On the shelves of archives, libraries and in private collections, a silent, self-sustaining process of destruction of our written heritage is taking place. This phenomenon, referred to in specialist literature as „slow fires”, leads to a drastic reduction in the mechanical strength of the pages and their irreversible degradation. Polish diagnostic data indicate that the problem of critical acidification affects as much as 90% of library materials produced on machine-made paper in the 19th and 20th centuries. To halt this process, extensive chemical intervention is required.

Understanding what deacidification of paper is, is fundamental to modern conservation. This treatment allows for extending the lifespan of documents, books, and artworks by tens, or even hundreds, of years. Learn why cellulose degrades so quickly, what the mechanism of acid neutralisation entails, and what standards determine whether a given material can be considered fully safe for historical collections.

Why does paper become brittle? The chemistry of „acidic paper”

The crisis in contemporary archival science has its roots in the Industrial Revolution. Until the turn of the 18th and 19th centuries, paper was made by hand, using cotton and linen fibres obtained from rags. This was a naturally durable and alkaline medium. The situation changed radically with the invention of the paper machine and the shift to mass production based on wood pulp.

However, the real catalyst for the problem turned out to be a new method of sizing paper. Industry began to widely use acid solutions, mainly aluminium sulphate (alum) and rosin. These compounds introduced a highly destructive environment into the carrier's structure. The process of acid hydrolysis began. – reaction in which cellulose chains undergo drastic shortening. Worse still, this process is autocatalytic. This means that the degradation of the structure generates the formation of further acids as by-products, which logarithmically accelerates further decomposition. The effect? Books and documents from this period turn a strong yellow, and their pages become so brittle that they break with the slightest fold.

The process of deacidifying paper involves neutralising the acids present in the paper, which are a major cause of its deterioration over time. These acids, often originating from the manufacturing process or environmental exposure, break down the cellulose fibres, leading to brittleness, yellowing, and eventual disintegration of the paper. The deacidification process typically works by introducing an alkaline substance into the paper. This substance reacts with the existing acids, neutralising them. In addition, the alkaline substance often leaves behind an "alkaline reserve" (or buffering agent) within the paper. This reserve acts as a protective barrier, neutralising any new acids that may form over time or be absorbed from the environment, thus slowing down future deterioration. There are several methods for deacidifying paper, broadly categorised as: * **Mass Deacidification:** This is a method used for treating large quantities of documents, typically in archives, libraries, or paper mills. The documents are exposed to a deacidifying agent, often in a gaseous or liquid form, which permeates the paper. Common agents include alkaline compounds like magnesium oxide, calcium carbonate, or diethyl zinc. * **Localised or Aqueous Deacidification:** This involves treating individual documents or pages. It can be done by immersion in a deacidifying solution or by applying the solution locally with brushes or sprays. This method is generally more controlled but can be time-consuming and may pose risks to certain types of inks or materials if not done carefully. * **Dry Deacidification:** This method uses deacidifying agents in powder form, which are dispersed onto or within the paper. It can be less invasive than liquid methods. The choice of deacidification method depends on various factors, including the type of paper, its condition, the inks and other materials present, and the scale of the treatment required. The goal is to preserve the integrity and longevity of the paper without causing damage.

Deacidification is an advanced chemical treatment based on two key pillars. The first is the complete neutralisation of acidic substances already residing within the cellulose fibre structure. The second, equally important from a preventative perspective, is the deposition of excess alkaline substances into the paper.

This precisely measured reserve is called the alkaline reserve (basic buffer). This reserve acts as a protective shield. Its task is to absorb and neutralise acids that may form in the future as a result of natural ageing of the material, as well as those originating from environmental pollutants (e.g. absorption of sulphur dioxide from the air). Most commonly, magnesium or calcium compounds act as buffering agents.

Survival Standards: ISO 9706 and ISO 11108 Standards

The struggle to halt the wave of cellulose decay has led to the development of strict regulatory frameworks. These have redefined the concept of durability for conservation and archival materials. The most important international indicator in this regard is the ISO 9706 standard, which defines the parameters for so-called permanent paper.

