For purifying metals and any objects made with them, Manu Smriti recommends the use of water, ash, mud, acids etc. See the following verses:
5.111. The wise ordain that all (objects) made of metal, gems, and anything made of stone are to be cleansed with ashes, earth, and
water.
5.112. A golden vessel which shows no stains, becomes pure with water alone, likewise what is produced in water (as shells and coral), what
is made of stone, and a silver (vessel) not enchased.
5.113. From the union of water and fire arose the glittering gold and silver; those two, therefore, are best purified by (the elements) from
which they sprang.
5.114. Copper, iron, brass, pewter, tin, and lead must be cleansed, as may be suitable (for each particular case), by alkaline (substances),
acids or water.
Gautama (1.29-31).—‘As regards the purification of things, objects
made of metal must be scoured, those of clay should be thoroughly
heated by fire, those of wood must be planed and those of yarns should
be washed.—Objects made of stone, jewels, shells or mother-o’pearl
must be treated like metallic objects.’
Baudhāyana (1.8.32, 46, 47).—‘Defiled objects made of metal must be
scoured with cow-dung, earth and ashes, or with one of
these:—conch-shells, horn, pearl-shells, and ivory with a paste of
yellow mustard; or they may be cleaned with milk.’
Bo. (1.14.45).—‘Vessels made of metal must be washed, after having
been scrubbed; the materials to be used for scrubbing arc cow-dung,
earth, ashes and the like.’
Āpastamba (1.17.11).—‘A vessel made of metal becomes pure by being
scoured with ashes and the like.’
Vaśiṣṭha (3.49-51).—‘Objects made of metal must be scoured with ashes;
those made of clay should be thoroughly heated by fire; those of wood
should be planed, and those of yarns should be washed. Stones and gems
should be treated like objects made of metal; conch-shells and
pearl-shells like gems.’
Yājñavalkya (1.182, 183).—‘Of vessels and cups made of gold, silver,
conch-shell, of stones, vegetables, ropes, roots, fruits, cloth,
bamboo, and leather—as also of other vessels of wood,
etc.,—purification is accomplished by means of water.’
Devala (Aparārka, p. 254).—‘Vessels not touched by liquids are
purified by water; those touched by liquids are regarded as purified
only when they are free from fatty stains and odour.’
Hārīta (Aparārka, p. 254).—‘Gold, silver, conch-shells and
pearl-shells are purified by water; if these are defiled to the extent
of being discoloured and losing their properties, then they should be
cleaned with the flour of barley, wheat, beans, lentils and cow-dung;
copper-vessels are cleansed by acids and salts; vessels of Kāṃsya by
ashes; iron-vessels by being scrubbed with stone, oil and sand;
vessels made of gems are cleansed by scrubbing with stone and
washing.’
Āpastamba (Do.).—‘Vessels of kāṃsya are cleansed by the ten alkalies.’
Śaṅkha (Do.).—‘A kāṃsya -vessel should not be heated; it becomes
purified by being washed twenty-one times. Vessels of Kāṃsya, lead and
zinc are purified by hot water. Kāṃsya and iron are cleansed by
alkalies; iron-vessels are purified by heating, also by ashes and
cow-dung. Vessels made of stone are cleansed by heating, scrubbing and
also by water; those of wood, by planing; also by earth, cow-dung and
water.’
Uśanas (Do., p. 255).—‘Vessels of gold, silver, copper, lead, zinc and
kāṃsya are cleansed by water mixed with ashes; those of metals in
general, if defiled, are cleansed by washing with ashes three times.
Gold, gems, silver, conch-shells, pearl-shells and stones, as also
diamonds, bamboo, ropes and leather, are purified by water. Vessels of
clay and weapons are heated for purification.’
Kāśyapa (Do.).—‘Ivory, horn, conch-shell, pearl-shell and gems are
cleansed by sand.’
Yama (Do.).—‘Silver, gold, copper, lead, iron, Kāṃsya and zinc are
purified by ashes.’
Ṛṣyaśṛṅga (Do.).—‘Pearls and corals are purified by washing; also
vessels made of conch and other shells, and also of all kinds of
stone.’
Viṣṇu (Do.).—‘Things made of copper, lead or zinc are purified by acid
and water;—all things made of metal are cleansed by being washed with
ashes and water twenty-one times.’
Smṛtyantara (Aparārka, p. 255).—‘Vessels of kāṃsya are cleansed by
ashes, if they have not been touched with wine; if so touched, they
can be cleansed only by heating and scrubbing; copper is cleansed by
acids if it has not been touched with flesh; if so touched, it can he
cleansed only by being heated over again.’
Ādipurāṇa (Do., p. 256).—‘Things made of gold, silver, conch-shells,
shells and gems,—also those made of kāṃsya, iron, copper, lead and
zinc,—if they are not smeared,—become cleansed with simple water.’
Śātātapa (Do.)—‘Gold, silver, copper, lead, iron and zinc are cleansed
by being scrubbed with stone,’