Masonry Magazine March 1968 Page. 7
A PEARL IN THE HEART OF CHICAGO'S LOOP
More than a billion years ago, the facing for the tallest building in Chicago's Loop was forged in tremendous heat and pressure deep within the earth under south central Texas.
Called "Pearl Grey," this distinctive new granite will adorn the exterior of the new headquarters building of The First National Bank of Chicago. Never before used on a major building, Pearl Grey was hewn from a quarry about 50 miles from Austin, Texas. The age of the quarry, a 15-acre exposed dome of coarse grained pink and white granite, has been set by the Texas Geological Survey at one billion, one hundred million years.
The granite is removed from the quarry by burning, drilling and sawing with twisted steel wire. Tremendously tough because of the nature of its birth, granite rates 6 to 7 on a scale of hardness, with talc as 1 and diamond as 10.
Taken from the quarry in 20-ton blocks, the Pearl Grey is then transported to a nearby fabricating plant and cut to a specified thickness. Carborundum or diamond saws are used in this process.
Next the Pearl Grey is face-finished by passing a 5000-degree Fahrenheit jet flame across each slab. This produces a textured but self-cleaning surface.
Slots are then drilled in the granite for anchors to tie the material to the bank's super-structure. The anchors are formed of non-magnetic stainless steel to prevent corrosion.
The granite was subjected to a three-month series of (Please turn page)
7