Archaeological iron objects are very vulnerable to corrosion if the relative humidity is too high, as no really effective and efficient means of stabilising archaeological has been found, despite many attempts by conservators and scientists. A few archaeological items seem to stabilise without intervention, but most are at risk from corrosion and eventual disintegration if they are exposed to moisture and oxygen.

Since oxygen is hard to exclude, archaeological iron objects are ideally stored in very dry conditions at 10%RH or less, using sealable plastic storage boxes and large amounts of dry silica gel. A simple humidity-sensitive colour card is used to visually monitor the RH in the box.

The effort involved in having to check and change the silica gel as the RH rises above 10% can be reduced by keeping dry storage boxes in a dry store where the RH is reduced to say 20% or 30% by the use of a dehumidifier.

Displays can be constructed for archaeological iron objects with well-sealed display cases and even more silica gel, but this is more difficult to deliver for many museums. Efforts have been made in the past to devise dry showcases using dehumidifiers and this has sometimes been very successful.

The main issue with such forms of preventive conservation is the need for regular monitoring and occasional intervention by museum staff. It is unfortunate but true that replacing bags of silica gel into boxes rarely features very high on anyone's "to do" list!




Conservation and storage of a corroding Viking iron spearhead

This spearhead was found in three fragments. Together with other items from the site it has been kept in dry storage with silica gel while they await study and publication. However the spearhead appears to be more prone to corrosion than the other items as despite the dry environment, or perhaps because of sporadic maintenance of the dry silica gel, some corrosion has occurred.

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The three fragments of the Viking spearhead in their storage box

A close-up of the tip of the spearhead shows the detail of the type of flaking corrosion that can lead to delamination and eventually the loss of the object.

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The tip of the Viking spearhead, showing flaking and delamination

After conservation cleaning and repair, the spearhead was stored in its sealed box with increased amounts of silica gel to keep it as dry as possible for as long as possible.

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A typical dry storage box for archaological iron; the large bags of silica gel can be seen under the lid; there is a humidity monitorcard inside for regular checking