Preservation of historical pewter in church and museum collections

Pewter refers to items made of tin and its alloys rich in tin. Valuable pewter from the 17th, 18th and 19th c. is to be found in both the Danish churches and in the museum collections. The museums also have many older pieces. The Conservation Department at the National Museum of Denmark regularly restores pewter items such as ewers (or pitchers) and font basins. In spite of age and historical value the pewter is often used in the baptismal service as well. The department also conserves pewter from museums, private collections as well as items coming from archaeological marine sites.

Tin is a shiny grey metal which is rather soft. To distinguish pure tin from lead the latter has a duller grey colour, is even softer and much heavier (a specific gravity of 11.37 compared to tin's 7.29). The pure tin was generally alloyed with small amounts lead and sometimes also copper to obtain better casting properties. The alloy is also stronger than pure tin. Even so, pewter was made of thicker parts than similar items of silver, copper or brass. Alloying with lead also made the pewter less expensive.

Pewter was usually constructed of cast parts joined by soft soldering to complete the object. The soft solder alloy of tin and lead was made with a slightly lower melting point than the cast parts to be joined. There have been regulations for craftsman and rules for pewter quality marking since the Middle Ages, to avoid products with too high a lead content being marketed. Holger Rasmussen (1) has described the history of Danish pewter in more detail.

Pewter darkens when the metal surface reacts with oxygen from the air. The lead content also influences the colour of the surface tarnish. From the middle of the 19th c., and especially in England, antimony started to be used for the alloying instead of the poisonous lead.

Surface oxides and deterioration mechanisms

Pewter should be kept in clean and not too humid surroundings. In a high relative humidity the oxide film grows thicker and the corrosion accelerates even more when dust settles on the surface. Dust contains corrosive salts, which keep the surface humidity high. The thick oxide layers can be seen on tin coffin plates from damp and dusty crypts and old chapels. The superficial tin oxide, which bears the inscription, can then easily flake off in thick layers. Archaeological items of tin can be totally converted to oxides. From archaeological marine sites it has been noted (2) that pewter containing some proportion of lead can survive under certain conditions.

A special form of corrosion on the insides of basins and ewers for baptising is caused by deposited calcium carbonate (limestone). If kept damp the carbonate coated tin surface will corrode.

There is another deterioration phenomenon that should occur in pure metallic tin, and in tin with a limited amount of lead, at temperatures below 13°C. The metallic tin should change to at grey crystalline powder. This allotropic structure change is normally referred to as tin pest and is described by Hedges (3). Proving the phenomena by analysis is however difficult, because mostly tin oxide is detected. Deterioration seems to start locally in spots or as lines where there is stress in the metal from bending or pressure. The attack is first seen as small volcano shaped craters opening on the metal surface. This is because the change results in expansion. The produced powder drops out of the craters, leaving holes, fig. 1-2.

Fig. 1-2 A dish from 1747 from the National Museum collections (mus.nr. M 835) bears typical holes from tin pest. On the back the dish is marked with three towers indicating that it is so called crown tin, with a maximum of 1/6 of lead.

Fig. 2 A close up of figure 1

Pewter is furthermore very sensitive to careless handling because it is a relatively soft metal. Scratches and dents come very easily and hinges and knobs can break. One should also avoid open fire and heating equipment close to the pewter. It will easily melt.

Preservation

The cold climate in the Danish churches can lead to deterioration by tin pest. A baptising basin often rests in a granite font, causing the metal to stay cold during the short periods the church is heated for services. Placing the pewter in a warmer place between services would eliminate deterioration, but this is not always possible or advisable. If one moves a basin from the church to another building there is a risk of dropping it. Insulating the basin from the font by lifting it slightly would at least minimize the danger of condensation from temperature differences. This, however, does not eliminate the danger of deterioration by tin pest in a cold church. Making a suitable support in the font has to be done with care. Individual answers for preservation are needed for each item so consulting an expert in conservation and climate is advisable.

Caring for pewter in private and museum collections is easier. Room temperature with good clean cupboards and shelves are all that is required. Lead objects kept in the same place would corrode if left near wooden materials, which release acetic and formic acids. That though, seems only to be a danger for lead or nearly pure lead items.

Pewter can be cleaned off for dust once in a while. If the surfaces are a bit greasy after the removal of dust, a cleaning in de-ionised water with a drop of detergent can be done. It is best to use a plastic container and afterwards dry the item with a soft cotton cloth or napkin right away. When putting the pewter away again do not make piles of plates or let big plates stand on edge, because the weight itself can distort them.

It is best to leave the pewter with the surface oxides undisturbed. Polishing it will only open the surface for further reaction with oxygen. Especially trying to polish off oxide layers built up over several years is not advisable, since the oxides are harder than the metal beneath. If it is thought necessary to remove oxidation from a historical piece, a trained conservator should do it. A controlled electrolytic treatment might be chosen. Repair jobs, including welding or removing dents, also require an expert. Some pieces can even be too deteriorated to stand a traditional repair treatment. If welding is done badly, irreparable damage can be the result. An example of a bad repair on an otherwise well-preserved pitcher is shown in figure 3 (left).

Fig. 3 A tin baptising pitcher from about 1842 belonging to Øster Skerninge shown before and after repair at the Department of Conservation.

Fig.4 A well-preserved plate excavated from the wreck of the Spee, a Swedish warship from 1677 that was found in 1994 off the coast of Skagen in North Jutland. A large number of plates were recovered and their good condition could be ascribed to a certain amount of lead in the alloy, as mentioned above. One plate was cleaned with a combination of electrolytic baths and mechanical cleaning.

References

2) North, N.A. and MacLeod, I.D.: Corrosion of metals. Conservation of Marine Archaeological Objects, Butterworths, London, 1987, pp. 68-98.

3) Hedges, E.S.: Tin and Its Alloys, Edward Arnold publishers LTD, London, 1960, pp 16-37, 50-72.