Float Glass (Annealed Glass)
Float glass, also known as annealed glass, is the fundamental form of glass produced by floating molten glass on molten tin, resulting in perfectly flat, uniform surfaces with exceptional optical clarity. It serves as the base material for most architectural glass products and can be further processed into various specialty glass types.
Composition
Float glass is composed primarily of silica sand (70-74%), soda ash (sodium carbonate, 12-16%), limestone (calcium carbonate, 5-12%), dolomite (2-5%), and small amounts of other materials such as alumina and cullet (recycled glass). These raw materials are melted at approximately 1500°C (2730°F) in a furnace, then the molten glass flows onto a bed of molten tin in a controlled atmosphere of hydrogen and nitrogen. The molten glass floats on the perfectly flat surface of the tin, spreading out to form a level sheet with uniform thickness and virtually no distortion. As the glass cools and solidifies, it's drawn off the tin bath through an annealing lehr where it's slowly cooled to prevent internal stresses. The continuous ribbon of glass is then inspected for defects using automated systems and cut to standard sizes. The resulting product is clear, flat glass with parallel surfaces and uniform thickness, typically ranging from 2mm to 25mm. Float glass in its basic form is referred to as annealed glass, indicating it has been slowly cooled to relieve internal stresses.
Properties
Visible Light Transmission
87-92% (varies by thickness)
Float glass allows approximately 87-92% of visible light to pass through, depending on thickness and iron content. Standard clear float glass transmits about 90% of visible light at 3mm thickness, decreasing slightly as thickness increases. Low-iron float glass can achieve up to 92% transmission with reduced green tint.
Tensile Strength
40-50 MPa (5,800-7,250 psi)
The tensile strength of annealed float glass is relatively modest compared to tempered variants. This property determines how much stress the glass can withstand before breaking when pulled or stretched. The theoretical strength of glass is much higher, but microscopic surface flaws significantly reduce practical strength.
Density
2,500 kg/m³ (156 lbs/ft³)
Float glass has a density of approximately 2,500 kg/m³, making it a relatively heavy building material. This density affects structural loading considerations, handling requirements, and support systems needed for installation.
Thermal Expansion Coefficient
9 × 10⁻⁶/K (9 × 10⁻⁶/°C)
This property measures how much the glass expands or contracts with temperature changes. The relatively high thermal expansion of float glass makes it susceptible to thermal stress breakage when subjected to uneven heating or rapid temperature changes.
Thermal Conductivity
1.0 W/m·K
Float glass is a relatively poor insulator with a thermal conductivity of approximately 1.0 W/m·K. This means it readily transfers heat, making single-pane float glass inefficient for thermal control in building envelopes without additional treatments or configurations.
Modulus of Elasticity (Young's Modulus)
70-75 GPa (10.2-10.9 × 10⁶ psi)
This property measures the stiffness of glass—its resistance to elastic deformation under load. The high modulus means glass deflects very little under load before reaching its breaking point, exhibiting brittle rather than ductile behavior.
Applications
Residential Windows and Doors
Float glass serves as the foundation for residential glazing, though rarely used in its basic annealed form due to safety concerns. In residential applications, float glass is typically further processed into insulated glass units, often with additional treatments such as tempering, laminating, or low-e coatings. For non-safety applications like fixed picture windows above head height, basic float glass may be used if building codes permit. In historical restoration, traditional float glass or restoration glass that mimics the slight distortion of older glass-making techniques may be specified. Float glass is also used in smaller residential applications such as cabinet doors, picture frames, tabletops (with appropriate edge treatment), and decorative elements where safety glass is not required by code.
Commercial Glazing
In commercial buildings, float glass serves as the base material for more complex glazing systems. While rarely used in its basic form due to safety and performance requirements, processed float glass is essential in curtain walls, storefronts, interior partitions, and display windows. Commercial applications typically require float glass to be further processed into tempered, laminated, or insulated units to meet building codes and performance specifications. Float glass may be used in its basic form for non-safety applications such as spandrel glass (opacified glass in non-vision areas) or as a component in laminated assemblies where the lamination provides the required safety performance. The optical clarity and flatness of float glass make it ideal for commercial applications where distortion-free views are essential.
