Perhaps the easiest to obtain material is welder's glass. But not just any density. Ask an expert, they will say use nothing but No. 14, the densest shade. Of course, back when No. 13 was the densest, they said to use nothing but it. Same thing before that when No. 12 was densest. So over the years I've collected all three shades in my kit. But be safe, and stick with No. 14, available where welding supplies are sold.