Regardless of composition, the main ingredient in almost all glass is silicon dioxide (aka sand). Because it has such a high melting point (1,720 degrees C), other chemicals, like sodium oxide, are used to lower the melting temperature of the mixture, making it easier to work with and cheaper to produce. Many of these chemicals also happen to imbue glass with specific properties, such as resistance to x-rays, tolerance for high temperatures, or the ability to refract light and disperse colors. Problems arise, though, when the composition is changed; the slightest tweak can result in a drastically different material. Throwing in a dense element like barium or lanthanum, for example, will decrease the melting temperature, but you risk not getting a homogeneous mixture. And maxing out the overall strength of a glass means you’re also making that glass more likely to fracture violently when it does fail. Glass is a material ruled by trade-offs. This is why compositions, particularly those that are fine-tuned for a specific manufacturing process, are fiercely guarded secrets.
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