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Miscellaneous Masonry Products: Glass Block
1) Development
2) The Manufacturing Process
3) Glass Block Data
4) Glass Block Properties
1. Development Of Glass Block
Glass, one of man's most useful products, is also one of the oldest (about 4,000 B.C.). In ancient times, formed pieces of colored glass were considered as valuable as precious stones. Today, glass, although valued chiefly for function, also takes artistic and beautiful forms. Glass Block units have a unique combination of aesthetic and functional properties which permit the vision, taste, and skill of the architect and designer to be utilized to the fullest. In the United States, glass block has been produced since 1938.
Various glass blocks
2. The Manufacturing Process
A glass block begins as a molten glob of glass leaving a glass tank at a temperature of 2300F. The hot glob falls into one of several molds on the rotating table of the glass block press. The hot glob is pressed to form one half of the glass block. After extraction from the mold, the still hot glass block half is placed on a moving belt which transports it to a sealing machine. Here, two block halves are positioned while a gas flame melts the glass again where the seal will be made. The seal is now made. Since the glass temperature at this point is about 900 F, the air trapped in the hollow block is quite hot. Upon cooling, the air will contract to form a partial vacuum in the enclosed space. This is the reason for the high insulation value possible with glass block. The blocks leave the sealer and are placed in a continuous Lehr or controlled cooling oven. The cool block exit the Lehr about three hours later. From the Lehr, the sealed block are transported to the Edge Coater where they receive a Poly-Vinyl edge coating. This coating provides a better bond between the glass and the mortar used to assemble glass block panels.
3. Glass Block Data
Glass Block Dimensions And Weight
| Dimensions (in.) | Average Weight per Block (lb.) | |
| Nominal | Actual | |
| Thickness 3 7/8 / 4 and 3 1/8" | ||
| 6 x 6" | 5 3/4" x 5 3/4" | 4 |
| 8 x 8" | 7 3/4" x 5 3/4" | 6 |
| 12 x 12" | 11 3/4 x 11 3/4" | (3 7/8" only) 16 |
| 4 x 8" | 3 3/4 x 7 3/4" | 3 1/2 |
| 5" sq brick | 5 x 5 x2 5/8" | 6 |
| 8" sq brick | 7 5/8 x 7 5/8 x 3" solid | 15 |
Glass block are used in traditional roles to:
- Control window area heat loss and gain
- Control light transmission and noise
- Increase building security
- Reduce window area maintenance costs
Recently, glass block are finding new and innovative uses such as:
- The complete curtain walls of a building
- To define and enclose interior spaces
4. Glass Block Properties
4.1 Heat Transmission
| Block Size and Type | U-Value (BTU/hr - Sq. ft. - degrees F) |
| 6" sq single cavity | 0.60 |
| 8" sq single cavity | 0.56 |
| 8" sq double cavity | 0.48 |
| 12" sq single cavity | 0.52 |
| 12" sq double cavity | 0.44 |
| 4 x 12" sq single cavity | 0.60 |
| 4 x 12" sq double cavity | 0.52 |
| For comparison: single sheet of glass |
1.10 |
4.2 Light Transmission
| Block Type | % Light Transmission |
| Glass brick | 80 |
| Clear block | 75 |
| Clear block with irregular glass | 43 |
| Diffusing block with solar control | 20 |
Popular "Thin" Block Pattern
4.3 Resistance to Surface Condensation
4.4 Impact Strength, Compressive Strength, Thermal Expansion
| Style of Block | Impact Strength in (lbs.) | Compressive Strength (psi) | Thermal Expansion Coefficient |
| Standard | 50-60 | 400-600 | .0000047/F |
| Thinline Series | 40-50 | 400-600 | .0000047/F |
| VISTABRIK® | 80-100** | 80,000 | .0000047/F |
**Produces surface mar only
The NW Masonry Guide Table of Contents
Masonry Institute of Washington
Washington State Conference of Mason Contractors
