The glass industry, a villain for the environment?
Glass is made from three main raw materials, namely sand (silica), soda (sodium carbonate) and lime (calcium carbonate). Of these three, the sand is the glass former, the substance that builds up the glass. Soda is added to lower the melting temperature and simplify the process and the lime is a stabilizer.
Glass can be made from sand only - pure quartz glass - but this requires temperatures above 2000 oC for the melting. Such high temperatures are both expensive and difficult to achieve. By the addition of fluxes, such as soda, the melting temperature is lowered typically to more manageable 12-1400 oC. But by choosing a different fluxing material or by varying the ratio of sand and soda, the melting temperature can be set within a wide range.
Many solid materials around us are composed of crystals. A crystal is a well-ordered structure where the atoms are set in fixed locations. When a crystal melts this strict order is broken and as it becomes liquid, the atoms and the molecules can move freely. In the innermost structure glass looks like a liquid - the atoms have no internal order. However, at room temperature, they cannot move relative to each other. Glass is a solid at room temperature, but it is not composed of crystals. Quartz, sand, stone, ordinary metals and most minerals are crystalline and there is always a risk that the melt in the glass furnace forms crystals when it solidifies. If it does, then it's not glass but another material. Therefore, one must add stabilizers to the glass, substances that prevents the formation of crystals during cooling and solidification. The cheapest, and therefore the most common, is lime.
Additives, often metal oxides, can color the glass, can enhance or improve special properties such as extra shine and luster, you can make it heavier or lighter, the glass can be made more or less resistant to chemicals and generally the properties of glass can be set within very wide limits.
The metals in the additives: barium, lead, arsenic, zirconium, antimony, strontium, bismuth ... are a major cause for environmental impact.
From the raw materials to the finished glass
- The first step is to melt the batch, a mixture of sand, melting temperature about 2000 degrees, soda, melting temperature 850 degrees and lime, 825 degrees.
- Then you want to use different methods to process the soft, semi-liquid glass. During processing, it has about the same consistency as syrup.
- The third step is to cool down the object slow enough to avoid internal tensions that can cause it to crack or even explode.
- The final step is (possibly) a cold decoration or finishing. Such processes include grinding, etching, sandblasting, painting and others.