{"id":28461,"date":"2026-09-28T20:14:21","date_gmt":"2026-09-28T17:14:21","guid":{"rendered":"https:\/\/artrestorationsupplies.gr\/?p=28461"},"modified":"2026-09-28T20:14:26","modified_gmt":"2026-09-28T17:14:26","slug":"arxeiaki-fylaxi","status":"publish","type":"post","link":"https:\/\/artrestorationsupplies.gr\/en\/arxeiaki-fylaxi\/","title":{"rendered":"Archival storage of documents and photographs: acid-free, lignin-free, buffered, PAT \u2013 what they mean and what to choose"},"content":{"rendered":"

Correct storage is the most effective \u2013 and the most economical \u2013 form of preventive conservation for archives, libraries and photographic collections. A document in an unsuitable folder or a photograph in a PVC plastic sleeve can suffer damage within a few years that no treatment can fully reverse. Yet many terms are used on the market \u2013 acid-free, lignin-free, buffered, unbuffered, PAT, ISO 9706<\/strong> \u2013 which are often used loosely. In this guide we explain what each one means, which standards apply and how we choose materials for documents, books and photographs.<\/p>\n

Why paper deteriorates<\/h2>\n

Paper consists mainly of cellulose. Its main chemical deterioration is acid hydrolysis<\/strong>: acids break the cellulose chains, the paper yellows, becomes brittle and eventually crumbles. Acids come from the paper itself (e.g. alum-rosin sizing in 19th- and 20th-century paper), from the lignin in wood pulp, from atmospheric pollution and from storage materials in contact with the document. The oxidation<\/strong> and photochemical deterioration accelerate the process, while high temperature and relative humidity multiply the rate of the reactions.<\/p>\n

Storage materials therefore have two aims: to do not add<\/strong> harmful substances to the object and, where necessary, to protect it<\/strong> from external factors (dust, light, pollutants, mechanical stress, sudden changes in humidity).<\/p>\n

The terms, one by one<\/h2>\n

Acid-free<\/h3>\n

Paper or board with a neutral or slightly alkaline pH (usually pH 7 or higher) at the time of manufacture. Note: the term describes the condition at the time of manufacture<\/em>. An acid-free paper containing lignin can become acidic over time. That is why the term \"acid-free\" on its own is not enough for archival use.<\/p>\n

Lignin-free<\/h3>\n

Lignin is a natural polymer of wood that oxidises and produces acids and chromophores \u2013 it is the reason newsprint turns yellow. Archival papers are made from pure chemical cellulose pulp (alpha-cellulose) or cotton (rag), with a minimal lignin content. ISO 9706 controls lignin indirectly via the kappa number (less than 5).<\/p>\n

Buffered (with alkaline reserve)<\/h3>\n

Paper to which has been added alkaline reserve<\/strong>, usually calcium carbonate (CaCO\u2083) or magnesium carbonate. The reserve neutralises acids that will form in future or migrate from the environment or from neighbouring acidic documents. According to ISO 9706, the alkaline reserve must correspond to at least 0.4 mol\/kg (about 2% calcium carbonate), with an extract pH between 7.5 and 10. Buffered materials are the general recommendation for most paper documents, especially acidic 19th\u201320th-century papers.<\/p>\n

Unbuffered<\/h3>\n

Acid-free and lignin-free paper with neutral pH, but without alkaline reserve<\/strong>. It is recommended for materials that may be affected by alkalinity: certain historical photographic processes (e.g. cyanotypes, albumen prints), certain colour photographs with dyes, and protein materials such as wool, silk, leather and parchment. In mixed photographic collections, many institutions prefer unbuffered materials that have passed the PAT as the safer general choice.<\/p>\n

PAT \u2013 Photographic Activity Test<\/h3>\n

PAT is an international test standard (ISO 18916<\/strong>) developed by the Image Permanence Institute. It checks whether a storage material (paper, board, plastic, adhesive, ink, label) causes chemical reactions with photographic materials \u2013 specifically, whether it causes image fading or gelatine staining. A material that \"passes the PAT\" is suitable for direct contact with photographs. The PAT not<\/em> is it an indicator of mechanical strength, nor does it replace the requirements for pH or lignin; it is a complementary criterion. The relevant standard for enclosures for photographic materials is ISO 18902.<\/p>\n

Archival paper to ISO 9706 \/ ISO 11108<\/h3>\n

The ISO 9706<\/strong> sets out the requirements for \"permanent paper\": alkaline reserve, pH, low lignin and minimum tear resistance. The ISO 11108<\/strong> (archival paper) adds requirements for high folding endurance, suitable for documents in very frequent use. For boxes and folders for the archival storage of paper documents there is ISO 16245<\/strong>, while the requirements for storage areas are covered by ISO 11799<\/strong> and the EN 16893<\/strong>.<\/p>\n

Plastics for archival use<\/h2>\n

Transparent enclosures allow examination without handling, but only certain plastics are suitable:<\/p>\n