This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
Make a bath of 10 parts of sulphuric acid and 90 parts of water, and let the coin lie in this until the crust of silver sulphide is dissolved. Prom 5 to 10 minutes usually suffice. Rinse in running water, then rub with a soft brush and castile soap, rinse again, dry with a soft cloth, and then carefully rub with chamois.
Into a wide-mouthed bottle provided with a good cork put the following mixture:
Cream of tartar...... 2 parts
Levigated chalk..... 2 parts
Alum.............. 1 part
Powder the alum and rub up with the other ingredients, and cork tightly. When required for use wet sufficient of the powder and with soft linen rags rub the article, being careful not to use much pressure, as otherwise the thin layer of plating may be cut through. Rinse in hot suds, and afterwards in clear water, and dry in sawdust. When badly blackened with silver sulphide, if small, the article may be dipped for an instant in hydrochloric acid and immediately rinsed in running water. Larger articles may be treated as coins are— immersed for 2 or 3 minutes in a 10 per cent aqueous solution of sulphuric-acid, or the surface may be rapidly wiped with a swab carrying nitric acid and instantly rinsed in running water.
If greasy, wash carefully in suds, or, better, dip into a hot solution of caustic potash, and then wash in suds with a soft rag, and rinse in running water. If not then clean and bright, dip into the following mixture:
Nitric acid.......... 10 parts
Aluminum sulphate .. 1 part
Water.............. 40 parts
Mix. Rinse in running water.
Rub first with jewelers' rouge made into a paste with oil; wash in suds, rinse, dry, and finish with chamois or wash leather.