a rough or unfinished drawing or painting, often made to assist in making a more finished picture.
"a charcoal sketch"
synonyms:drawing, preliminary drawing, outline
Here is a small selection of some of John's sketchwork. It is a fascinating part of the process.
As you can see he works in a variety of mediums form pencil to fountain pens, whatever is on hand at the time. These are invaluable reminders that assist him to create the correct look and feel for the scene he is painting.