Preserving Gilded Age Grandeur
- eva_hatzaki
- 5 hours ago
- 9 min read
The Restoration of the Convent of the Sacred Heart’s Landmark Houses
By Alafia Akhtar, Senior Project Manager; Senior Architectural Conservator
The Convent of the Sacred Heart School, located on Upper East Side of New York City is comprised the former Otto Kahn House (1 East 91st Street) and neighboring James A. Burden House (7 East 91st Street).
Photo: Convent of the Sacred Heart
The Otto Kahn House is a five story, Neo-Italian Style building constructed between 1914-1918 in St. Quentin limestone, sourced from St. Quentin, France. Similarly, its neighbor, the James A. Burden House is a five story, Beaux Arts style building constructed in 1905 in an American, Indiana limestone. The two buildings were combined as part of the expansion of the Covent of the Sacred Heart School in 1940. Both 1 and 7 East 91st Street are vital parts of New York City history, particularly as part of the Gilded Age; an era known for its excess and extravagance particularly with the rising industrialists of America. Both buildings are an illustration of this opulence in their use of building materials, size of houses and location on the Upper East Side, directly across from Central Park in Manhattan. The historical significance of these homes is recognized as they are listed on the United States National Register of Historic Places, recognized as a New York State Historic Preservation site, and are individual New York City Landmarks.
The project’s success hinged on scientific testing, global material sourcing, and precise execution within a living, operational school environment.
Investigation
CANY was brought to site to document and provide recommendations for repair of existing deteriorated conditions consisting of spalled limestone at the street facing façades. A conditions assessment of the existing limestone façades was performed as part of the investigation alongside ACM testing and localized probes to document existing backup conditions. Both the St. Quentin limestone and Indiana limestones were documented along with supplementary materials at both buildings inclusive of brick masonry, copper, clay tiles, slate shingles, and a decorative plaster ceiling within the carriage way of the Otto Kahn house. The types of conditions noted at the Otto Kahn house were severe erosion and weathering of the stone resulting in the sugaring of the surface, spalling and cracked stone, deterioration of previous repairs, open mortar joints, and localized areas of biological growth.
Previous interventions at the building, where replacements were performed, were not matched in kind. Instead, elements such as railing balustrades were replaced in domestic Indiana limestone and ashlar units were replaced in a Miami based oolite stone. Given that the St. Quentin limestone was from a different climate that does not experience the freeze thaw of New York City, the weathering of the original material appeared to be severe as compared to its domestic Indiana limestone counterpart at the James A Burden house. The deterioration at the James A Burden House was minor, with localized areas of open mortar joints, cracks, small scale spalls, and minor stone displacement due to adjacent embedded steel components. Observed deterioration was deemed to be typical given the age of the building and expected weathering overtime.
Photos: Skyline Restoration
Invisible repair of 1 East 91st Street Carriage Way plaster ceiling panel, expertly performed by Evergreen's artisans.
Repair Documents and Testing
Repair documents and specifications were assembled for both buildings once the investigation was concluded. During this phase, ownership emphasized the importance of schedule and the priority of all work and demobilization concluding prior to graduation in June the following year. Repair documents and bidding phases were fast tracked and maintaining a schedule was prioritized. During the bidding phase, each contractor was assessed on the basis of knowledge and experience with historic buildings and repair along with their respective methodologies for mobilization keeping in mind the communicated schedule which allowed for 15 months for filing of drawings with Department of Buildings (DOB), submission of samples and approvals with Landmarks Preservation Commission (LPC), inspections and required repairs, and demobilization of all scaffolding including sidewalk shed.
Though both the Otto Kahn House and James A Buden House are part of the Convent of the Sacred Heart school complex, they are recognized as individual buildings on their own block and lot. Therefore, two separate sets of documents were created for both buildings. Filings for each of the buildings were also performed separately with both the DOB and LPC. To accurately specify materials for the repair of both buildings, testing of the existing limestone, mortars, and plaster (at Otto Kahn House) was required.
