Items of Interest January 21, 1933 Issue

Items of Interest

Christian Scientists in Boston, who have noted the wooden staging about the finial at the very top of The Mother Church dome, the pineapple as it is often called, have surmised that some repairs were being made. These repairs have now been completed. The base of the pineapple was formerly of terra cotta, like the dome itself, and the pineapple was of carved limestone. Running through the pineapple in the center and projecting several feet above is a lightning rod to conduct lightning into the ground without damage to the building.

Last fall the pineapple was inspected and found to be cracked, due to the action of frost, wind, and weather. Then began a study of how best to make repairs which would be permanent. New developments in building materials have of late brought into use materials not before available. Because of the exposed position of the pineapple, two hundred and twenty-five feet above the street, it was felt that both it and its base should be of material least affected by temperature changes. Eventually those charged with solving the problem decided upon cast shale for the base course at the apex of the dome and cast aluminum alloy for the pineapple. This plan provided that the pineapple be cast in a shell of three parts varying from one quarter inch to one half inch in thickness. The molds were made from carefully prepared new plaster patterns corresponding exactly in outline and design to the original. Within the pineapple is an ingenious arrangement to take care of expansion and contraction. It consists of a series of cross braces at various levels, welded together, which strengthen the shell; also, a system of reamed rings of steel lined with reamed fiber insulating rings, the cross bracing being secured to the rings and the rings to the steel rod already mentioned. This use of insulating rings avoids the contact of other metals with the aluminum shell. A set of large and heavy steel springs acts upon the rings in such a manner as to make it possible for the rod to lengthen and shorten as a result of temperature changes without distorting the outer sheell. To further insure freedom of movement because of temperature changes, the base of the finial is set upon a bed of nonhardening elastic cement, and has an apron covering the shale capping course below to provide for taking up the slacking. The lightning rod, secured into the top of the steel center rod, is completely insulated also by means of reamed fiber rings.

The shell unit which takes the place of the original vitrified terra cotta capping course under the finial structure is made up of a composition of approximately one hundred per cent crushed shale which has been fired. It is set in mortar cement firmly encircling the steel rod, and the joints are calked with nonhardening elastic cement. Shale was selected because it is very little affected by temperature changes. Of course, care was exercised in casting it that in color and texture it should conform reasonably well to the existing roof covering of terra cotta. The aluminum for the pineapple and its base above the dome was chosen partly because of its lightness and toughness, but mainly because it is said to be noncorrosive except under certain extraordinary circumstances; and since the salts of aluminum are practically colorless the use of this metal should eliminate discoloration of adjacent parts of the structure. To eliminate reflection and make a surface of a matching color and texture, a particular surface finish was given the aluminum by mechanical sandblasting, which brought the desired result. Great care was taken by the aluminum company which did the casting to turn out a perfect job. Thus the finial of the lantern topping the dome of The Mother Church is now complete and expected to withstand many years of service.

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