As a young man Corot, like all other artists, heads to Rome. Artists had been going to Rome since the Renaissance to build their resume, find inspiration, and study art. At the Louvre Valenciennes advised painters to go out to study the landscape. Study the geology, the flora, the architecture and study it in Italy. French, Germans, Americans, and English artists were all Italy bound. Michallon had studied under Valenciennes and Corot had studied under Michallon. They painted the same subjects.
Example 1 presents Valenciennes’ oil sketch of the Coliseum. He relies on earth tones, architecture, a bright sky and a distant sacred mountain. Examples 2 and 3 present two of Corot’s paintings from 1826. Like Valenciennes’ earlier work they are small studies in oil on board, 7 inches by 9 or 11 inches. These works are for personal experience and collecting evidence for later use in larger exhibition paintings. Occasionally these studies are brought to a high degree of finish. Some artists (Germans especially) are more precise in their approach and begin with a careful drawing which they color in from the top down. Others like Corot sketch more loosely and paint all at once to build a unified tone.
example 1. Valenciennes. 
example 2. Corot.
example 3. Corot.
I will reprise an example of their process and palette. I begin with a small canvas 18×18 which is larger than they would consider. And, I begin on top of older painting whose tone suits my project (example 4). They painted over unsatisfactory works as well. My subject was well known to the traveling artists of their time, Herculaneum in Naples. The mountain behind is Vesuvius. When it blew in 79 AD it blanketed both Herculaneum and Pompeii. I start cutting in the horizon, the shape of the silhouette of the tenements of Naples (example 5). Next, I develop shapes within the larger shapes. Big shapes first, smaller shapes later. I am working from the top down following a common approach for 1826 (example 6). In the last step I have brought my architecture and trees down to the ground plane( bottom of the picture) and, I have begun variegating the light on the sides of buildings and the light on the foliage (example 7). Notice that some of the original underpainting’s colors, stains and effects are still showing through augmenting the theme of the new overlaying painting.
example 4. older painting, substrate.
example 5. step 1, making the silhouette.
example 6. step 2, progressing down the picture plane.
example 7. step 4. reaching the ground plane and filling in smaller shapes.
Fredric Neuwirth
Usually you show a photograph and develop the painting from the design using elements of the photograph. In this case are you simply developing the painting from the elements of the original discarded painting and simply working down. If so is this the message & the point of the blog or is there more to it that still needs to be amplified and expanded?
Kay Halcrow
There is something that really works about the way this painting develops – I like the flow downward and the buildings having grown up like a new crop, from the old volcano. Don’t get FN’s comment at all.
Gail Ingis
Your spatial relationships are amazing. The way you developed that finished work from the underlayment of the older work was clever. To be sure that what I saw is correct, I compared the parts of the older spaces to the new ones, and they appear right on. I like your finished, structured work. Thank you for sharing your methods.
Keith Brooks
David, this seems very much like painting from back to front, or from distance to close, which I believe you taught me years ago.
For sure, your result in this case is great!
Keith