Lead sulphide problems

In issues 12/07, 1/2008 and 2/2008 of the journal “Philately” the so-called lead sulphide damage to classic postage stamps after storage in plastic film was reported and discussed.

The assessment of philatelic items submitted for an opinion requires, inter alia, the identification of colours, as well as, where appropriate, changes in colour. The fact that the colours of certain stamps can change under specific conditions was in some cases described about a hundred years ago. The most common example is the formation of blackish-brown lead sulphide by the exposure of yellow lead chromate, which may be contained in inks, to hydrogen sulphide or inorganic as well as organic sulphides.

Because even the slightest amounts of sulphides are enough to cause visible changes to the appearance of a postage stamp, the exact origin of these substances is difficult to determine. Thus, problems in determining the colourexist in individual cases.

Members of the Federation of Philatelic Expertisers e. V., professional philatelists and collectorshave observed over a number of years a greatly increased occurrence of changes incolour, not only in the first affected area of classic stamps, but also in more recent ones as well in modern collecting areas.

This is especially noticeablein the case of specific stamp issues which have been stored in protective covers or album sheets made from hard PVCfilm. Significant colour changes were observed especially in the long-term storage of specific stamp issues in hard PVC film. The effect occurred on individual issues but also relatively quickly, and even more clearly under anaerobic conditions or in low air circulation – as within welded film packaging or during storage in a Bank safe.

Plastic films manufactured from various synthetic materials (polyethylene “PE”, polypropylene “PP”, polyester, polyvinyl chloride “PVC”) require the use of additives, such as the so-called stabilisers or plasticisers,either in the production process or to achieve the desired properties. Stabilisers for the production of PVC film may contain complex tin-sulphur compounds (sulphides). The nature and quantity of these additives can also have changed over the years – for example due to processing machines. Accurate, retrospective observation would be very difficult here also.

Notwithstandingthe partly conflicting scientific investigations into the chemical reactions of certain film additives on certain components of the stamp colour (also the gumming, according to the latest findings), there remain the numerous observations, made by users,of colour impairment during storage in some products made from hard PVC.

The BPP recommends the storage of postage stamps only in films that were produced without the addition of sulphide-containing stabilisers as a precautionary measure to prevent sulphide impairment.

Information as to whether the film previously used by collectors containsulphur compounds can and should be obtained by the individual collector from the respective manufacturers.

The BPP considers it essential that protective covers that are declared “free of sulphur compounds” are offered in future in the philatelic accessories market. We therefore appeal to dealers, manufacturers and collectors’ associations.

The Board of Directors of the BPP.