Cleaning of paintings
The Venice painting was very dirty, partially cleaned in this image and had to be remounted. Remounting is to remove the folded edge of the canvas (were it folds around the frame) and glue, with wax, it to another canvas to support it.
Finally it is varnished following cleaning and touching in.
Cleaning of paintings requires choosing a medium that will work with the dirty surface, and not attack the paint.
The top image shows cleaning in progress on a very dirty canvas.