Historic Material and Installation Specifications

Early 20th century advertisers often promoted pigmented structural glass as a new panacea of the building materials industry. Their claims were not without substance. Unlike masonry units such as terra cotta, pigmented structural glass would not warp, swell, or craze. Nor was the glass highly susceptible to staining, fading, or burning. Like most glass products, it was impervious to moisture and could be easily maintained and usually cleaned with a damp cloth. Adaptable to a wide range of uses, the glass could be colored and textured to attain brilliant visual qualities. Perhaps most important, when compared to marble, the glass was easier to handle, less expensive to use, and simpler to install. The key to proper preservation and repair of both interior and exterior pigmented structural glass is a thorough understanding of the original material specifications and detailed installation techniques. Fortunately, these specifications and techniques remain virtually unchanged from their first early 20th century application. In the installation process, hot asphaltic mastic was applied to the ribbed or textured back of each glass panel. The panel was then pressed against a masonry wall and supported by metal shelf angles which were usually bolted to the masonry substrate on 3-foot centers. The joints between the panels were filled with cork tape or joint cement. The end result was a modular veneer of clean, uniform glass panels.

Exterior Installation

Essentially, the glass veneer was applied to a dry, smooth, and solid masonry or plaster-on-masonry substrate using an asphaltic masonry adhesive. Manufacturers recommended against affixing the glass directly to wood, either lath or paneling. Glass thicknesses of 1-1/32 inch or 7/16 inch were most common for commercial storefronts.

Shelf angles--18-gauge brass or stainless steel, 3 inch square with a 1/2 inch leg fastened directly to the masonry substrate--were used to provide additional support. Inserted along the bottom edge of the panels, they supported every second course of glass and were thus spaced not more than 3 feet apart. Horizontally, the angles were spaced approximately one every 18 inches with at least two used for any piece.

Basic exterior construction details. Photo: Courtesy, Bulletin for the Association for Preservation Technology, 13 (1981).

Actual installation involved applying daubs (2 inches to 3 inches in diameter) of hot asphalt-based mastic adhesive to the glass and then attaching the glass directly to the substrate. Manufacturers of the mastic recommended coverage of about 50 percent of the glass panels. A full 3 inch width of mastic coverage was recommended around detail edges or any holes in the panels. The mastic was applied in a molten state after being melted in an electric "hot cup." (Hot cups are still manufactured for this specific purpose and are made to hold enough mastic for a single daub.)

The next step in the installation procedure was to push the glass panel onto the masonry substrate. Every horizontal seam and abutment was separated by a 1/16 inch thick adhesive cork tape recessed from the front surface by 1/8 inch. Vertical edges were kept apart at a uniform 1/32 inch. In either case, the joint opening was then buttered with a joint cement which was colored to match the surrounding glass.

Proper detailing at the edges of the veneer could prolong the life of the pigmented structural glass. For example, to prevent possible chipping and cracking of the glass where it met the sidewalk, a cushion of neoprene or leather was provided and the exposed surface then caulked. The side edges of the glass were detailed in a variety of methods or the glass simply terminated at the desired location with the ends ground smooth. In either case, the edge was secured to the substrate with a mastic and the joints or void filled with joint cement or caulking compound. Where the edge of the glass abutted another material, such as the brickwork of a neighboring storefront, the glass was held back 1/8 inch to 1/4 inch from the adjacent material. The gap was usually filled with pliable caulk to permit expansion and to prevent moisture migration.

Interior Installation

White pigmented structural glass was used in this remodeling of the U.S. Custom House, Denver, Colorado. Photo: Gregory D. Kendrick.

Construction methods and materials were quite similar for interior and exterior uses of pigmented structural glass. Most interior veneers were the same thickness and approximate dimension of those used for exteriors. Minor differences did, however, exist. For example, joints between the pieces of glass could be reduced to little more than hairline cracks for interior applications due to the limited thermal expansion of the substrate. On the other hand, the use of glass as an indoor ceiling material created unusual installation requirements.

Ceiling slabs 11/32 inch in thickness were attached to 1 inch x 4 inch wood furring strips with mastic (a full 4 inch width coverage was recommended around the edge of the panels). Brass wood screws and small rosettes, protected with felt exterior covers, provided additional support.

Owners of the Hadley Dean Glass Company Warehouse, St. Louis, preserved the unusual polychrome pigmented structural glass interior lobby. Photo: Paul Marshall.

As a non-veneer material, pigmented structural glass was generally used for counter and table tops and restroom partitions. Counter tops presented little or no unusual installation problems. Partitions, however, often involved formidable installation challenges; for example, enormous glass panels, weighing up to 16.25 pounds per square foot and measuring 1 inch to 1-1/4 inches in thickness were used. The desired thickness was obtained by cementing two 7/16 inch slabs together with mastic.