A chemical cleaning composition for silver- or copper-containing surfaces consists of a water-soluble solid or powder comprising thiourea or a derivative thereof, and is formulated in unit application form, e.g. in tablet or sachet form. A preferred composition comprises up to 60% thiourea or a derivative thereof; up to 4% surfactant; not more than 40% acidifier; less than 20% disintegrant; up to 10% binder; 0-1% lubricant; and up to 40% diluent.