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Crystal glass
5-8 usd/kg
Crystal glass is known for its incredible clarity and brilliance. Also called lead alkali glass, the inclusion of lead oxide (PbO) dramatically improves optical properties compared to regular glass, as well as increasing density, workability and machinability. The lead acts as a flux, reducing the softening point of the glass to around 600 degC, and so makes it relatively easier to blow, mould and manipulate. It also reduces the surface hardness, which allows for greater freedom with cutting, grinding and polishing processes. The optical qualities of crystal are often emphasised with facets, cuts and engraving that reflect and refract light – typified by Swarovski crystal.
The use of lead is problematic, because it is toxic and a heavy metal – particulates from crystal glass manufacture may have a lead content of 20-60%. Prolonged exposure to the element is potentially very harmful, especially for babies and children. As part of the EU REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) regulation, the lead content of jewellery and similar items is restricted.
In the case of finished crystal glass, the lead is locked into the structure and so does not present the same health concerns in use. For example, crystal glass is exempt from REACH regulation. Small amounts are thought to transmit from the glass to stored contents through diffusion ionic exchange, and so it is not recommended for long term storage. For short term storage, such as during a meal, the rate of leaching is so minute that it is not thought to be a risk. Acid polishing is a technique used to enhance surface brilliance, whilst also reducing the concentration of lead at the surface, further reducing the risk of contamination.
Sustainability concerns
Non-renewable ingredients
Raw material generates polluting by-products
Potentially toxic in use
Crystal glass contains up to 25% PbO. More than that and it becomes lead crystal glass. Full lead crystal contains at least one-third PbO. These types of glasses are used in lenses, prisms, stemware, tableware, decanters, jewellery, awards, trophies, plaques and lighting. It is used for high-index optical and ophthalmic glass and high electrical resistance glass for lamps and display technologies – the lead content imparts excellent electrical insulation properties. It is also popular as a glaze for ceramics.
Similar to soda lime glass, it does not have good high temperature properties, and it has a poor thermal shock resistance. For applications that demand these attributes, borosilicate is preferred.
Glass with more than 50% is used in radiation shielding in hospitals and laboratories, because of the ability of lead to protect against gamma and X-ray radiation. It is laminated with high performance glasses in blocks up to 500 mm thick to provide additional types of protection, such as against impact and scratching.
Glass food and drinks packaging, often referred to as flint glass (due to the original source of silica), is typically made from soda lime glass. It is less expensive and more readily available. The transparency of materials can be measured by refractive index (RI) and Abbe (V). RI is the relative speed of light through the material. V is a measure of the separation of light into its individual colours.
Regular transparent packaging glass is around 1.51-1.52 and 58.6 respectively. Flint is loosely defined as anything >1.6 and <55. By controlling the ingredients in regular glass, it is possible to drop well below this Abbe number, even with recycled content. Polymethyl methacrylate (PMMA), acrylic, which is used in many of the same applications as glass, measures 1.49 and 57 - very close to soda lime glass. For reference, lead glass is 1.7 and 30.9.
