Patina and original silvering
Patina is the layer formed by time on the surface of the metal. It protects the coin, it recounts where the coin has been, and it accounts for part of the pleasure of the object.
Bronze patinas
On bronze, patina ranges from dark brown to green, according to the soil the coin lay in and the composition of the alloy. A patina formed slowly is smooth, even and adherent. It follows the relief without drowning it.
Some corrosion, by contrast, is destructive. Bronze disease, an active corrosion recognisable by its powdery light-green spots, eats into the metal and spreads. An affected coin must be treated, not merely wiped.
The silvering on nummi
Tetrarchic nummi and reformed aureliani were covered with a fine layer of silver, obtained by treating the surface of the flan. That silvering gave a bronze coin the appearance of silver.
It has almost always gone through rubbing. Where some survives, it is seen chiefly in the hollows, around the letters and at the bottom of the field, pale grey with a slight iridescence. A coin that retains part of its original silvering is markedly more sought after than one cleaned down to the metal.
Why not to clean
An ancient coin cleaned aggressively loses its patina, its silvering, and a great part of its value. The brightness obtained with a product or a brush never comes back.
If a coin looks dirty to you, leave it. Dry dusting and storage in a neutral capsule are enough. Everything beyond that belongs to professional conservation.