Glass block, also called glass brick, is a building product that combines the optical properties of glass with the structural behaviour of masonry. Glass blocks are used to create partitions, curtain walls, windows, and decorative features where light transmission is desired but transparency is not. They appear in styles ranging from Art Deco commercial buildings of the 1920s to contemporary minimalist bathrooms and facades.
Pittsburgh Corning and Seves
Pittsburgh Corning, founded in 1937 as a joint venture between Pittsburgh Plate Glass and Corning Glass Works, was the dominant glass block manufacturer in the United States for most of the twentieth century. The company developed and popularised many of the standard glass block patterns, sizes, and installation methods still in use today.
In 2017, the Pittsburgh Corning glass block business was acquired by Seves Group, an Italian manufacturer of glass block and other architectural glass products. Seves operates production facilities in Europe and the United States and markets glass block under the Seves Glass Block and Pittsburgh Corning names. The combined entity produces millions of glass blocks annually for distribution worldwide.
Hollow Glass Block Construction
The most common type of glass block is hollow, manufactured by fusing two pressed glass halves together at high temperature. The fusion process creates a partial vacuum inside the block containing roughly 80% air by volume. This trapped air provides thermal and acoustic insulation far superior to a solid glass pane of equivalent thickness.
A standard hollow glass block measures 190 by 190 by 80 millimetres, or approximately 8 by 8 by 3 inches, though half-blocks, end-blocks, corner blocks, and bullnose shapes are also produced. The face of the block is typically patterned with ribs, waves, or bubbles that diffuse light and obscure vision. Different patterns offer varying levels of privacy, from fully obscure to semi-transparent. Light transmission values range from approximately 50% for heavily textured privacy blocks to over 80% for lightly patterned blocks.
The thermal performance of standard hollow glass block is modest. A single 80 millimetre thick block has a U-value around 2.8 to 3.0 W/m2K, roughly comparable to a single-glazed window. Special energy-efficient blocks with internal low-e coatings can improve this to approximately 1.3 W/m2K.
Solid Glass Block and LuxCube
Solid glass blocks differ fundamentally from hollow blocks. They are cast as a single solid piece of glass, typically annealed slowly to relieve internal stress. Solid blocks are heavier, roughly 4 to 5 kilograms for a 190 by 190 by 80 millimetre unit, compared to roughly 2.5 kilograms for a hollow block.
Solid blocks transmit light with far less diffusion than hollow blocks, making them suitable for feature walls and high-end architectural applications where a more crystalline, jewel-like appearance is desired. LuxCube is a brand of solid glass block manufactured in Europe, known for high clarity, tight dimensional tolerances, and a range of colours including clear, blue, green, and amber. Solid blocks are also used as glass floor pavers and in load-bearing glass walls when designed by a structural engineer.
Fire-Rated Glass Block
Certain glass block assemblies carry fire-resistance ratings under ASTM E119 or equivalent standards. The fire resistance comes not from the glass itself, which will eventually soften and deform, but from the way the block assembly resists fire penetration for a specified period. Fire-rated glass block panels using specially formulated blocks and fire-resistant mortar or caulk can achieve ratings of 45, 60, or 90 minutes. These assemblies are used in fire-rated corridor walls, stairwell enclosures, and other applications where both light transmission and fire compartmentation are required.
Fire-rated glass blocks are typically solid rather than hollow and may incorporate wire mesh within the glass for additional integrity during fire exposure. The specific products and installation methods must be tested as a complete assembly, and the rating applies only to the tested configuration.
Installation Methods: Mortar vs Silicone
Glass block can be installed using either traditional mortar or a modern silicone system. Mortar installation uses a Portland cement and lime mortar with reinforcing ladder wire embedded in the horizontal joints every two or three courses. Mortar joints are typically 6 to 10 millimetres wide. This method produces a wall with substantial structural capacity but is labour-intensive and requires the masonry skills of setting, jointing, and striking.
Silicone installation uses clear or colour-matched structural silicone applied from a caulking gun. The blocks are set with small plastic spacers that create a uniform 2 to 3 millimetre joint. Silicone systems are faster to install than mortar and produce a sleeker, more contemporary appearance. Movement capability is higher with silicone, making it better suited to areas subject to vibration or thermal movement. However, silicone joints have lower compressive strength, limiting panel sizes unless a supplementary framing system is used.
Panel Size Limits and Structural Considerations
Glass block panels installed with mortar are typically limited to 7.5 square metres in area, with no dimension exceeding 6 metres, per building code guidelines. Panels installed with silicone may have different limits depending on the manufacturer's tested data. Expansion joints must be provided around the perimeter of every panel to isolate the glass block from the surrounding structure. Glass block is not load-bearing and must not be used to support building loads from above unless specifically engineered for that purpose.