Foxing, Tanning, and Sunning: A Visual Guide to Common Book Defects

Every book ages. Paper yellows, leather dries, cloth fades, and adhesive weakens. But there is a difference between the graceful aging of a well-kept book and the degradation caused by poor storage, harmful materials, or environmental exposure. Three of the most common forms of visible deterioration — foxing, tanning, and sunning — have distinct causes, distinct appearances, and distinct implications for a book’s collectibility and value.

Foxing

Foxing is the appearance of small, irregular brown or reddish-brown spots on the pages of a book. The spots range from tiny pinpoints to larger blotches, and they typically appear in scattered patterns rather than uniform discolouration. The name may derive from the fox-brown colour of the spots, or from an old English word for “fermentation.”

Causes

The exact mechanism of foxing has been debated by paper scientists for over a century. Current research identifies two primary causes:

Iron contamination. Tiny particles of iron, introduced during paper manufacture (from water, machinery, or raw materials), oxidise over time when exposed to moisture. The oxidised iron produces the characteristic brown spots. This form of foxing tends to produce smaller, more sharply defined spots.

Fungal growth. Certain fungi colonise paper in damp conditions, producing brown metabolic byproducts that stain the fibres. Fungal foxing tends to produce larger, more diffuse spots and may be associated with a musty odour.

In practice, both mechanisms may operate simultaneously in the same book, and distinguishing between them without laboratory analysis is difficult.

Where foxing appears

Foxing is most common in:

• Books printed between approximately 1820 and 1940, when paper manufacturing processes introduced more metallic contaminants than earlier handmade papers or later modern papers.
• Pages with lower ink coverage (blank pages, margins, and endpapers), where the foxing is most visible.
• Books stored in damp or humid conditions, which accelerate both iron oxidation and fungal growth.
• Books printed on cheaper paper grades, which tend to contain more impurities.

Impact on value

Foxing reduces value, but the degree depends on severity and location:

• Light foxing to preliminaries (half-title, endpapers): Minimal impact. This is expected in pre-1940 books and is tolerated by most collectors.
• Scattered foxing throughout the text: Moderate impact. Reduces the grade by one step (a book otherwise Fine might be graded Very Good).
• Heavy foxing affecting text readability or illustrations: Significant impact. Heavily foxed copies are appropriate only as reading copies or for very rare titles where no better copy is available.

Treatment

Professional paper conservators can reduce foxing through chemical bleaching (hydrogen peroxide or calcium hypochlorite solutions) or enzymatic treatment. These procedures lighten the spots significantly but carry risks — they can weaken the paper, alter its colour, and affect inks or illustrations. Treatment is generally recommended only for high-value items where the foxing materially affects value.

For prevention, maintaining relative humidity below 55% and ensuring good air circulation around bookshelves significantly reduces the risk of new foxing developing.

Tanning (Toning, Browning)

Tanning — also called toning or browning — is the overall darkening of paper from its original white or cream colour to yellow, tan, or brown. Unlike foxing, which produces discrete spots, tanning is a uniform discolouration that affects entire pages or sections of the book.

Causes

Acid hydrolysis. The primary cause of tanning in books printed after approximately 1850. During this period, paper manufacturers switched from rag-based stock (cotton and linen fibres, which are pH-neutral) to wood-pulp stock (containing lignin and other acidic compounds). The acids in wood-pulp paper cause the cellulose fibres to degrade over time, producing the characteristic yellowing.

This process is self-accelerating: as the paper degrades, it produces more acid, which accelerates further degradation. Books printed on highly acidic paper will eventually become so brittle that the pages crumble at the touch — a fate that has already overtaken millions of volumes in library collections worldwide.

Oxidation. Exposure to atmospheric oxygen darkens paper independently of its acid content. This process is slower than acid hydrolysis but contributes to the overall browning of all papers over time.

Light exposure. Light accelerates both acid hydrolysis and oxidation, causing paper to darken faster in exposed areas. A book whose fore-edge has been exposed to light while shelved will show a tideline of tanning along the visible edge.

Where tanning appears

• Pages of books printed on acidic wood-pulp paper (roughly 1850–1990)
• Page edges (most exposed to air and light)
• Dust jacket flaps and spine (most exposed to light)
• Books stored near heat sources or in hot environments

Impact on value

Tanning is so common in books from the acidic-paper era that moderate toning is accepted as a normal characteristic of the period. The impact on value depends on severity:

• Light toning to page edges: Expected and tolerated. Does not significantly affect grade.
• Moderate toning throughout: Noted in descriptions (“pages toned”) but accepted for books of the period.
• Heavy browning with brittleness: Significant impact. Indicates advanced paper deterioration that threatens the book’s long-term survival.

Treatment and prevention

Deacidification is a professional conservation treatment that neutralises the acids in paper and deposits an alkaline buffer to slow future degradation. It does not reverse toning that has already occurred, but it significantly slows further deterioration.

Modern books (roughly post-1990) are increasingly printed on acid-free paper, which resists tanning indefinitely. The acid-free movement — championed by conservators and adopted by major publishers — has largely solved the tanning problem for new books.

Sunning (Fading)

Sunning is the fading or bleaching of colour caused by exposure to light — particularly ultraviolet radiation from sunlight or fluorescent lighting. Unlike tanning (which darkens paper) sunning lightens or shifts the colour of dyes, pigments, and inks.

Where sunning appears

Dust jacket spines. The most common and most visible form of sunning. Because books are shelved spine-out, the jacket spine receives the most light exposure. A sunned jacket spine is immediately recognisable: the spine colour differs dramatically from the front and back panels, which were shielded by adjacent books.

Cloth bindings. The cloth on the spine can fade from its original colour, particularly in brightly dyed cloths (reds and blues are most susceptible). Faded cloth spines are common on books from the nineteenth and early twentieth centuries.

Exposed page edges. The fore-edge of a shelved book receives ambient light, which can cause differential toning — the exposed edge darkens (from oxidation) or bleaches (from UV) relative to the interior pages.

Leather bindings. Leather darkens or lightens unevenly when exposed to light, particularly when one side of the book faces a window.

Impact on value

Sunning is one of the most significant condition issues for dust-jacketed books:

• Light spine sunning (slight tonal shift, original colour still recognisable): Moderate impact. Reduces grade by half a step.
• Moderate spine sunning (noticeable colour change, original colour identifiable but altered): Significant impact. May reduce grade by a full step.
• Severe spine sunning (spine is a completely different colour from the panels): Major impact. Severely sunned jackets are considered damaged, and the book’s value is substantially reduced.

Treatment

Sunning is irreversible. The chemical bonds in the dyes and pigments have been broken by UV radiation, and no treatment can restore the original colour. The only “treatment” is prevention: store books away from direct sunlight, use UV-filtering window film, and choose LED lighting (which emits minimal UV) over fluorescent lighting.

The Practical Takeaway

All three defects — foxing, tanning, and sunning — are caused or accelerated by environmental factors within the collector’s control. Stable humidity below 55% prevents foxing. Acid-free storage materials slow tanning. UV-filtered lighting prevents sunning. A collector who controls these three environmental variables protects their collection from the most common and most value-destroying forms of deterioration.

When evaluating a book for purchase, examine it for all three conditions under good lighting. Check the pages for foxing (hold pages at an angle to see spots more easily), the page edges for toning (compare edge colour to interior page colour), and the dust jacket spine for sunning (compare spine colour to front panel colour). These three checks take seconds and reveal the environmental history of the book more accurately than any seller’s description.