A charcoal fire fed with air from the bottom is hot enough to melt the combination of those materials into glass but not hot enough to make it truly liquid, so bubbles tend to remain and make the glass cloudy. I mixed the finely ground ingredients together and heated them in a cast-iron pot, then poured the molten glass into a graphite mold and pressed it down with a graphite stamp. Soda-lime glass has the lowest melting point but must be cooled slowly to avoid shattering from thermal stress. Borosilicate glass, commonly known as Pyrex, melts at a higher temperature but can be cooled more rapidly. I made a medallion out of each and just left them in the fire as it died down over a few hours. Although making glass from sand is satisfyingly primal, starting with actual glass is more practical. Old test tubes yield high-grade borosilicate glass; wine bottles, colored glass. Watch out—it’s all very sharp and hot when molten!
Click here for a slideshow of the glass-making in action. And turn the page for video.
Like what you see? For more Gray Matter, head to popsci.com/graymatter. And for Theodore Gray’s one-of-a-kind periodic table poster, check out periodictable.com/posters
