What are Didymium Glasses?
Didymium glasses are frequently recommended for use in lampworking, flameworking and glassblowing as they are effective for blocking the bright yellow sodium flare that occurs when a torch flame is used to heat glass.
The first use of didymium glass lenses to filter the sodium flare light in glass torchworking is attributed to Sir William Crookes in the 1870s. In fact, while didymium was originally considered a novel metallic ore, it is actually a blend of two elements: praseodymium (element #59), and neodymium (element #60). Interestingly, most "didymium glass" lenses sold today actually contain predominantly or exclusively neodymium and are not actually "didymium at all", whereas only the neodymium component contributes to the sodium flare filtering and there is no practical benefit for including praseodymium into the glass blend.
The First Danger: Invisible Light
While at first glass a pair of didymium glass lenses appear to be highly effective at filtering out the annoying sodium flare light, these glasses come with a hidden danger: the sodium flare light while annoying is actually of minimal danger, rather the exposure risks to the glassworker's eyes actually primarily come from invisible components of light that didymium glasses may fail to adequately filter out. These invisible components include ultraviolet and high-energy blue-light which can cause corneal burns (sunburn) and macular degeneration, and short wavelength infrared light, which can cause a condition known as the "glassblower's cataract".
Did you ever wonder why the torch flame burns with a blue light? And have you ever heard of lampworkers complaining of sensitive skin or a sunburn after working? Well, it turns out the torch itself is a source of both ultraviolet light and high-energy blue-light. These wavelengths of light are emitted by the excitement of certain electronic transitions that occur when burning hydrocarbons in the torch flame, which are also called "Swan Bands" named after the Scottish physicist who first studied them in 1856, William Swan. Because these emissions cluster around short wavelengths they cause the flame to appear blue in color, but, the short wavelengths also cause the individual photons to carry a high energy payload which can cause damage to the skin and eyes including sunburn, corneal burn, lens cataracts and macular degeneration.
Meanwhile, just past the other end of the visible spectrum are invisible wavelengths of infrared light that are generated at high levels by intense heat. The hallmark of infrared emission is when something is glowing "red hot"; the red light you can see is just a portion of the light being emitted - it is often the case that a much greater amount of light than can be seen is actually coming off the object, at wavelengths just past the edge of what the eyes can see. Industrial safety experts have identified the so-called "near infrared" band as being particularly dangerous to the eyes, which consists of invisible light having wavelengths between about 780 nanometers to 2000 nanometers. Long term exposure to this near-infrared light causes undesirable heating of the internal eye tissues and is linked with formation of cataracts called "glassblower's cataracts".
The Second Danger:Â Glass Breaks
"Glass is glass. And glass breaks." -Â JerryRigEverything.
A second hidden danger of didymium glasses is the fact that the lenses are made from actual mineral glass, which is prone to shattering when stressed or impacted by hard or sharp objects, or when subjected to thermal shock.
Historically, spectacle lenses were made from glass, however due to uncontrolled manufacturing processes, the risk of eye injury from wearing a glass spectacle lens was significant. In the 1970s, recognizing the threat to public health from glass lenses, the FDA regulated the eyewear industry by requiring that lenses pass a "drop-ball" impact test to certify sufficient toughness for general use. To pass the drop-ball impact test with a glass lens, lens manufacturers had to adjust their manufacturing tolerances to increase the thickness of the lenses, and incorporate a glass tempering step prior to mounting the lenses in a frame. As a result, glass lenses became significantly heavier and uncomfortable to wear, which in turn drove a rapid transition to plastic lenses across the US and globally. Today, the use of actual glass lenses in eyewear accounts for less than 1% of eyewear sold and is largely considered to be an anachronism.
Following in the footsteps of the FDA recommendations, the American National Standards Institute ANSI committee responsible for setting standards for industrial safety eyewear (the Z87.1 standard) went even further by requiring a high-mass and high-velocity impact test to certify use of glasses for use in industrial environments. These tests are virtually impossible to pass using a traditional glass lens, except in some rare cases with extremely thick lenses (over 3.0mm) and with advanced chemical tempering methods.
Alternatives to Didymium Glasses
Within the lampworking, flameworking and glassblowing trades, the use of traditional didymium glass lenses does remain commonplace today, however for serious users it is always recommended to use a didymium lens that is bonded to a secondary welding shade lens having an ANSI Z87.1 rating of at least Shade #3. It is only by combining a didymium lens with a welding lens, one can be sure of sufficient filtering of the "invisible" wavelengths of light in the ultraviolet and infrared spectrum. Furthermore, when the two lenses are truly bonded together using a suitable optical-grade adhesive, the combined structure can enhance the strength of the lens significantly, although the weight of the resulting glasses can become a serious issue for long term use.
Comprehensive Eye Safety with Didymium-Free Polycarbonate Lenses
Fortunately, in the past two decades new organic dye compounds have been discovered that can perform the same type of sodium flare filtering, and are compatible with high-impact ophthalmic plastics used in safety glasses such as polycarbonate. These new dye compounds can be combined with ultraviolet and infrared absorbers without increasing the weight of the lens, ensuring excellent comprehensive eye safety in a light-weight, comfortable product that includes protection against radiation and impact. The use of the didymium-free plastic lenses represents a significant step forward in safety lens technology for the needs of lampworking, flameworking and glassblowing techniques wherever protection against sodium flare light and exposure to ultraviolet radiation and heat (infrared) radiation is encountered. Through the use of these new polymer lens technologies, in combination with suitable full-coverage frames, it is possible to deliver a special purpose safety glasses for engaging in these activities that is fully compliant with the ANSI/ISEA Z87.1 standards for industrial eye protection. To check your eyewear's compliance, examine the lenses and frame for a permanently engraved "Z87" mark or consult with the manufacturer regarding what standards their products are test against.