Palazzo Valmarana



Detail from XVII/4 showing foxing

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Detail from sheet XVII/4 showing foxing

Andrea Palladio (c.1566)

RIBA Library Drawings Collection


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Design for a tomb for the Countess of Shrewsbury

Robert Smythson (c.1600)

RIBA Library Drawings Collection

When old paper becomes covered in brown spots, this is known as 'foxing'. The main causes of foxing are:

  • fungal activity
  • metal-induced degradation

When chemicals have not been removed during the paper's manufacture process, then foxing can occur. The growth of fungi is stimulated, and this is responsible for by-products rich in insoluble iron salts.

The chemical reaction depends on how much moisture is present. A reaction occurs between the iron impurities (iron hydroxide and iron oxide) in the paper's substrate and the organic acids released by fungi.

If the paper's properties consist of almost pure cellulose it will produce little foxing. Earlier papers like those used by Palladio have a high natural presence of alkalinity in the form of calcium carbonate and so are unlikely to be severely affected. You can see the difference in the images shown here of sheet XVII/4 and of the later ink drawing by Smythson, which dates from ca.1600.

The characteristic brown/reddish colour is an accumulation of the decomposition of water soluble products in damp surroundings. The visible areas of foxing are just part of a wider area of damage and they stand out as darker spots under UV light. The amount of moisture needed to produce foxing is less than that required for mould growth.

The later the paper the higher its content of impurities. If environmental conditions of storage are also poor, the damage due to foxing is likely to be more extensive.

The treatment of foxing

Bleaching can be used to treat foxing, although there is no guarantee that the foxing will not return at a later date. There are risks associated with this treatment as chemicals can remain within the paper and alter the tonal qualities of some pigments.

Bleaching was not considered as a treatment for the Palladio drawings because he used iron gall ink and this would have reacted chemically with the bleaching agent. The foxing on the Palladio drawings was not treated.