Occasionally the question of ‘Glass or Plastic’ comes up in an e-mail or on-line discussion, and I always advise the questioner that plastic (or polycarbonate) lenses are not a desirable choice for the glass working studio.
The reasons for this vary, but boil down to a couple of fundamental issues:
1) Durability. Plastic/polycarbonate lenses WILL scratch, regardless of any anti-scratch coatings. Let’s face it, the glass studio is about the worst place to wear plastic/polycarbonate lenses if you want to avoid scratches. Glass chips and dust are everywhere even if you are the cleanest glassworker in the world. All it takes is one small scratch (and it always is dead square in the center of the lens!).
2) Filtration issues. Despite the high-tech ability of plastic manufacturers, no one has been able to invent a dye that exactly replicates the multi-notch filter that didymium and Blue Didymium/ACE provide. Let’s be clear (sorry, pun not intended): the notch filter at 575-590 nanometers is an absolute requirement to filter out sodium flare. This notch needs to be sharp and well-defined. If the notch at that point is too wide, you remove the surrounding wavelengths of light, which, of course, removes your ability to see those colors, which affects your color perception.
3) Fading issues. ALL plastic/polycarbonate dyed lenses fade. The manufacturer may claim that they won’t — but under what conditions are they making those claims? In a medical office or surgical suite? In an industrial setting? Have they tested the dye in front of a 2800-degree F torch for 8 hours per day, 6 days a week for a month? Most likely not. All the dyed lenses that I have ever seen fade over time and exposure to torchlight/heat, something that these lenses were NEVER designed for.
Plastic or polycarbonate lenses are inexpensive, and you get exactly what you pay for. You will have to replace them frequently as they get scratched and faded. As often as every 4 months, depending on how much time you spend behind the torch. A good pair of glass lenses will last for years. An average pair of plastic so-called “borosilicate glassworker” filters cost $60.00. An average pair of glass full-coverage borosilicate glassworker filters cost $235.00. If you (conservatively) replace your plastic filters every 9 months due to scratching/fading, you will have paid for a pair of glass filters in 36 months, and the glass lenses highly resist scratching and absolutely will not fade.
The average pair of glass borosilicate glassworker filters will last (if you take diligent care of them) many years. Plastic/polycarbonate filters will last maybe as long as 9 months.
And, as a bonus, you don’t require a separate add-on lens holder if you require a prescription. Glass lenses quite easily can have your prescription ground into the filter. Plastic/polycarbonate filters usually cannot.
The choice is always yours, and the best choice is the one that is made with all the information available.