Glass block

Glass block (also called glass brick or hollow glass tiles) are two hollow rectangular cups that are sealed together at their open faces in high temperatures. Glass block structures are built like masonry construction. Partitions and curtain walls made of glass block are not used as load-bearing walls but have an impressive compression strength ranging between 400 and 600 pounds per square inch. These blocks are excellent insulators and can reduce thermal and sound transmission. There are a variety of patterns for either directing or diffusing light. The standard size of a glass block is either 8 or 12 inches square and usually 4 inches thick.

History
Glass block is a term that is loosely applied to describe a variety of different but related products and applications including pavement lights, prismatic glass, hollow glass blocks, and glass-concrete construction. The term is often differed from every manufacturer and changed frequently. Inventions and technical advances in the glass industry led to what is known today as hollow glass block. The idea of the hollow glass block as a building material can be traced back to Gustave Falconnier, a Swiss architect and engineer, who patented hexagon shaped hollow glass blocks that were manufactured individually by being blown in a mold then sealed air-tight with a molten glass plug while still hot. Soon his products were being produced under patent agreements by several European manufacturers and were exhibited at the 1893 World’s Columbian Exposition in Chicago. These blocks endured problems with stability and condensation but had great success with young French architects. In the 1930s, these blocks were still on the market and seen occasionally in the United States.

Experimentation to find more stable alternatives continued and in 1903, hollow, open glass bricks emerged in Germany. These German bricks were manufactured by pressing glass into a mold and remained open on the bottom. The stability of these blocks were also questionable but were still available in Europe and the United States until the late 1920s. The use of conical or prismatic glass slabs for lighting dark interiors and basements had been common in European and American urban centers throughout the 19th century. In the last quarter of the 19th century, these illuminating blocks were used frequently to reinforce concrete settings. In 1911, Friedrich Keppler, head of the German Luxfer Company, developed a vertical version for partition walls and windows. Solid tiles were usually between 4 and 6 inches and were cast in sections of dumbbells. The edges contained the supporting reinforced concrete bars, covering them completely between their flanges and rendering them nearly invisible due to reflection. This system was highly popular during the rise of modern architecture in the 1920s. In order to improve sound and heat insulation, producers experimented with two layers of glass tiles using a tongue-and-groove system to fit both halves onto each other, leaving a hollow space between. Internal condensation still proved problematic. American glass producers experimented with solid and partially evacuated blocks and in 1929, the Structural Glass Corporation introduced several sealed hollow glass blocks. Four years later, the Owens-Illinois Glass Company introduced the first pressed glass block. The Owens-Illinois block was a five-sided dish that was sealed with flat plate glass. It was first introduced for demonstration at the Century of Progress Exposition in 1933.

Advances in pressing techniques led to the development of functional hollow glass blocks. The machinery pressed two identical concave halves together under heat and pressure, which made the glass block airtight. Machine-made hollow glass blocks had a higher degree of insulation and a more uniform look. Also, these blocks were stronger and easier to install that their ancestors. The introduction of Insulux, from Owens-Illinois in 1935, was one of the first widely used hollow glass bricks, which was sealed with lead. It was advertised as being ideal for exterior windows and partition walls for factories, offices, and apartments. Owens-Illinois’s No. 1 series was an attempt to make these glass blocks matched to brickwork. The blocks bonded with two courses of ordinary brickwork and could be laid in a similar bond. Insulux 200, 300, and 400 blocks soon took No. 1’s place and were more economical. These blocks came in 6-, 8-, and 12-inch sizes. Pittsburgh Corning, in an attempt to surpass Owens-Illinois, formed the Corning Glass Works and the Pittsburgh Plate Glass Company in 1936 to develop similar products. Corning Glass Works had presented its Corning Steuben Block in 1935 but the block did not reach perfection until 1938. This perfected block, known as the PC block, was made from Corning’s heat-resistant Pyrex, which could reduce its expansion and contraction considerably. The establishment of the new glass block as an ideal aesthetic treatment followed quickly after the block’s initial introduction. 20 million blocks were sold just two years after being introduced. The block lost popularity in the late 1970s, caused U.S production to nearly cease. Glass blocks are still produced in most of it original forms (but limited quantities) by the Pittsburgh Corning Company.

Manufacturing Process
Glass blocks were originally hand-blown or cast by molds. The use of hexagonal shapes helped to reduce pressure on the corners. An airtight seal was created between the two halves initially by the addition of a cementatious and solder-based sealant. In the automated process, the first step to producing glass blocks starts with melting the glass in a tank furnace. The molten glass is then deposited in measured amounts into a pressing machine. Once the appropriate amount of glass for a half block is release, it is cut and poured into a mold on an automatic pressing machine. The pressing machine has a revolving table carrying 12 female molds. Each mold consists of two machined cast-iron parts. One cast-iron part established the face size while the other determines the walls and thicknesses of the flange. Each mold is moved into position under a plunger that is equipped with a male mold machined to produce the configuration for the block’s interior face. Air is blown into the mold and glass, which should cool the glass slightly. At the last station, the block is moved to the sealing machine where the two halves will be sealed. The edges of the blocks are then heated and brought together with less than ½ inch between them, which allows for air to escape, and fused. During the cooling process, the air will contract and a partial vacuum will be formed. The final piece of glass is placed on a conveyor belt and passed through a furnace. Traditionally, the block would have been inspected and placed on revolving spindles as the sidewalls were covered with a resilient coating of synthetic resin and a fine spread of sharp-grained sand or marble dust. Now, the blocks undergo a plastic coating to improve bonding.

