The surface is part of what we see
A before-and-after conservation photograph can be startling. Its lesson is not simply that an old painting was dark and a restored one is bright. The image records a treatment involving several possible layers, and those layers need to be identified before the difference can be explained.
The Smithsonian American Art Museum documents discoloured varnish and surface dirt on Charles Bird King's Miss Satterlee, dating to about 1830–1839. Its treatment record says those materials were removed. The museum describes other paintings with active flaking, losses or distorted supports that required different work. A single word such as cleaning hides these distinctions.
Sources: s1
Read the treatment, then the photograph
The museum's painting-conservation studio distinguishes cleaning from inpainting areas of lost paint. Cleaning may remove accumulated grime, darkened varnish and discoloured earlier retouching. These are specific interventions on an object's history; they are not evidence that every darker passage was unwanted. A treatment description should identify what was removed and what was added.
For a useful comparison, record which layers were treated, whether losses were filled or retouched, and whether the photographs were made under comparable conditions. This is a proposed reading checklist. The image alone cannot supply answers that the treatment record does not give.
Keep appearance and intention separate
A changed surface can make a composition easier to read, but 'this is exactly what the artist saw' remains a stronger claim. Materials age and documentation varies. A careful caption should describe the known treatment and its visible result, then leave uncertainty about original appearance where the evidence cannot resolve it.