Compression and absorption testing guided a material strategy that balances historical authenticity with long-term durability.
As a result of the observed conditions, requirements of LPC, and to aid in accurate repair details/specifications various material testing was performed. At the Otto Kahn House, due to the sugaring surface of the existing limestone, two stones were removed from discreet locations to undergo Compression (a laboratory test, per modified ASTM C170) and Absorption (a laboratory test, per modified ASTM C97) testing in a local laboratory. The results of the test conveyed that the existing stone was highly absorptive with low compression strength. The existing limostone mortars at both homes were digested using Gravimetric Mortar Analysis to determine the components of each historic mortar for the purposes of replication. Along with being a requirement of the New York City Landmarks Preservation Commission, understanding if existing materials used were appropriate for the building materials, especially the St. Quentin limestone, given its absorptive and compressive nature was important. All mortars were determined to be typical Type N (lime, sand, cement) mortars.
As a result of the compression and absorption testing of the St. Quentin limestone, a natural hydraulic lime mortar and patching material was specified for the project. These types of mortars typically dry at a longer rate and are harder and less absorbent than a Type N mortar typically used for historic structures. This would help reduce the amount of water absorbed by the stone and allow for a slower weathering process. In contrast, since the Indiana limestone was a domestic stone, quarried from an area that experiences similar freeze thaw cycles to New York City, the stone appeared to have sustained less deterioration, and the use of a Type N Mortar and single component cementitious repair patching and crack repair material was deemed appropriate.
Photos: Skyline Restoration
Existing deteriorated stone stair panels replaced with Vaio Galina limestone. Reconstruction here takes care to preserve original coping stones and ornamental newel post element.
For all materials, but especially the NHL mortar and patching, the Contractor, Skyline Restoration, was required to perform onsite mockups for CANY review, documentation and where applicable, submission to LPC. Mockups were especially important for the installation of the NHL mortars as the curing of the material required extra attention to specific processes.
In addition to stone and mortar testing, the existing plaster ceiling within the carriageway at the Otto Kahn House was also tested. To properly cast replacement sections of deteriorated/spalled plaster, and repair cracked areas with compatible materials, knowledge of the historic plaster was deemed important. A section of plaster was removed for Gravimetric analysis of each subsample of the decorative element. The removed section also functioned as a probe for documentation of underlying anchorage and “hanging” components, which assisted in design of replacement sections. Digestion of the plaster revealed that the historic material was a gypsum plaster with two coats. The base coat was reinforced with twisted fibers to increase the tensile strength of the plaster, and the second coat was the exposed finish coat.
Stone matching and lead times varied for both buildings given that one stone was domestic (James A Buden House) and the other foreign (Otto Kahn House). Given that both buildings are individual New York City landmarks, LPC was particularly stringent about sample submissions for any stones to be used at either building for replacement or dutchman repair.
Photos: Skyline Restoration
Approved alternative to St. Quentin Limestone, Vaio Galina Limestone tested well for strength, color and appearance.
At the Otto Kahn House, Skyline Restoration, originally provided St. Quentin limestone to LPC for approval given its known historic use at the building. Once the stone was approved by LPC, Skyline Restoration attempted to contact the quarry for information regarding lead times which were a key driving factor for the project given the tight timeline, however, contact with the quarry was challenging. To prevent stalling of the project, Skyline Restoration suggested the use of an alternate limestone from Italy, Vaio Galina, which was a close match to the existing. Prior to submitting to the LPC for approval, the compression and absorption rates of the substitute stone were reviewed to ensure that it was like the existing St. Quentin, in order to avoid any negative interactions between the replacement and existing stones after installation.
