Unit
- Soda-lime glass: The most common family, composed of approximately 73% silica (SiO2), 14% soda (Na2O), and 9% lime (CaO). This composition yields a low melting temperature and good workability, making it the standard for windows, bottles, and jars. The remaining 4% includes minor additions of magnesia, alumina, and iron oxide for color and durability.
- Borosilicate glass: Contains about 81% silica, 13% boric oxide (B2O3), and 4% soda. The boron replaces some of the alkali compounds, producing a glass with very low thermal expansion (3.3 x 10^-6 /K). This property allows it to withstand rapid temperature changes, making it suitable for laboratory glassware, cookware, and pharmaceutical packaging.
- Lead crystal glass: Incorporates lead oxide (PbO) typically at 24% to 32% by weight. Lead increases the refractive index and density, creating high brilliance for decorative tableware and optical components. Due to toxicity concerns, lead-free alternatives using barium or zinc oxides are increasingly adopted for consumer products.
- Aluminosilicate glass: Replaces much of the soda with alumina (Al2O3) in the range of 20% to 25%. This composition provides exceptional mechanical strength and chemical resistance. It is used in fiberglass, display covers for smartphones, and high-temperature industrial sight glasses where soda-lime would soften.
- Fused silica glass: Made from pure silicon dioxide without any fluxing agents. It has the highest melting point (around 1700 deg C) and transmits ultraviolet light, used in semiconductor optics and high-purity laboratory apparatus. The manufacturing cost is significantly higher due to the extreme temperatures required.
- Color control through metal oxides: Adding chromium oxide produces green glass; cobalt oxide yields blue; iron oxide gives a green-brown tint; and selenium with cadmium sulfide creates red. The amount added is typically less than 0.5% of the batch but dramatically affects both aesthetic appearance and solar heat rejection properties.
- Batch calculation principle: The batch formula must account for weight losses during melting due to decomposition of carbonates and sulfates. For every 100 kg of glass produced, approximately 115 kg of raw materials are needed because carbon dioxide and sulfur dioxide gases escape. This factor is called the batch yield factor and is calculated from the R2O - RO - SiO2 ternary diagram.
- Influence of minor constituents: Alumina in amounts as low as 0.5% improves chemical durability and prevents devitrification. Magnesia (MgO) reduces the tendency for glass to crystallize during forming. Boric oxide lowers viscosity without reducing the working range. Each 0.1% addition must be balanced to avoid batch segregation during melting.
Process Flow
- Step 1 - Define target glass composition: Specify the oxide percentages required for the intended application, referencing standards such as ASTM C162 for soda-lime or ISO 3585 for borosilicate compositions.
- Step 2 - Convert oxides to raw materials: Calculate the quantity of silica sand, soda ash, limestone, dolomite, feldspar, and other minerals needed. Each raw material is expressed as a percentage of the batch, and its purity must be accounted for to avoid excess impurities.
- Step 3 - Apply batch yield factor: Multiply the target glass weight by the yield factor (typically 1.10 to 1.18 depending on gas evolution) to determine the total batch weight. This compensates for the loss of CO2 from limestone and SO2 from sulfates during melting.
- Step 4 - Adjust for cullet addition: Determine the percentage of recycled glass cullet to add. Each ton of cullet replaces approximately 1.2 tons of raw batch materials and reduces energy consumption. The cullet must be sorted by color and composition to avoid contamination.
- Step 5 - Verify with laboratory melt: Prepare a small pilot batch, melt it in a laboratory furnace, and measure the resulting composition using X-ray fluorescence or inductively coupled plasma analysis. Adjust the batch formula if the composition deviates from the target by more than 0.1% for critical oxides.
Quick Revision
- Soda-lime glass is the dominant family for commodity products due to its low melting point and adequate durability.
- Borosilicate glass uses boric oxide to achieve low thermal expansion for temperature-resistant applications.
- Lead crystal is being replaced by barium or zinc alternatives for health and environmental reasons.
- Batch yield factor compensates for gas losses during melting; typical values range 1.10 to 1.18.
- Cullet addition reduces energy consumption and raw material costs but requires careful sorting.
- Laboratory verification of batch composition is essential before production-scale melting.