Tarnishing

Assay Office Birmingham is actively involved in the research and development of tests to identify the causes of tarnishing in jewellery items. Dippal Manchanda, technical director of Assay Office Birmingham, explains how some elements of packaging can cause or accelerate tarnishing and how to avoid it.

Tarnishing is an ongoing problem for jewellers, particularly for silver and low carat gold articles. Silver naturally interacts with oxygen and sulphur-bearing pollutants to create silver sulphide, resulting in a visible discoloration of the metal’s surface. Silver tarnishes in environments containing various sulphuric gases, even in very low concentration. The amount of tarnishing is determined by the relative humidity, ambient temperature, gas concentration, and the length of time the silver is exposed to the gases.
After extensive research, paper and cardboard materials used for packaging have now been identified as one of the factors that can accelerate tarnishing. High sulphur and other tarnish-causing compounds released from paper, cardboard, certain cloths, foams and adhesives are a common problem. Most types of paperboard in contact with silver, copper and certain copper-zinc based low carat gold alloys cause localised tarnish stains on the metal surface.

The tarnishing agent is most likely to be free sulphur or a volatile sulphur-liberating compound, which can still cause problems even if not in direct contact with the product. The blackening of silverware wrapped in tissue paper and packed in paperboard boxes is not due to the direct effect of any corrosive substances in the tissue but to the vapourphase action of sulphur compounds from the boxes, which penetrate through the paper to the metal surface.

Moisture may also be a vital ingredient. Paper/paperboard containing 0.5 or 2.5 per cent sodium sulphide tarnishes silver only in the presence of moisture. Similarly, various types of paper containing sulphur compounds which, on alkaline extraction and acidification, liberate hydrogen sulphide do not blacken silver unless they are moist. On the other hand, filter papers impregnated to contain 0.006 per cent of free sulphur can cause tarnish stains, even when the papers are completely dry. The tarnishing of silver, copper and certain copperzinc alloys can be avoided with certainty only if a free sulphur content of less than 0.0008 per cent can be assured in the paper or paperboard with which the metal articles are in contact.

Recycled paper and cardboard may be helpful as they have lower sulphur content than virgin paper, as well as lower levels of other harmful tarnish-causing constituents. As far as paper/cardboard packaging material is concerned, it is not only sulphur that is involved. All wood products have some level of formaldehyde present, which converts to formic acid over time and thereby increases the acidity of the paper product. Unless manufacturers employ rigorous quality control practices to their input stock paper and board, large variations in the tarnish-causing constituent can occur.

If the pH of paper is low, for example 4.0 to 4.5 (cold extraction), as little as 0.0002 per cent of reducible sulphur may cause tarnishing, whereas if the pH is higher, even a much higher quantity of sulphur may not cause tarnishing.

So-called ‘acid free’ paper has been found to contain high concentrations of sulphur and other contaminants. ‘Acid free’ and ‘acid neutral’ have very different meanings: ‘acid free’ is a process to remove the acids from the board and often relies on non-wood components (rag is typical). ‘Acid neutral’ is simply a pH buffering material (which leaches out over time, leaving behind an acidic board or paper) added to the paper or board to bring the pH of the system to neutral. ‘Acid free’ paper/cardboard is therefore preferable.

In the UK, the average level of atmospheric sulphur is only a few parts per billion – not sufficient itself to cause rapid tarnishing. However, when an item is packaged in a small, sealed environment with sulphur-releasing agents such as sulphurous adhesives etc, the levels of sulphur can become concentrated, especially in warm, humid conditions. Experimental evidence shows that even one small adhesive label can have dramatic effects.

Some synthetic foam, besides sulphur, contains high levels of chlorides and fluorides – both of which will cause tarnishing of silver and other metals. Some plastic bubble wraps in direct contact with silver objects can also encourage tarnishing. Two kinds of tarnish have been noted; firstly an even clouding with a mild orange colour where the smooth side of the bubble wrap made contact with the silver, and secondly a heavier layer of tarnish with clean voids where the bubble surfaces had pressed against the silver. The bubble wrap is made from polyethylene coated with ‘liquid saran’ to keep the air from escaping from the bubbles. The saran is deduced to be the culprit causing tarnishing.

Testing packaging to identify tarnish accelerants
The accelerated tarnish test from the Analytical Laboratory for testing packaging materials involves the identification of the presence or absence of components responsible for causing tarnishing or staining of products. This test is primarily for packaging that is in direct contact with silver, low carat gold and base metal items. Test results indicate the type of tarnishing or staining and the relative distribution of the materials causing it. Test conditions are such that if any volatile sulphides or any other compounds which cause tarnishing are present, then they will cause the control sample of uncoated silver to tarnish. This can be a very valuable ‘early warning’ system for retailers, preventing unsuitable packaging from causing large shipments to tarnish and become unsaleable as has often happened in the past.