Once it was determined that both stones had similar properties, the replacement was submitted and approved by LPC. Given that Vaio Galina stone was an international stone, another hurdle that was overcome was the long lead times for the stone. To prevent any delays, CANY surveyed the entire building, identifying replacement and dutchman stones which were all included within a primary order once shop drawings were approved. In addition to the required stones, Skyline also acquired allowance slabs of the stone for any additional work that may result from demolition of areas where repair work was required. Lead times for the stone were upwards of 5-6 months due to shipment of stone via cargo ships and customs inspections. In contrast, sourcing the Indiana limestone through local stone vendors was easier given the extensive use of the material throughout New York City’s built fabric. The James A Burden house had two types of textures: striated and flat, at the building, both were submitted and approved by LPC. Stones required for the minor dutchman at the building were obtained within weeks of shop drawing approval.
CANY’s meticulous investigation revealed the contrasting resilience of imported St. Quentin limestone and domestic Indiana limestone.
Ongoing Repairs
At the start of the project, ownership had requested that the project be concluded, and all scaffolding and sidewalk shed be demobilized prior to graduation in June of 2025. To achieve this CANY and Skyline worked systematically at both buildings to complete required repairs. CANY reviewed all existing conditions and required work was scoped out early into the project, which was facilitated using pipe scaffold. The scaffolding allowed for access to all areas of the building for the entirely of the project. While replacement stones and dutchman were in the process of being produced and delivered, Skyline proceeded to complete all other work consisting of patching, crack repair, repointing, etc. This work was subsequently signed off by CANY to allow for any areas that did not require replacement units to be demobilized preventing additional rental fees for the owner and working towards the goal of full mobilization before graduation in June 2025. At locations where replacement units and dutchman repairs were pending, all backup masonry work was completed and protected to allow for a seamless replacement stone installation.
Photo: Skyline Restoration
Innovative Corner Stone shoring solution, shown here, permits future stone resetting while maintaining full upper stone support.
Part of the required work was the rebuilding of an existing deteriorated corner located within the northwest courtyard of the Otto Kahn House. As part of the rebuilding, existing steel was exposed to assess the conditions and prepare for the scraping, priming, painting, and waterproofing of the steel. However, once sections of the corner began to be exposed, modern day introductions of water tanks above the corner, brought into question the capacity of the existing steel structure to hold the additions. To understand the existing steel capacity prior to designing a potential structural reinforcement, Coupon Testing was performed. Coupon Testing helps in understanding steel strength and its ability to be welded to; especially given that historic steel is not as strong as modern-day steel. Results of testing revealed average tensile strength of material, which reduces the amount of potential flange and web reinforcement required to supplement localized steel loss from corrosion. However, the steel did contain high phosphorous content which is not conducive to welding and can result in cracking. As a result of the coupon testing, welding procedures were modified to ensure that the steel would not be damaged during required repairs.
Ownership was kept abreast of work progress via weekly project meetings. Since the buildings function as a school, close coordination between CANY, Skyline, and ownership was required. Noise mitigation plans were implemented on dates where school events were underway and most of the work which required demolition was performed when the school was closed for various holidays and breaks.
With the combined coordination efforts of CANY and Skyline, all work was submitted and approved by LPC in a timely manner allowing for work to progress and conclude ahead of the required schedule in May of 2025.🀰
Project Team
Owner
Convent Of Sacred Heart
Maurice Pinard, Director of Operations
Jamie Bowen, Chief Administrative Officer
Consultant
CANY
Alafia Akhtar, Senior Project Manager; Senior Architectural Conservator
Nathanael Morejon, Architect of Record
Lynne Vu, Senior Engineer
Jason Chen, Project Engineer
Xsusha Arogunmati, Director of Architectural Conservation
Contractor
Skyline Restoration
Spiro Markatos, Account Executive
Rafal Horodecki Sr, Project Manager
Krzysztof Zajac, Assistant Project Manager
Sergio Cruz, Project Superintendent
Supported by
Alpha Glass
BIR Restoration
Cathedral Stone
EverGreene Architectural Arts
Glen Gery
Gotham Building Supply
Limeworks (de Gruchy's)
Prosoco
Robert Young & Sons
Spec Mix
Spring Scaffolding
Vaio Galina
Vesta Cast