Furniture and Interior Elements
Float glass is widely used in furniture applications including tabletops, shelving, cabinet doors, and decorative elements. For these uses, the edges are typically polished or beveled to remove sharp edges, and the glass may be tempered for safety depending on the application. The perfect flatness and optical clarity of float glass make it ideal for display cases in retail and museum environments. In high-end applications, ultra-clear low-iron float glass may be specified to eliminate the slight green tint of standard float glass. Float glass is also used for mirrors, with a silver coating and protective backing applied to one surface. Interior designers often use float glass for room dividers, shower screens (tempered for safety), and decorative panels, sometimes incorporating treatments like acid-etching, sandblasting, or digital printing to add visual interest or privacy while maintaining light transmission.
Base Material for Processed Glass Products
Perhaps the most important application of float glass is as the starting material for nearly all architectural glass products. Float glass serves as the substrate for numerous value-added processes including: tempering, which increases strength and safety; laminating, which improves safety, security, and acoustic performance; coating, which enhances energy performance, aesthetics, or functionality; insulated glass unit assembly, which improves thermal performance; and decorative treatments such as fritting, printing, etching, or sandblasting. The exceptional flatness, clarity, and consistency of float glass make it the ideal base material for these processes. The float process allows for precise thickness control, which is critical for many processing techniques. The global float glass manufacturing infrastructure, with plants on every continent, provides the foundation for the entire architectural glass industry.
Specialty and Technical Applications
Beyond standard architectural applications, float glass is used in numerous specialty and technical fields. In the solar energy industry, low-iron float glass serves as the cover material for photovoltaic panels and solar thermal collectors due to its high light transmission. In electronics, ultra-thin float glass is used as substrates for displays, touchscreens, and other electronic components. The optical industry uses high-quality float glass for lenses, prisms, and other optical elements. Float glass is also used in automotive applications, though almost always laminated or tempered for safety. In scientific and laboratory settings, float glass is used for observation windows, sample slides, and containment systems. The consistent quality and optical properties of float glass make it suitable for these demanding applications, often with specialized compositions or treatments for specific performance requirements.
Art and Architectural Features
Artists and architects use float glass for its aesthetic qualities and versatility. In architectural features, float glass can be incorporated into balustrades, canopies, floors, and stairs (always properly processed for safety). Artists use float glass as a medium for sculpture, etching, painting, and mixed-media works. The transparency, reflectivity, and interaction with light make float glass a compelling artistic material. Architectural glass art often incorporates float glass with treatments such as sandblasting, acid-etching, painting, or laminating with interlayers containing fabric, metal, or organic materials. In religious and public buildings, float glass serves as the base material for stained glass, either traditional leaded assemblies or modern laminated interpretations. The perfect flatness of float glass provides an ideal canvas for these creative applications, while its optical clarity allows for dramatic interactions with light.
Advantages
- Exceptional optical clarity and distortion-free surface
- Perfectly flat parallel surfaces with uniform thickness
- Available in various thicknesses from 2mm to 25mm
- Relatively cost-effective compared to processed glass types
- Can be cut, drilled, and edge-worked after manufacturing
- Serves as the base material for most architectural glass products
- Excellent substrate for coatings and films
- Available in large sheet sizes up to 6m × 3.21m
Limitations
- Breaks into large, dangerous shards (safety hazard in many applications)
- Relatively low strength compared to tempered or laminated glass
- Vulnerable to thermal stress breakage from uneven heating
- Poor insulating properties when used alone (high U-value)
- Limited sound attenuation capabilities
- Standard float glass has a slight green tint visible on edges
- Susceptible to surface scratching and abrasion
- Not suitable for safety glazing applications without further processing
Sustainability Profile
Float glass has a mixed sustainability profile. On the positive side, it is made from abundant natural materials (primarily silica sand, soda ash, and limestone) and is 100% recyclable at end of life. Glass recycling is well-established in many regions, and cullet (recycled glass) can constitute up to 30% of the raw material in float glass production, reducing energy requirements and CO₂ emissions. However, float glass production is energy-intensive, requiring temperatures of approximately 1500°C (2730°F) maintained continuously in large furnaces. A typical float glass plant consumes significant energy and produces substantial CO₂ emissions. The industry has made progress in reducing environmental impact through improved furnace efficiency, increased cullet use, and emissions control technologies. From a life cycle perspective, float glass can contribute to building energy efficiency when used in appropriate glazing systems, potentially offsetting the embodied energy over the building's life. Low-iron float glass variants require specially sourced raw materials with lower iron content, which may have additional extraction impacts. When specifying float glass, consider the manufacturing location relative to the project site to minimize transportation impacts, and prioritize manufacturers with documented environmental initiatives and transparency about their production processes.