He primed the wall with a material that he hoped would accept the tempura and to protect the paint against moisture. Although the painting itself is beloved, leonardo's tempura on stone experiment was a failure. By the early 16th century, the paint had started to flake and decay, and within 50 years, The Last Supper was a ruin of its former glory. Early restoration attempts only made it even worse. In 1980, a 19 year restoration effort of that artwork began