interior.”

Downward deflection of a leaded panel under its own weight.

Reduced height of the panel within the rebate; gaps at the head; compression at the base; sometimes associated lead fatigue.

Lead fatigue, lack of support, original lead composition and gravity over long periods may contribute.

• extent of sagging
• evidence of associated lead fatigue
• condition at head and base of the panel
• implications for design legibility

“Sagging was recorded in the upper light, with the panel approximately 8 mm lower at the centre than at its edges.”

Distortion of stone or metal tracery affecting the geometry of the glazed openings.

Cracking in tracery, displaced sections, gaps at junctions with glazing, distorted panel shapes.

Movement of the surrounding fabric, corrosion of embedded iron or original construction may contribute.

• location and type of distortion
• associated cracking in tracery
• implications for restraint of glazing
• evidence of progression

“Tracery distortion was recorded at the head of the south-facing window, with cracking visible adjacent to embedded ironwork.”

Adherent atmospheric soiling on the external face of glass and leadwork.

Dark, often greasy deposits on external surfaces; concentrated in sheltered or run-off zones.

Atmospheric pollution, traffic, proximity to ground and lack of cleaning may contribute.

• extent and pattern of soiling
• association with run-off pathways
• whether soiling obscures condition assessment
• any associated chemical staining

“External soiling was recorded across the lower lights, with darker deposits concentrated in sheltered zones beneath the hood mould.”

Soiling on the internal face of glass associated with internal sources.

Dark or greasy deposits on the interior face; sometimes following condensation patterns or proximity to heating.

Candles, oil lamps, heating systems, condensation and lack of cleaning may contribute.

• location and extent
• association with heating or lighting sources
• any associated condensation pattern
• stability of paint beneath

“Internal soiling was recorded across the chancel glazing, with darker deposits concentrated above the historic heating pipes.”

Discolouration following the path of rainwater across the external face of glass and leadwork.

Linear streaks following gravity; sometimes with concentrated deposits beneath ferramenta or stonework.

Defective rainwater detailing, run-off from soiled fabric above and ferramenta corrosion may contribute.

• direction and extent of staining
• source of run-off where identifiable
• association with corrosion staining
• implications for ongoing soiling

“Run-off staining was recorded on the external face of the lower lights, with streaks consistent with run-off from the soiled stonework above.”

White or pale crystalline deposits on glass, leadwork or adjacent fabric.

Whitish powdery or crystalline material on surfaces; sometimes seasonal; can be associated with damp adjacent stone.

Moisture movement, salt-bearing water reaching the surface and de-icing salts may contribute.

• location and extent
• association with damp stone or run-off
• seasonality where known
• any associated corrosion or surface loss

“Salt deposits were recorded on the internal stonework adjacent to the lower lights, with associated dampness in the rebate.”

Green or dark biological growth on the external face of glass.

Greenish, brown or black film on glass and leadwork; particularly in damp or shaded conditions.

Sustained surface dampness, sheltered orientation and proximity to vegetation may contribute.

• extent and location
• association with shading or damp surfaces
• stability of paint beneath
• implications for cleaning regimes

“Algae was recorded on the external face of the lower lights of the north window, where the orientation receives little direct sunlight.”

Lichen colonising lead cames or adjacent stonework.

Crusty, leafy or powdery growths on cames and adjacent fabric; varied colour.

Long-term exposure, clean atmosphere, sheltered orientation and lack of disturbance may support lichen growth.

• extent and type of growth where identifiable
• association with weathering surfaces
• any apparent effect on the substrate
• significance of the affected area

“Lichen was recorded on the external face of several cames in the upper lights, with patches across an area approximately 200 mm wide.”

Higher plants colonising joints, rebates or adjacent stonework around the glazing.

Mosses, ferns or small plants rooting in damp joints; sometimes visible above the glazing line.

Sustained moisture, debris accumulation and lack of maintenance may contribute.

• type of growth
• extent and location
• association with water retention
• implications for the glazing system

“Plant growth was recorded in the rebate above the western window, with small ferns established in retained debris.”

An applied lead came added across a cracked piece of glass as a strap repair.

Narrow lead strip soldered across a crack; sometimes obscuring painted detail; visible on internal or external face.

Past repair to stabilise cracked glass without removing the piece may have introduced mending leads.

• number and location of mending leads
• any obscured painted detail
• condition of the underlying crack
• stability of the repair

“Several mending leads were recorded across cracked pieces in the lower light, partly obscuring the figure of the donor.”

Cracked glass stabilised by adhesive applied along the crack edges, intended to avoid the use of mending leads.

Visible repaired crack with thin adhesive line; sometimes minimally visible from one face.

Past conservation interventions using reversible or semi-reversible adhesives may have introduced edge-bonded repairs.

• location of repairs
• condition and stability
• any evidence of yellowing or discoloration of the adhesive
• comparison with adjacent unrepaired pieces

“An edge-bonded repair was recorded on a cracked piece in the central light, with the adhesive line stable and minimally visible.”

Evidence that a panel has been wholly or partly releaded at some point in its history.

Differences in came profile, alignment or surface; sometimes with associated rearrangement of pieces.

Past programmes of conservation, restoration or repair may have introduced replacement leadwork.

• extent of releading
• condition of replacement leadwork
• any associated rearrangement of glass
• documentary evidence where available

“Evidence of earlier releading was recorded in the lower border, where calme profiles differ from those in the main figural panels.”

Pieces of glass that are visibly later than the surrounding original fabric.

Differences in colour, surface texture, paint style or weathering compared with surrounding pieces.

Past replacement following damage, loss or alteration may have introduced non-original glass.

• location and number of replacement pieces
• differences from original fabric
• condition of replacement pieces
• documentary references where available

“Replacement glass was recorded in two quarries of the upper border, with brighter colour and less surface weathering than the surrounding fabric.”

An earlier repair that appears inappropriate in material, extent or compatibility with the historic glazing.

Use of unsuitable adhesives, mismatched glass, oversized mending leads or visibly degraded repair material.

Past repairs of limited specification, emergency measures or use of generic materials may contribute.

• type and extent of repair
• condition of repair material
• compatibility with surrounding fabric
• implications for the affected glazing

“An apparently inappropriate previous repair was recorded across the lower border, with discoloured adhesive and a non-matching infill piece.”

Issues with the performance of a protective glazing system installed externally to the historic glazing.

Condensation between layers, debris accumulation in the cavity, biological growth, damaged or distorted external panes.

Design of ventilation, original installation quality and maintenance regime may contribute.

• type of protective glazing in place
• evidence of condensation or biological activity in the cavity
• condition of external panes and frames
• ventilation arrangement

“Protective glazing performance issues were recorded at the south-facing window, with seasonal condensation between layers and debris accumulation along the lower edge.”