Abstract: A method of dissolving either a precipitate or deposit that contains essentially calcium carbonate comprising contacting the precipitate or deposit with a mixture comprising a first acid, the first acid being a Lewis acid in a concentration of 10 to 1000 mmoles/l, and a second acid, the second acid being a medium-strength to weak protonic acid or a partly deprotonated acid that yields at least one proton with the molar ratio of the second acid to the Lewis acid being between 1:1 and 6:1, the Lewis acid being a solution of at least one salt of a strong acid with cations selected from the group consisting of cations of a 3-valence main-group element, cations of a 3-valence transition element, cations of a 3-valence lanthanide, cations of a 3-valence actinide, cations of a 4-valence main-group element, cations of a 4-valence transition element, cations of a 4-valence lanthanide and cations of a 4-valence actinide.