The work of the famous French-Baroque painter Philippe de Champaigne is one of many still lifes that has a skull. He emphasizes it by putting it between two other things , a lily and hourglass.
The painting reminds us that all things have its ends, the lily simply symbolizes resurrection or after life. The hourglass depicts the time we spent with our lives or the journey to our death, good or bad. The three subjects are related to one another when you put them together, it's just the painting is telling us that we have to make our lives useful because time is limited, we can no longer experience how to be contented and happy.
De Champaigne used oil on panel as a medium for the painting.
Paul Cézanne, he only used skulls as subject and add some drama on it like he stacked the four skulls and make it look more gloomy, reminding again our death.
Cézanne used oil on canvas for the artwork.