Do Glass Lenses Scratch?
Glass lenses sometimes get a bum rap.
They’re often eschewed by all but the people who absolutely must wear them – those who require a prescription too strong to be cut from any material other than glass. It is true that glass has some innate drawbacks, including two that really come into play when glass is used to make eyewear lenses: weight and brittleness. Heavier glasses tend to slip, indent the nose, or become uncomfortable when worn for long periods of time, and the higher rate of breakage when compared to plastic lenses is an obvious disadvantage.
Glass lenses to have some redeeming qualities, however, which are often overshadowed by the above-mentioned traits. For one, no plastic lens is capable of providing the optical clarity of a glass lens. Another is the scratch-resistant property that is inherent in a glass compound – so much so that an additional scratch resistance coating is not necessary for a glass lens as it is for a plastic one. But does this property completely inoculate glass from scratches?
To answer that, let’s first examine the process of making glass lenses. Glass is formed by the melting of silica formed from sand or pulverized sandstone. An alkali stabilizer is added to lower the melting point, as is cullet (recycled, pulverized glass of the exact same composite) to expedite the melting process. Other additives are based on the type of glass being manufactured. Optical glass usually includes barium to increase the refractive index, metallic oxides if color is desired, or manganese for maximum decolorization and clarity. Additional substances depend on the manufacturer and the exact purpose of the glass being created. This mixture is then subject to the following steps:
- Melting – a batch of silica is mixed with the appropriate amount of cullet and melted down in a furnace, at temperature range of 1,110° – 1,500° Celsius
- Fining – this stage boosts the temperature to about 1,600° Celsius to convert the mixture to a liquid state and filter out gasses still present in the mix
- Conditioning – a mixing and blending process that stirs the liquid mixture non-stop at predetermined speeds, rendering it to a thick viscosity
- Pouring and Molding – the conditioned glass mixture is pumped into molds of various sizes to produce lens blanks of appropriate thickness and shape
- Annealing – this step again heats the blank to temperature of between 550° – 700° Celsius to reduce internal stresses, then cools it down at a controlled rate
This entire process creates the glass blank, into which a prescription is machine-cut, prepped, edged, and polished. This creates a lens with a hardness level of about 470 HK (Knoop Hardness, a standardized micro-hardness test), which makes it extremely resistant to scratches.
So, does this make glass completely scratch-free? No, glass lenses can still be scratched, but not nearly as easily as untreated plastic lenses. A particulate or physical object would have to strike the glass at a high rate of impact to gouge the surface. This can and does happen to glass lenses in the real world, but usually only if they are mistreated or subjected to unusually punishing conditions. Under normal circumstances glass lenses prove remarkably resilient against scratches. This is why no additional scratch-resistant coating is required for a glass lens.