Oil paint
By 1540, there were very few painters who still worked in tempera, the previous method for painting on panels.  In Italy, many artists continued to decorate walls and ceiling with frescos. However, it was discovered that oil paint, unlike tempera was flexible (it could bend). This meant that it could be used on flexible surfaces like cloth without breaking away and falling off. Once painting on canvas (heavy linen cloth) became usual, artists were able to do enormous pictures. If the painting was too big to fit through a doorway, the artist could just roll it up.