Damage to your eyes is very permanent, and it is so easy to prevent. Most old-time furnace glass workers developed cataracts. The intense light including infrared and ultraviolet rays generated by glass in a flame can be dangerous. The simple didymium filter glasses are intended to allow you to see what you are working on, through the yellow flare of the glass in flame. They were not designed to protect your eyes any more than typical safety glasses.
More sophisticated filters are available.
Our best advice to avoid eyestrain is to keep your face as far back as possible while working, and look around as much as you can. Avoid staring intently at the flame, and take breaks often. If you can afford them, buy the best glasses available, but at minimum use quality Rose Didymium safety glasses. The second top priority in any safety program is ventilation. If you are breathing fumes while working, you are literally killing yourself.