Cleaning and Preservation of Coins - A Complete Guide, page 19
HOW TO CLEAN ZINC and TIN COINS: Effective Methods and Applications
NOTE: All chemical cleaning methods are relatively safe. However, their use and application are entirely at the reader's risk. We assume no responsibility for damage to property or personal health.
HOW TO CLEAN ZINC COINS
Zinc was used for coinage only in emergencies and only for coins destined to have a short period of circulation.
For example, zinc money occurs principally in many emergency coinages of German cities after the First World War and also in the small change of the Third Reich from 1940.
It must be assumed that the raw material available was not always satisfactory. Zinc contains traces of lead, bismuth and iron. These impurities cause zinc coins to vary in behavior with chemicals. That is why removal of the unsightly zinc oxides by various chemical methods can cause unexpected discoloration on coins. This discoloration often occur in different forms, even with coins of the same type.
The following methods can be used for cleaning zinc coins:
1) Mechanical Cleaning of dirt from zinc coins can be done more vigorously than for silver. A hard brush is best, but a glass brush and an ink eraser can be used advantageously too.
2) Wet Cleaning: Dirt on zinc coins is most easily removed by softening in warm soapy water. This is done for as long as necessary. The coins then can be brushed with a stiff brush or a glass brush. It is also possible to rub the coins with the common salt, sprinkled on from a salt shaker, with the fingers or a cloth.
3) Immersion into a Hydrogen Peroxide Bath, described on page 11, is applied briefly to remove the dirt.
4) Chemical Cleaning with Vitrolin Copper Soap: zinc coins can be cleaned very simply by rubbing them with a moist cloth that has been passed over Vitrolin Copper Soap. If the coating is heavy, the zinc coins can be steeped in Copper Soap Paste for five to ten minutes. Thorough soaking for a quarter to half an hour must follow.
5) Immersion into Sulfuric Acid (H2SO4) Solution: the best and at the same time the simplest method of removing grayish-white zinc oxidation is placing the coins in approximately 5% Sulfuric acid (diluted 1:20) and leaving them there for 10 to 20 minutes.
CAUTION: DILUTION OF SULFURIC ACID MUST ALWAYS BE DONE BY POURING ACID INTO WATER IN A FINE STREAM, NEVER by pouring water into concentrated sulfuric acid!
The coins are turned over several times with two wooden sticks. Neutralization is accomplished by immersion in 5% Sodium Hydroxide (caution: caustic soda) and rinsed thoroughly. They are then brushed.
Boiling in 5% Sulfuric acid or brief immersion of heated coins in concentrated sulfuric acid is not recommended as the coins at the conclusion appear far more spotted.
Zinc coins must be protected unconditionally from the effect of the atmosphere and its constituents, or they will darken again in a short time (a few weeks). This is best done by lacquering with Japanese lacquer. An oil film can be provided by kerosene or Ballistol.
HOW TO CLEAN TIN COINS
Tin has been rarely used as a coin metal. For example, as an attempt to boost the tin mining industry and deter forgers, bimetallic tin coins, Charles II farthings and halfpennies with copper plug in the center, were struck in England for a short period in the 17th century.
In another case, tin coins were struck in Thailand and Japan in the 1940s because the warm climate would prevent the influence of Tin Plague (described on page 4) on coinage. Tin has been widely used only as an addition to copper alloys.
The following methods can be used for cleaning tin coins:
1) Removing Tin Coatings: it can be done best in dilute Hydrochloric acid. After thorough rinsing, tin coins are rubbed bright with prepared chalk; harder agents are not used because tin is very soft.
Removal of very tightly adherent coatings can be accomplished by immersion in concentrated Hydrochloric acid, thorough rinsing must follow.
2) Treatment of Tin Plague. Tin plague is not a chemical, but a crystalline, decomposition of tin. One precautionary measure is the avoidance of storage of tin objects in rooms below 68 °F ( 20 °C), unless the objects are removed or used frequently.
Lacquering does not fully protect against tin plague, although sometimes it retards heat loss in the tin.
If tin plague appears, the affected coins, medals and other objects immediately must be separated from pieces not yet affected to prevent infection as much as possible. Then the tin must be heated to 212 °F (100 °C), but never over 321.8 °F (161 °C), or a new type of tin will result.
This heating can be done only in an electric oven equipped with a sufficiently accurate thermometer. A simple expedient is to boil affected tin pieces for several hours in water. After boiling, the tin pieces can be rubbed with a moist cloth and prepared chalk. They can be brushed off mechanically also.
To arrest the progress of tin plague, it is advised to brush the affected areas with dilute Hydrochloric acid and alternately washing it off again with water. The hydrochloric acid "etches" away the affected regions and the water protects the remaining sound areas. It is proper to warm the tin object beforehand to about 122 °F (50 °C).