What is glass made of?
Glass is composed of about 70% silica (silicon oxide, or, basically, sand), 15% soda (sodium carbonate), 15% lime (calcium oxide, usually made on site from limestone), and minor amounts of other additives. Thus the name of the common glass: soda-lime glass. Glass is used in huge variety of product, but vast majority is made into windows or into containers. For many uses glass needs to be made with different additives. For example, replacing the lime with lead oxide produces "crystal" glass, replacing soda and lime with boron oxide produces borosilicate glasses such as Pyrex, etc.
The major environmental impact of glass production is caused by atmospheric emissions from melting activities:
· The combustion of natural gas/fuel oil and the decomposition of raw materials during the melting lead to the emission of CO2. This is the only greenhouse gas emitted during the production of glass.
· Sulphur dioxide (SO2) from the fuel and/or from decomposition of sulphate in the batch materials can contribute to acidification and formation of SMOG.
· Nitrogen oxides (NOx) due to the high melting temperatures and in some cases due to decomposition of nitrogen compounds in the batch materials also contribute to acidification.
· Evaporation from the molten glass and raw materials can cause release of particles in the atmosphere.
Other environmental issues are water pollution, the use of non renewable natural raw materials such as sand and minerals, production of solid waste and emission of volatile organic compounds (used in production of mirrors and coatings).
The need for extremely high temperature furnaces to melt the mixture of substances makes the melting stage of the glass making process very energy intensive. It is estimated to take 15.2 million BTUs of energy to produce one ton of glass. During any one of the formation processes the glass may need to be reheated to keep it in liquid form. This means the heat in the furnace must be kept up until the process is complete.
Discharges from the glass making process may find their way into the aquatic environment during the cooling and cleaning processes where the most significant amounts of water are used. Discharges may contain some pieces of glass, some soluble used in the production like sodium sulfate, lubricant oil used in the cutting process, dissolved salts and water treatment chemicals.
An increased risk of lung, stomach, and colon cancer as well as of brain tumours has been reported in previous studies. It is postulated that the excess risk of lung cancer, detected in this study, can also be accounted for by lifestyle, and not only by possible occupational exposures, because a similar excess risk of lung cancer has been found previously for all industrial workers in Finland. Although the risk of stomach cancer in this study was increased among glass blowers, it was not high in the largest groups of plain glass workers. The risks of tumours of the central nervous system and colon were not increased either.
Glass blowers face respiratory hazards from the materials used to make the glass. Hazards can take the form of fumes or inhaled particulates. For example, dirty glass and quartz produce harmful fumes when heated. Asbestos tapes present a particulate risk, while some minerals that give the glass its beautiful color are highly toxic no matter how they are ingested. While a glass blower's canopy hood may capture heat and very light gases, the hood does not offer protection against most fume and particulate inhalation hazards.
Heat represents an obvious glass blowing hazard, as glass blowers work around extremely hot furnaces and superheated glass. Even surfaces not directly in contact with the furnace or glass, such as metal work bench rails, can absorb enough heat to make them dangerous to touch. Equipment surface temperatures of several hundred degrees are common, meaning a glass blower must use proper protective gear and extreme caution at all times.
The initial crushing and grinding step sends particulates of metals, chemicals, acids and dust into the air. These are easily inhaled causing irritation to the nose and throat, potentially causing damage to the lungs. The particles of metals are hazardous to the environment as they can find their way into surrounding soil and water.