The invention in 1840 of an alloy comprised mainly of silver, tin, and mercury, called amalgam, was the turning point. In its preliminary form, amalgam is a liquid metal at room temperature because of its mercury content. However, when it’s mixed with its other components, a reaction takes place between the mercury and the silver and tin that results in a new crystal, which is fully solid, hard-wearing, and tough. This miracle material could be squirted into a cavity while it was liquid, and then left to set hard. As it solidifies, the amalgam also expands slightly, wedging the filling into
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