page_title: Bald arch
section_title: Weathering
hierarchical_section_title: Bald arch / Causes / Weathering
caption_attribution_description: Lindisfarne Priory
context_page_description: A bald arch is an arch featuring decay on the crucial keystones in stone or masonry buildings. Left unchecked, the progression of the condition will eventually lead to the failure of the arch and any structures supported by it.
context_section_description: The weather plays an important role in destroying stone structures. Daily or annual freeze/thaw cycles cause water trapped in cracks to expand, breaking up the stones or causing them to shift from their original setting. Frost heaving in the ground may move the structure, making it unstable. Precipitation can also be detrimental for certain materials. Limestone and cement are subject to weathering from rain. Natural rainwater has a pH of 5.6 due to carbon dioxide in the atmosphere. This is low enough to dissolve the calcium carbonate that constitutes limestone and cement. Air pollution can greatly enhance this effect by dropping the pH of the rainwater. In the eastern United States, the pH of rainwater can be as low as 2.0.
  Rain can also dissolve salts from the stone or other sources and redeposit the salt into cracks in the stones or between them. The salts will then crystalize as the water evaporates, putting pressure on the edges of the crack and causing the stones to move or chip. This effect is most pronounced in coastal and urban environments. Salt damage is also prevalent in cities that employ salts to deice roads and sidewalks.
  If an arch includes mortar between the stones, the mortar becomes a likely point of weathering. As the mortar decays, the stones on either side will become unsupported and lead to failure.
caption: A very exposed arch at Lindisfarne Priory on Holy Island, Northumberland.
exif: {"Image Make": "Canon", "Image Model": "Canon EOS 30D", "Image Orientation": "Horizontal (normal)", "Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "Aperture 3.2.4", "Image DateTime": "2012:10:08 15:50:16", "Image ExifOffset": "194", "EXIF ExposureTime": "1/250", "EXIF FNumber": "13", "EXIF ExposureProgram": "Program Normal", "EXIF ISOSpeedRatings": "200", "EXIF ExifVersion": "0221", "EXIF DateTimeOriginal": "2012:10:08 15:50:16", "EXIF DateTimeDigitized": "2012:10:08 15:50:16", "EXIF ShutterSpeedValue": "3027/380", "EXIF ApertureValue": "15157/2048", "EXIF ExposureBiasValue": "-1/3", "EXIF MaxApertureValue": "4", "EXIF MeteringMode": "Pattern", "EXIF Flash": "Flash did not fire, compulsory flash mode", "EXIF FocalLength": "12", "EXIF FlashPixVersion": "0100", "EXIF ColorSpace": "sRGB", "EXIF ExifImageWidth": "2336", "EXIF ExifImageLength": "3504", "EXIF FocalPlaneXResolution": "233600/59", "EXIF FocalPlaneYResolution": "233600/59", "EXIF FocalPlaneResolutionUnit": "2", "EXIF CustomRendered": "Normal", "EXIF ExposureMode": "Auto Exposure", "EXIF WhiteBalance": "Auto", "EXIF SceneCaptureType": "Standard"}