Really, pretty much all the time! You need safety glasses when you cut and when you grind. They don’t have to be expensive, but you don’t want to be scratching up your prescription glass or getting a glass shard in your eye.
The time to pay the big bucks for your glasses is if you are doing torch work or you are doing raking in a kiln. Then you really need didymium glasses.
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Not only are they a fashion statement but with the right ones you will be able to see the colors you are working with.
This is the technical reason these are great glasses.
Schott Glass Technologies has developed a new contrast enhancement glass for spectacle lenses. The new material, called ACE, short for Amethyst Contrast Enhancer, utilizes rare earth oxides in the glass composition to achieve the unique color enhancing characteristics. Basically, the color enhancement concept works by selectively positioning transmission in the blue, green, and red spectral regions whereby one can improve the color discrimination between different color objects. These are the same colors used to produce a color television picture. They optimize the readability of such displays under high ambient light conditions. Phillips-202 Glass utilizes rare earth oxides in its composition to provide the same filtration. This glass is very efficient in filtering sodium flare in addition to providing ultraviolet protection to 390 mm. The Phillips202 (ACE) is becoming a preferable choice of hot glass workers. It is very comfortable for viewing under high ambient light conditions such as when working with hot glass or glowing heat sources for a duration of time. Also, although it has a relatively low luminous transmission of 38%, it is not recommended for sun protection use unless UV filtering coatings are applied.