The canvas supports are what are called "archs". They're studs. These are made of stainless steel (diameter 25 or 30) or aluminium (diameter 25).
For inox, apply a stainless steel cleaner paste twice a year to the surface to be cleaned with a sponge and rub vigorously. The polishing action begins when the product becomes dry. Repeat the operation several times if necessary. Wipe the holder with a clean cloth, the metal then regains its full shine.
For aluminium, just rub the materials twice a year with solvent-free cleaner.