A revolution in glass
The Hall of Mirrors, completed in 1684, was successful economic warfare. In the 17th century, the Republic of Venice held an absolute monopoly on high-quality mirror manufacturing. Venetian artisans on the island of Murano were the only ones who knew how to produce "cristallo," a clear glass, and their techniques for applying a reflective tin-mercury amalgam were state secrets. The republic went to extreme lengths to protect this knowledge, forbidding glassmakers from emigrating.
Jean-Baptiste Colbert, Louis XIV's finance minister, established the Manufacture royale de glaces de miroirs in 1665 with the explicit goal of breaking this monopoly. Through a campaign of industrial espionage, French agents secretly recruited several important Venetian artisans to Paris. This act of technological theft allowed France to master the Venetian methods. Soon, the French company, which became Saint-Gobain, innovated further. They developed a new casting process that allowed them to produce much larger, flatter panes of glass than the Venetians, who relied on blowing glass cylinders and flattening them. This new method involved pouring molten glass onto a special metal table and rolling it flat, a breakthrough that made the massive mirrors of the Hall possible. By 1672, the quality of French mirrors was so high that Colbert banned the import of Venetian glass altogether.
The physics of imperfect reflection
The Hall of Mirrors stretches 73 meters long and contains 17 massive, arched windows overlooking the gardens. Directly opposite these windows are 17 matching arches filled with 357 individual mirrors, a number chosen to maximize the reflection of light and the display of royal power. The primary optical purpose was to capture the daylight from the west-facing windows and fill the grand corridor with light, an invaluable commodity in an era before electricity.
The reflective backing on these mirrors was not silver, but a tin-mercury amalgam. To create it, artisans would spread a thin sheet of tin foil on a flat table and cover it with liquid mercury. The tin would dissolve into the mercury, forming a reflective paste. A polished plate of glass was then carefully slid onto the amalgam, squeezing out the excess mercury. This process was incredibly toxic due to mercury vapors and produced an inherently unstable alloy that degrades over time.
The glass itself was not perfect. Made with potash and sand, it had a slight greenish tint and contained small bubbles and distortions. These imperfections, combined with the unique properties of the amalgam backing, created a shimmering quality of light rather than a crisp, perfect reflection. At night, lit by thousands of candles, the hall would have been filled with a soft, moving glow, multiplying the light and creating a bright effect. The reflections were meant to merge with the expansive views of the gardens, symbolically extending the king's dominion over nature.