When Holy Roman Emperor Charles IV commissioned the Charles Bridge in 1357, he needed a structure tough enough to withstand the raging Vltava River, which had previously destroyed the Judith Bridge. Master builder Peter Parler faced an immense engineering challenge: binding massive Bohemian sandstone blocks without modern hydraulic cement. According to widespread Czech folklore, the emperor ordered towns across the realm to ship thousands of raw eggs, milk, and wine to Prague to be whipped directly into the slaked lime mortar.
The organic proteins were believed to create a durable, water-resistant binding agent. Legend hilariously claims that the anxious citizens of Velvary, fearing the eggs would crack during transport, sent them pre-boiled, leaving masons with an unexpected lunch instead of construction supplies. While modern historians long dismissed the story as pure myth, material scientists in 2010 conducted mass spectrometry on original mortar samples.
Surprisingly, they detected unmistakable traces of egg proteins, casein from milk, and fatty acids. Medieval masons frequently experimented with organic additives—such as ox blood, lard, or curd—to slow down the drying process of lime mortar, preventing shrinkage cracks and improving hydraulic setting. Combined with deep-oak cofferdams to drain the riverbed and meticulous block-interlocking mechanics, this bizarre breakfast-blend mortar created a bridge that has weathered river ice, wars, and six hundred years of traffic.