For an archive medium to be considered compliant with the ISO 9706 standard, it must meet stringent conditions:

• Appropriate pH: The pH of the aqueous extract must be between 7.5 and 10.0.
• Alkaline reserve A buffer equivalent to at least 0.4 moles of neutralised acid per kilogram is required, which in practice means adding at least 2% of calcium carbonate.
• Fibre purity (Kappa number): This indicator must be below 5, which guarantees the absence of easily oxidisable materials (e.g. residual lignin responsible for darkening).
• Tear strength Minimum 350 mN in both directions of web forming.

For the most valuable systemic and historical artefacts, an even more restrictive ISO 11108 standard for archival paper is applied. It requires the paper to be produced from fine, long fibres of cotton, linen or hemp. When choosing acid-free paper For repair purposes, including protective cardboard and dividers, strict attention must be paid to compliance with the above standards.

Mass deacidification and work in a conservation studio

The problem of fragile paper has forced large institutions to implement mass technologies. Deacidification of entire book collections on an industrial scale is now carried out in giant vacuum chambers, using non-aqueous liquid-phase technologies. The elimination of water from the process protects the paper structure from warping and inks from blurring.

In Poland, as part of a major government programme implemented by, among others, the National Library and the Jagiellonian Library, Bookkeeper technology has been employed. It is based on introducing micrometric particles of magnesium oxide (MgO) suspended in an inert perfluoroalkane solvent into the paper structure. In turn, other European technologies (e.g. Papersave) are based on fully homogeneous titanium-magnesium ethoxylate solutions, which penetrate the fibres at a molecular level, yielding even higher mechanical stability coefficients.

However, mass installations have their limitations and are not suitable for every type of collection. In conservation workshops, where work is carried out on single, often unique objects or sensitive works of art, an individualised approach is used. This requires the use of precise paper deacidification preparations, most often in the form of manual aerosols or liquids applied with a brush. In such cases, the paper conservator must carefully assess the risk – as studies show that introducing alkaline magnesium into sensitive pigments (e.g. in watercolours or pastels) can cause undesirable colorimetric shifts (changes in colour).

Busting myths: What deacidification CANNOT do

In the world of collectors and restoration enthusiasts, a harmful myth often persists: the belief that applying an deacidifying agent will restore a brittle card to its former flexibility and original strength.

From a physicochemical perspective, the deacidification process serves a purely stabilising, not reconstructive, role. The resulting alkaline reserve acts as a „chemical brake”.” – prevents further cracking of cellulose polymer bonds. However, technology cannot mend chains that have already broken. A document that crumbled in one's fingers before treatment will remain extremely brittle after deacidification. The only change will be that the rate of its further deterioration will be drastically reduced.

Therefore, for physically weakened paper stocks, simply neutralising acids is not enough. After the chemical process, physical consolidation and strengthening of the weakened sheets are necessary. Professional services are used for this. Tissue and Japanese papers (e.g., washi, kozo), which thanks to their long fibres create an invisible mesh reinforcing the structure of torn documents.

Materials list: How to protect acidified crops in practice?

If your studio, company archive, or private collection contains items showing signs of acidification (characteristic yellowing of edges, brittleness), you must take appropriate preventative and remedial steps. To carry out basic protective treatments, you will need professional materials that comply with conservation standards:

1. Acidifying solutions and sprays: Specialised non-aqueous preparations (e.g. based on magnesium oxide) that allow for safe topical or whole-card application without the risk of smudging waterproof inks.
2. Materials for physically strengthening tears: Brittle paper is inherently associated with mechanical damage. For mending tears, ultrathin Japanese tissue paper and suitable Specialist adhesives (e.g. methylcellulose, pure wheat starch). For quicker repairs, acid-free Filmoplast tapes are ideal.
3. Protective barriers Acidic paper tends to „contaminate” (migrate acids) adjacent, healthy materials. Therefore, it is essential to use interleaving made from certified papers with an alkaline reserve.
4. Secure storage environment Even an object undergoing deacidification requires isolation from variable environmental conditions and pollutants, as well as UV radiation. Suitable acid-free boxes from reputable manufacturers (such as KLUG) and archive folders will create the necessary microclimate on the shelf.