The plan to move services from the Crypt to the Crossing caused several changes. Temporarily abandoned was the erection of the Lantern and Spire. Enclosing the openings to the future North and South Transepts was also necessary. Work began on a temporary dome over the Crossing, America’s largest dome.
The Guastavino Company was pioneering the erection of domes and vaulted ceilings of thin terra cotta tiles. They had been involved with the Cathedral since 1900 starting with the ceiling in the crypt. They also had put a barrel vault in the choir and the chancel.
The dome, begun in 1909, is one of the largest masonry domes in the world. It is about 135 feet in diameter, measured across the lower part of the spherical surface. The crown is 200 feet above the floor of the building. It took 3 months and 16 days to complete the job. The dome consists entirely of burnt clay slabs 6 x 15 x 1 inch. Portland cement bonded the clay pieces. This therefore created a monolithic dome shell of unprecedented thinness. The dome construction didn’t involve any scaffolding or falsework.
The Process
The workers laid up the thin terra-cotta slabs in successive circular layers. The joints broke vertically and laterally and formed a hemispherical dome which surmounts the four great arches. The novel method of construction saved great expense by avoiding the erection and removal of heavy scaffolding and falsework. As well, no scaffolding meant reduced risk to workers associated with these tasks.
The lower portion of the dome springs from the four corners of the great granite arches. From these four starting points upward the successive layers of tile widen rapidly over the curve of the great arches. They eventually converge in a perfect circle at their crown. This was not the use intended for the arches. Rafael Guastavino Jr. had to design the dome using those existing structures.
Finding and Using the Center Point for America’s Largest Dome
They threw 1/4 inch cables across from the the center of the crown of each of the four granite arches. This then established the center point for America’s largest dome. They intersected at right angles exactly in the central point of the dome’s diameter. Engineers accurately determined the point with transit and level observations. Turnbuckle attachments rendered it possible to adjust for temperature variations. The engineers attached a 4 inch square steel plate with a center hook to the intersection of the cables. From this hook a fifth cable extended to an 800-pound weight at the floor. This weight acted as a guy to maintain the center point. The threaded end of the hook extending through the plate served for the attachment of the steel tapes. These tapes indicated the circumference of the dome and the hemispherical curvature.
Stretching these steel tapes determined the vertical and lateral curvature for laying each course. They went from the established center point to the interior dome surface. These tapes were marked for one half the diameter of the hemisphere. They instantly indicated the exact position where each tile should be laid to form its part of the hemisphere. The dome is six tiles in thickness at the base, or about seven and a half inches. This decreased to three courses or four inches at the top.
The Unique Dome Completed
The work progressed rapidly and the materials sustained their weight and that of the mechanics without the slightest mishap. According to every known theory, work created in this manner would fall of its own weight. Each morning, as the artisans resumed work, the material laid the previous day was found to have acquired such rigidity as to be capable of supporting their weight and the fresh construction. The dome was originally intended to be up 10 years. America’s largest dome has lasted over 100 years.
This post was not about divine stone, but about its cousin, divine clay
- Archives of the Cathedral
- Irma and Paul Milstein Division of United States History and Genealogy, The New York Public Library. Manhattan: Amsterdam Avenue – Cathedral Parkway
- Museum of the City of New York
- Zawinsky, N., Fivet, C., & Ochsendorf, J. (2017). Guastavino design of the 1909 thin brick dome of the Cathedral of St. John the Divine. Construction History, Vol 32(2), 39-66 dog:10.2307/26476167
- Scientific America, October 30, 1909, Scientific American Supplement N0. 1765
- L. Ramazotti, La cupula para San Juan el Divino de Nueva York de Rafael Guastavino. Las bóvedas de Guastavino en America, S.Huerta (ed.) Madrid Inst. J de Herrera, 2001