Another common glass ingredient is crushed alkali glass or "cullet" ready for . The recycled glass saves on raw materials and energy. Impurities in the cullet can lead to product and equipment failure. Fining agents such as , , or may be added to reduce the number of air bubbles in the glass mixture. is the method by which the correct raw material mixture is determined to achieve the desired glass composition.
Most common glass contains other ingredients to change its properties. or is more 'brilliant' because the increased causes noticeably more and increased . Adding also increases the refractive index. gives glass a high refractive index and low dispersion and was formerly used in producing high-quality lenses, but due to its has been replaced by in modern eyeglasses. Iron can be incorporated into glass to absorb energy, for example in heat absorbing filters for movie projectors, while can be used for glass that absorbs wavelengths.
In science, and many polymer familiar from everyday use are glasses too. These sorts of glasses can be made of quite different kinds of materials than silica: metallic , ionic melts, , molecular liquids, and . For many applications, like or , polymer glasses (, or ) are a lighter alternative than traditional glass.
Glass has the ability to , reflect, and transmit light following , without scattering it. It is used in the manufacture of and windows. Common glass has a around 1.5. This may be modified by adding low-density materials such as boron, which lowers the index of refraction (see , or increased (to as much as 1.8) with high-density materials such as (classically) lead oxide (see and ), or in modern uses, less toxic oxides of , , or . These high-index glasses (inaccurately known as "crystal" when used in glass vessels) cause more chromatic dispersion of light, and are prized for their diamond-like optical properties.