Brownstone’s Susceptibility To Accelerated Deterioration and Restoration

Brownstone is a type of sandstone. Sandstone is common and prolific, found in many parts of the world.

Sandstone is a sedimentary rock, formed by granules of silica, essentially small granules of sand, in some cases with small amounts of quartz crystal, feldspar and other materials.   Sandstone can be found  in a variety of colors including light colors such as yellow and dark colors such as brownstone and black or dark colors.

Sandstone is extremely easy to carve. Some of the most ornate architectural carvings are done in sandstone, in many cases as reliefs. However, the negative tradeoff to sandstone is that it’s also very delicate and can easily be damaged from exposure to several common types of environmental elements:

  1. https://franksbeveragenmore.com/shop/Precipitation.  Exposure to precipitation,  in general, creates subsurface hydration which leads to disintegration of cohesive natural binders.
  2. Acid rain.   Since the industrial revolution, humans have been significantly and increasingly polluting the Earth’s atmosphere. The burning of fossil fuels for combustion has created toxic and poisonous byproducts such as sulfur and nitrogen oxides.   Once released to the atmosphere, these chemicals lower the ph level of atmospheric precipitation which leads to acid rain. Acid rain deteriorates masonry and many types of stone which destroys the architectural surface of carvings and building cladding.
  3. Freeze-thaw cycles.  Washington DC is a climate with significant precipitation and corresponding Autumn, Winter, and Spring temperatures constantly cycling above and below freezing.   Water  molecules consist of one oxygen and two hydrogen atoms. When frozen, below 32゚F, the atoms, in molecules, form a 6 sided crystalline structure which is less dense but larger in size. That transformation, and expansion between the states of liquid and frozen ice, changes the physical shape of water in a way that has immense expansive force. That force is strong enough to break and crack stone and concrete. As stone masonry deteriorates naturally, gaps and cracks will increase in size and therefore allow greater entry of water. As that water freezes between temperature cycles, increasing damage can occur, without proper upkeep and maintenance.
  4. Salts.  Salts common in de-icing streetways and walkways seep into foundations and affect masonry walls through saline rising damp.   These salts react with calcium hydroxide in the stone (and concrete as well) and can cause significantly accelerated duration.
  5. Biocolonization.  As stone and masonry surfaces deteriorate with time and exposure to the elements, especially in the presence of moisture, plant and fungal growth can occur on the surface of masonry.  As growth begins to occur  deterioration increases and growth continues and the cycle continues,  increasingly.

all of the items in the list above are characteristic examples of the nonlinear nature of the deterioration curve over time.  Essentially as time goes by, deterioration increases at an increasing rate.

The picture below shows an example of a historic brownstone building.   Like many historic brownstones, only the front facade of this building is actually clad in brownstone. The side facades and rear facade building are actually built with a historic common brick.  (It should be noted that a common brick is different from the common bond used in brickwork.)

In the 19th century, even into the early 20th century most buildings, even in many cities around the United States were made with mostly wood building framing  and even wood cladding materials.  By comparison, brick and stone masonry were considered luxurious. Brownstone and other types of sandstones were considered even more luxurious than brick work. Unlike brick masonry units, brownstone had to be cut.    In historic times, the modern power tools of today simply didn’t exist.  Such work, of stone cutting,  requires very special skills whereas brick can be made with manufacturing processes. For these reasons, brownstone was considered beautiful and special, rich and luxurious.

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But unlike other stones, such as granite, sandstone and brown stone are softer, even easier to cut and chisel than some types of soapstone, alabaster and some marble stones.

The picture below shows an example of brownstone columns. Corinthian ornature, from the last of the three Greek and Roman architectural classical orders, at the capital of the columns, is carved into the stone.   Incredible detail has been sculpted and carved into this example.

Sandstone, and therefore Brownstone, is a type of stone which is comparatively soft and easy to carve.

The picture below shows an example of four large rowhomes, side-by-side, all built with brownstone facades.   The buildings at the left of the photo have been built entirely with split face masonry at the facade. By contrast, the buildings to the right have been built with a flush face masonry unit at the upper levels. The difference is noticeable and throughout the day, the contrast will change. The split face masonry units will stand out and be more pronounced as the sun moves across the sky, as that light source direction changes on an hourly basis.  The appearance or aesthetic of the front facade changes correspondingly at the buildings with split face masonry units.

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In the closer view below, you can see that the ashlar stone used in the facade construction of this building is a split face masonry unit.    The cantilevered projecting bay, at the oriel window at the ground level, in this case with a picture window above under a Roman archway, is decorated with hand carved ornature at closed balustrade system.

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The bedmould at the underside of the oriel window is dressed with a dental crown, an architectural element from Greek and Roman classical architecture. This detail of the dental crown, as seen in the photo below, repeats throughout the historic neighborhoods of Washington, DC.

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At the ground level a stairway leads from the front entrance door to the public walkway. A guard-less (not permitted by modern Code requirements, without an allowance through grandfathering or similar) copper handrail with an antiquated patina adorns the brownstone facade and stairway.

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Similar to the underside of the oriel window bay, at the bedmould of the front entrance stairway guard rail capstone, dentil cornice pattern is used again.

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As discussed, there are several advantages to sandstone, compared to alternative types of facade cladding which make sandstone and brownstone an almost ideal choice. But sandstone is not perfect, it has some inherent downsides. Mainly, sandstone is a sedimentary stone which has formed over millions of the years of settling silica debris and particles. Those particles settle and form sheets that are part of the structure of sandstone and brownstone.

