Biodeterioration: the invisible enemy of paper-based heritage
Paper-based cultural heritage — antique books, manuscripts, historical archives, parchments, photographs — are organic materials vulnerable to a silent, progressive form of decay: biodeterioration. Unlike damage caused by traumatic events such as fires or floods, biodeterioration acts slowly, often for years, before its effects become visible.
Biological deterioration of paper is caused by microorganisms — fungi, bacteria and wood-boring insects — that feed on the cellulose, starches, animal glues and organic substances present in paper and bindings. The result is the irreversible loss of consistency, legibility and structural integrity of the document.
Microbiological sanitization and preventive conservation are not optional: they are the necessary condition for guaranteeing the long-term survival of documentary heritage.
Recommended climate parameters
| Material type | Parameter | Recommended value | Note |
|---|---|---|---|
| Paper archives | Temperature | 13–18°C | ISO 11799:2003. Every 10°C increase doubles the rate of chemical reactions that degrade cellulose. |
| Paper archives | Relative humidity | 45–55% | Values above 65% favor fungal spore germination. Fluctuation must be ≤ 5% daily. |
| Parchment and leather | Relative humidity | 45–50% | Animal-based materials, particularly sensitive to relative humidity fluctuations. |
| Photographs and negatives | Temperature | 5–13°C | ISO 18911:2010. Acetate negatives require 0–5°C to slow vinegar syndrome. |
Mold, fungi and insects: the biological agents of deterioration
The mold in antique books is one of the most widespread problems in Italian archives. The most frequent species found in Italian archives are Aspergillus, Cladosporium and Penicillium: they produce organic acids that attack cellulose, chromatic stains (foxing) that make documents illegible, and mycotoxins hazardous to staff who work there.
Wood-boring insects — particularly silverfish (Lepisma saccharina), dermestid beetles and booklice — feed on cellulose, starches and glues. Their damage is mechanical in nature: holes, tunnels, and loss of text surface. Their presence almost always indicates uncontrolled environmental conditions.
Bacteria, less visible but equally destructive, produce cellulase enzymes that depolymerize paper fibers, reducing their mechanical strength. Archives with a history of flooding or water infiltration are particularly at risk.
Mold and fungi
Aspergillus, Cladosporium, Penicillium: they produce organic acids that attack cellulose, foxing stains and mycotoxins hazardous to staff. They require biocidal treatment and microbiological sanitization.
Wood-boring insects
Silverfish, dermestid beetles and booklice feed on cellulose and starches. They cause irreversible mechanical damage. Their presence signals uncontrolled environmental conditions. Treatment with cold (-20°C) or modified atmosphere.
Bacteria and dust
Bacteria produce cellulase enzymes that depolymerize paper fibers. Dust is hygroscopic and carries spores: every layer of settled dust increases biological risk. HEPA H14 filtration is mandatory.
Risk factors: humidity and temperature
Relative humidity is the most critical parameter for the conservation of paper-based cultural heritage. Values above 65% constitute the critical threshold for fungal spore germination: within a few hours of favorable conditions, a latent contamination can become an active infestation.
The challenge is not reaching an absolute value, but maintaining a minimal fluctuation over time. Rapid changes in temperature and humidity cause mechanical expansion and contraction that damage the layered structures of documents: bindings, pigments, ink layers and paper substrate react differently to the same environmental changes, creating internal stresses that accumulate over time.
Recommended relative humidity by document type
Source: ISO 11799 / ICRCPAL
| Type | Recommended RH |
|---|---|
| Documentary archives (paper) | 45–55% RH |
| Antique manuscripts and books | 45–55% RH |
| Parchment and leather | 45–50% RH |
| Photographs (b/w and color) | 30–40% RH |
| Acetate negatives | 25–35% RH |
HEPA H14 filters and air quality in archives
Dust is the primary vector for fungal spores and bacteria in archival environments. Every grain of settled particulate increases biological risk: it is hygroscopic (retains local moisture), carries viable spores over long distances, and creates a microenvironment favorable to microbial proliferation.
The solution is HEPA H14 filtration: high-efficiency filters capable of capturing 99.995% of particles ≥ 0.3 µm in diameter — including fungal spores (3–5 µm) and bacteria (1–3 µm). Class H14 is the only class acceptable for long-term conservation storage: class H13, though 99.95% effective, lets through a share of biologically active particulate incompatible with preventive conservation standards.
An HVAC system with HEPA H14 filtration, combined with a regular program of professional dust removal from shelving and documents, reduces airborne fungal spore concentration in archives by 95% and blocks the primary vector of biological biodeterioration.
Beyond biological particles, air quality in archives requires controlling gaseous pollutants: SO₂, NO₂ and ozone accelerate the acid hydrolysis of cellulose. In archives located in urban or industrial contexts, chemical filtration with activated carbon is essential in addition to HEPA mechanical filtration.
Preventive conservation: prevention is better than restoration
Restoring a document damaged by biodeterioration is costly, lengthy, and rarely fully restores the original value of the item. Preventive conservation — that is, the set of environmental, operational and monitoring measures that prevent deterioration before it manifests — is economically and culturally superior to restorative intervention.
An effective preventive conservation program for archives and libraries includes: continuous climate control (temperature, relative humidity, air quality), periodic professional dust removal with HEPA H14 vacuum units, microbiological monitoring through air and surface sampling, and an integrated emergency management plan for events such as water infiltration or active biological infestations.
The frequency of dust-removal interventions depends on the environment and intensity of use: generally, closed storage facilities require an annual intervention, while frequently accessed reading rooms require intervention every six months or quarterly.
Porcacchia Egidio’s scientific approach
Every Porcacchia Egidio intervention on paper-based cultural heritage begins with a technical assessment site visit: an analysis of the collection’s conservation condition, the local microenvironment, and the biological nature of any contamination present. Only after this assessment is the specific operational protocol defined.
Our technicians use certified HEPA H14 vacuum units with adjustable flow potentiometers, to adapt suction pressure to the fragility of the treated material — a critical parameter for antique documents, where excessive suction can cause detachment and micro-tears. All activities are documented with a final technical report, essential for managing public-sector grants and for the traceability of interventions required by tender specifications.