An important material for packaging.
The glass industry has to continually monitor the emissions that
are given off when glass is melted. Under European Directives, they
are obliged to test their product for toxic elements. Although analytical
tests are regularly performed on glass products, the results were
often unreliable. A team of scientists from glass research institutes
and the glass industry put their measurements to the test when they
compared their results of several interlaboratory studies. They
went on to produce a Certified Reference Material that will help
other laboratories to validate their own measurement methods.
Glass is an important material for packaging as well as being used across industry. Made from mainly silica that comes from sand, glass manufacturers vary the properties of glass by adding different chemicals such as lead and barium. They use a number of processes as well, depending on the final product. Blown or moulded glass forms bottles and jars while sheet glass is used for windows. The latter is made either by rolling molten glass into ribbons or by "floating" it on molten tin.
Another key industrial glass process is recycling which reduces the energy consumption per unit of the product compared to making the product from scratch. Nearly everyone is familiar with this early part of the 'green' movement. Glass bottle manufacture depends on graded glass fragments or off-cuts suitable for re-melting. In the trade, this is known as cullet.
All these processes have at least one thing in common. Whenever glass is melted, gases are given off. Depending on the glass, these emissions can contain heavy metals and contribute to air pollution. The industry has to monitor and control the process to comply with emission limit values. They perform many analyses to evaluate situation, but the quality, comparability and reliability of these measurements is questionable. One way to monitor the maximum possible emissions is to test the glass itself for contamination.
Glass reference materials, certified for element content, is one part of the answer. With these materials, industry and regulatory testing laboratories can check the accuracy and reliability of their measurement methods and procedures.
Such materials are already available from the United States' National Institute for Standards and Technology, but in many instances the substances listed in the EC Directives on emission control and regulating packaging and packaging waste are not certified. The substances that must be regulated in Europe include antimony, arsenic, barium, cadmium, chromium, cobalt, lead and selenium as well as fluoride and chloride.
Responding to industry's needs
In response to the glass industry's demand for a Certified Reference Material, a project was initiated under the Measurements and Testing programme. It brought together national glass research laboratories from seven Member States as well as large glass manufacturers like Pilkington plc in the UK, Schott Glasswerke in Germany, St Gobain in France and Glaverbel in Belgium. Glass research institutes from the Czech Republic and Turkey also joined the team. A total of 17 organisations participated in the project.
Starting with different methods and ideas about testing, the partners first evaluated the present state-of-the-art of glass
analysis. Through assessing different test procedures, they identified and removed possible sources of error in the experiments. Altogether, the group went through three interlaboratory comparisons. In their first study, they did not even use glass. Instead, they each analysed a specially prepared solution with carefully defined levels of metal contamination. For each element, up to five different measurement techniques were used.
Practice makes perfect
In the second and third studies, each partner tested two different glass samples - a float glass sample from industry and a specially prepared glass sample - for the trace elements. This required more complex analytical procedures. The solution is measured directly but, with glass, the samples have to be treated to extract the relevant element into solution. To make matters even more complicated, some of the chemicals are very volatile and have to be converted to less volatile relatives.
After each study, the partners analysed their results in technical meetings. The laboratories' measurements improved significantly as the studies progressed, showing the worth of the study. However, the variation of results in early work emphasised the need for a glass Certified Reference Material.
Schott Glasswerke prepared 800 samples of the material from 120 kg of glass which had been contaminated, in the range of 1-100 parts per million, with all the elements that need to be monitored by the glass industry. After testing for constant composition across the samples, the company shipped samples out to the project partners. Over six months, they each performed six measurements for each of the ten elements and at the end, once again, they analysed the results statistically and discussed any errors or problematic results. Eventually, in mid-1998, the partners were satisfied that the variation in their results was small enough for certification. Owing to technical problems, the glass fluoride level could not be certified.
The glass industry now has access to this reference material and it will help them to improve the reliability of their measurements. This means better process monitoring, including cullet testing for recycling. Studies on the environmental impact of the glass industry could also benefit from these results. Not only this, but both public and private glass testing bodies from across Europe had the opportunity to work together to develop both their expertise and a tool to help other laboratories. This is an important step forward on the road to standardisation.