Chemistry of Lime Mortar – Neighborhood Preservation in Philadelphia

19th Street Baptist Church
19th Street Baptist Church

The Chemical Heritage Foundation released an episode of their award winning podcast Distillations.  In this episode, Randy Ruth, formerly of, discusses the chemistry behind lime mortar, historic masonry buildings and the ongoing efforts to preserve one of the finest works of notable Philadelphia architect, Frank Furness. The 19th Street Baptist Church located in South Philadelphia has been in a state of disrepair for decades.  This episode sheds light on the chemistry behind its construction and the ongoing preparation to save it.

Inside the 19th Street Baptist Church
Inside the 19th Street Baptist Church



Photos © Sean K Maxwell
Other stories related to the 19th Street Baptist Church:

Yes We Can Save The 19th Street Baptist Church

Progress at 19th Street Baptist

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Audio Transcript Excerpts:

Randy Ruth, at the time, was the lab tech at He bridged the gap between architects, conservators and masonry practitioners on what types of building materials to use in reconstruction and restoration of masonry projects throughout the country.

“Randy and his colleagues at based in Quakertown, Pennsylvania were brought in to try and save the 19th Street Baptist Church

We took a look at at the types of mortars that were there and by knowing that the building was built in 1874 we could gauge what those original binders were without even necessarily doing an analysis. Architects, engineers and of course masons would use the local available materials which were at the 19th Street Baptist Church primarily lime based binders.

Lime is a very general term for inorganic materials that contain calcium and it’s been use in concrete and building materials since the Roman Empire. It’s derived from limestone and other rocks that are primarily made of calcium carbonate.

Basically lime is that calcium carbonate stone and it’s put into a kiln with fuel. These kilns would then be fired up to 900 degrees Celsius or slightly above and then the resulting product is a limestone that loses about a third of its weight because it’s driving off carbon dioxide so that calcium carbonate is now becoming calcium oxide.

This is all part of what’s called the Lime Cycle. Calcium carbonate is heated to produce calcium oxide and then it’s combined with water to form calcium hydroxide. Different amounts of water is added to form different consistencies.

That process is called slaking, if just a small amount of water is added it’s going to turn into a powder. If a little bit more water is added it’s still calcium hydroxide but it’s in the form of a putty.

Once the lime is in this putty form it’s spread over bricks and stones and hardens into a solid bond.

That limestone then stays malleable and flexible in a wall system in comparison to more modern materials.

Modern materials like cement. Portland cement is the most common type of cement used around the world. It’s made of primarily alite or tricalcium silicate, a more rigid and less permeable material than lime.

You’re walking down the sidewalk and wondering why are they sectioned off in four food squares or six foot squares and that’s because Portland cement is very brittle and we have to tell it where to crack and where to go versus the lime buildings like the 19th Street Baptist Church. There are no control joints and things were able to move freely and accommodate small bits of movement because lime unlike Portland Cement has the ability to heal itself, something called autogenous healing.

This autogenous healing is part of why the 19th Street Baptist Church is still standing today but the original lime mortar is all but dissolved now and that’s why the building is crumbling.

Lime mortars as they age and as they wear, they’re meant to be replaced so there’s always a maintenance issue associated with them. I hope that there is a lot of architectural salvage that can occur to pay homage to the traditional materials that were originally by Frank Furness and his architectural team.

In essence they should be able to replenish the lime mortar in the structure, but after so many years of neglect it’s getting late in the game.”

Photos © Sean K Maxwell

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