When painting a picture the colors may be too bright and intense. If these paintings are placed next to others which are already muted by the effect of the passage of time, the contrast is greater. For this reason, and in order to fine-tune or somewhat refine these vibrant hues, many painters throughout history, often finished their paintings with colored varnish applied as a glaze.
These dark varnishes, generally oil-based and glue mixed with bitumen or dark smoke pigment, is what some ancient writers called altramentum (or atramento).
For some of these treaties we know, for example, that the painter Apelles in the s. III BC, used these finishes to harmonize the colors in his paintings.