Over time, with exposure to the elements, when applied in thin sheets of cladding stones, like tile, especially laid with the grain parallel to the face of the facade,  The outer layers of sandstone or brownstone can flake off,  This process of deterioration to the point of flaking is what structural engineers, and architects, and geologists could refer to as delamination.

Sandstone and brownstone are very similar to flagstone in the way that the rock textures and layers are made from a sedimentary formation process starting several millions of years ago.

In the sections above, we talked about typical common causes of deterioration in brownstone which leads to disintegration of cohesive natural binders in sandstone and brownstone.  Some of the main typical types of environmental pressures are acid rain, freeze-thaw cycles, salts, and Bio-colonization.   These types of environmental pressures lead to accelerated deterioration.  As you walk through the historic streets of Capitol Hill, Georgetown, Dupont Circle and other parts of Washington, DC you can see many examples of sandstone and brownstone buildings. Within the architectural details of those buildings you will see many points of deterioration.

In the picture below you can see a brown stringer knee wall wall of a front staircase. This is an exterior element, and of great historic value. But there are large shapes of discoloration in the side of the stone knee wall.  This area of discoloration is caused by a combination of surface delamination, salt and calcium carbonate dissolution and  hydration / efflorescence, and by patching without a wholesale parging and without a close or proximate matching of color.

 

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Smaller individualized split face brownstone masonry units have similar issues at this building and others nearby.

Similar to the underside of the oriel window bay, at the bedmould of the front entrance stairway guard rail capstone, dentil cornice pattern is used again.

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You look close at the brownstone repairs and issues of spalling, you can see the stratified layers of delamination, common in sedimentary stone.

As discussed, there are several advantages to sandstone, compared to alternative types of facade cladding which make sandstone and brownstone an almost ideal choice. But sandstone is not perfect, it has some inherent downsides. Mainly, sandstone is a sedimentary stone which has formed over millions of the years of settling silica debris and particles. Those particles settle and form sheets that are part of the structure of sandstone and brownstone.

Over time, with exposure to the elements, when applied in thin sheets of cladding stones, like tile, especially laid with the grain parallel to the face of the facade,  The outer layers of sandstone or brownstone can flake off,  This process of deterioration to the point of flaking is what structural engineers, and architects, and geologists could refer to as delamination.

Sandstone and brownstone are very similar to flagstone in the way that the rock textures and layers are made from a sedimentary formation process starting several millions of years ago.

In the sections above, we talked about typical common causes of deterioration in brownstone which leads to disintegration of cohesive natural binders in sandstone and brownstone.  Some of the main typical types of environmental pressures are acid rain, freeze-thaw cycles, salts, and Bio-colonization.   These types of environmental pressures lead to accelerated deterioration.  As you walk through the historic streets of Capitol Hill, Georgetown, Dupont Circle and other parts of Washington, DC you can see many examples of sandstone and brownstone buildings. Within the architectural details of those buildings you will see many points of deterioration.

In the picture below you can see a brown stringer knee wall wall of a front staircase. This is an exterior element, and of great historic value. But there are large shapes of discoloration in the side of the stone knee wall.  This area of discoloration is caused by a combination of surface delamination, salt and calcium carbonate dissolution and  hydration / efflorescence, and by patching without a wholesale parging and without a close or proximate matching of color.

Smaller individualized split face brownstone masonry units have similar issues at this building and others nearby.

You look close at the brownstone repairs and issues of spalling, you can see the stratified layers of delamination, common in sedimentary stone.

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In the picture below you can see a historic window header and brownstone corbel at the National Building Museum, near Chinatown in Washington, DC. This set of architectural elements has also experienced deterioration and damage but in this case the property has been nonetheless maintained and preserved.

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From a conservation and preservation perspective, several approaches can be taken to improve conditions related to deteriorated historic brick masonry. Primarily, lime mortar brick joints and low temperature fired soft red clay bricks should be inspected and checked on a routine maintenance schedule, either seasonally or at least annually.   If brick masonry is kept in good condition, the life of embedded wood elements can be significantly extended.  Hire a professional contractor which specializes, understands and appreciates historic construction elements and buildings.

In this article we talked about the following terminology and  concepts, follow the links below for more related information from the IDS website:

  • Sedimentary rock
  • Silica
  • Sandstone
  • Architectural carvings
  • Reliefs
  • Precipitation
  • Soapstone
  • Alabaster
  • Marble
  • Corinthian ornature
  • Column Capital
  • Split face
  • Flush face
  • Ashlar stone
  • Cantilevered
  • Bay
  • Oriel window
  • Picture window
  • Roman archway
  • Balustrade system
  • Oriel window
  • Dental crown
  • Grandfathering
  • Copper handrail
  • Patina
  • Bedmould
  • Capstone
  • Knee wall
  • Calcium carbonate
  • Spalling
  • Stratified layers (of sedimentary stone)

These concepts are part of the fundamentals of historic masonry restoration, tuckpointing, and brick repair.

The links in the list above will take you to other articles with more information on defects, failures, preservation and repair of historic masonry.  You can learn a lot more on our blog.  Feel free to check it out.  If you have questions about the historic masonry of your building in Washington DC, fill out the webform below and drop us a line.  We will be in touch if we can help.