Uses and Installation
In the United States, glass blocks were used for industrial, commercial, and residential architecture. Glass blocks were laid by masons in the same style as brick. Portland cement mortar was the most popular type of mortar used in this process and walls required proper expansion joints at jambs and heads. With large areas of glass block, metal reinforcement was added to ensure strength and stability. Owens-Illinois’s Insulux block was often use for exterior windows or partitions in factories, schools, and apartments. Proponents of glass blocks thought that glass blocks were also ideal material for improving lighting conditions in the workplace. The blocks were nonporous and therefore had a natural barrier to dirt, odors, grease, water, and air. Glass blocks were the ideal construction material for areas that required high levels of sanitation. In 1939, 40 manufacturers of glass blocks began creating blocks that would refract light through prismatic ridges. The light would be redirected towards the ceiling and bounce back into the work space. No-glare blocks were introduced for lower portions of block wall installations. In the 1940s and 1950s, glass blocks were a staple for interior walls in schools in the United States. By 1958, ceramic-coated glass blocks could be manufactured with blue, yellow, or red hues.

Conservation
Glass blocks are a relatively durable building material and seem to weather well. If the blocks become cracked or fogged, they may require replacement. Historic glass blocks are likely to be expensive to replace due to their rarity.

Deterioration
In the beginning, glass blocks were traditionally sealed by dipping the edges of the two pieces in a molten metal and pressing them together. Seal failure is frequent with these kinds of blocks. Once a seal has been broken, moisture can seep into the block and the inside glass can become soiled. The need for a molten metal seal was later eliminated thanks to advancements in glass block technology. Failure in glass blocks can be attributed to the bond between the side walls and the mortar being inadequate. Earlier structures may lack the structural integrity needed to withstand exterior wind and climatic conditions. These walls can be strengthened by adding a sanded adhesive coating to the side walls during manufacturing. Refinements in the 1960s included corrugated sides with silica sand side walls to improve the bond. Today, paint-like or polymer coatings are often applied to increase the bond adhesion. Glass block structures are not meant to be load bearing walls. To prevent weight being shifted onto the structure from above, glass blocks are usually installed in isolated small panels that are subdivided with vertical and horizontal expansion joints. If there is a long section of a glass block wall panel that is uninterrupted by joints, it is possible the wall will experience bowing or crushing. Perimeter joints and expansion joints rely on preformed metal edging that should retain the block laterally while allowing differential movement in the wall plane. The earliest application used joints between the block and wall to support the structure. These joints were often caulked with oakum and sealed with mastic. The joints are not typically sealed with backer rod or elastomeric sealants. These materials should be replaced with contemporary sealant materials if they become depreciated.

Conservation Techniques
In order to ensure a long life, each glass block and its surroundings should be inspected at close range. Bowing and cracking are signs that there may be inadequate expansion joints of differential settlement. Fogging internally is a common problem seen in historic glass blocks but may not entail structural issues. Lead seals are likely to lose their air tightness over time. Small chips and isolated cracks are common. Replacement should only be done if the structure’s integrity is in question, cracks that have separated, or that have chips and cracks that allow moisture to enter. Internal staining is common and a strong indicator of water infiltration.

Common maintenance of a glass block structure usually includes repointing if joints are deteriorating or missing. Small chips, minor cracks, and slight fogging are common and do not usually affect the structure’s performance. These problems are not usually grounds for replacing historic pieces. Cleaning should be done after replacement of damaged blocks. Small walls and partitions that are made of glass blocks can be cleaned with water and a mild detergent. Stubborn dirt or graffiti can be moved with a solvent that is made for glass. On larger installations, mist water spray and a cotton cloth can be used, but the cloth should be rinsed regularly to remove excess dirt that may scratch the blocks. Bowing or extensive cracking is usually a signifier that the wall may require expansion joints.

Replacement
Finding replacements for glass blocks can be difficult because old blocks may have patterns that are no longer produced. Pittsburgh-Corning is the only company in the United States that produces glass block, though it imports most of its products from Germany, Holland, and Japan. Custom dies can be cast for historic patterns but may be too expensive for large projects. Finding similar historic blocks may be possible but the use of recycled blocks from other locations or buildings may be prohibited by building codes. The damage caused to the brick during the salvaging process may affect the new bond. Chips and other defects in used blocks make them more prone to cracking after reuse. The type and quality of mortar used should be checked because it will affect the durability of the construction. Masonry construction for glass block walls should be similar to that used for traditional brick masonry. Type S mortar, which uses Portland cement and lime, is more commonly used and is mixed stiffer and dryer than ordinary masonry. Repointing or replacement mortar should match the original mortar’s color but should not be stronger than the original mix. Conventional ladder-type joint reinforcement is used in glass block panels and should be installed every 16 inches on center vertically. Metal panel anchors should be embedded often to hold the block glass panel to adjoining construction but also allowing for differential movement. Contemporary reinforcement products are not identical to their historic counterparts but they serve the same purpose and are suitable for replacement work or new walls.