Acids slowly break down the cellulose fibres in paper.© GettyImages

Tiny particles of a strong alkali are helping preserve historical documents.

Piero Baglioni of the University of Florence and his coworkers have treated manuscripts dating back to the fourteenth century with a sprinkling of calcium hydroxide grains just 200 millionths of a millimetre across.

The nanoparticles of what is commonly called slaked lime penetrate between paper's fibres. They combat the ravages of acids introduced when paper is made, without altering documents' apperance¹. The technique is cheap and green and could also be used on canvas.

The researchers subjected one nineteenth-century sample to accelerated ageing by keeping it in a hot, moist environment for three weeks. Untreated, the paper turned a deep brown because of acid damage. A treated sample showed very little discolouration.

Neutral territory

Before the nineteenth century, paper was made primarily from shredded rags; thereafter wood pulp has been used. Acids slowly break down the cellulose fibres in paper, making it yellow, brittle and weak.

Currently museum conservators and librarians use chemical treatments to prevent or retard this degradation. A common method, the Book-keeper process, involves scattering grains of magnesium oxide over the paper. These stick to the fibres and react with water to form an alkali that neutralizes acids. But one problem is that the grains are rather large (around a micrometre- a thousandth of a millimetre), which can make the acid-neutralization process slow and risks altering the appearance of the manuscript.

Baglioni's team penetrate paper more thoroughly by spraying it with nanoparticles dispersed in propanol, a volatile alcohol. Paper could even be brushed with or immersed in this liquid, they suggest.

Water would simply dissolve the calcium hydroxide to form a strong alkali solution that might itself harm the document. Water could also dissolve and smudge the inks and paints used in some old manuscripts.

Once stuck to the cellulose fibres, the nanoparticles react slowly with carbon dioxide in the air to form calcium carbonate - in effect, a very fine powdered chalk. This creates a stable and durable reservoir of alkali to neutralize acids.

Paper samples from the fourteenth to the twentieth centuries, some of which were initially mildly acid, became near-neutral or mildly alkaline after nanoparticle treatment. They will probably stay this way, the researchers suspect. 

• References

  1. Giorgi, R., Dei, L., Ceccato, M. , Schettino, C. & Baglioni, P. Nanotechnologies for conservation of cultural heritage: paper and canvas deacidification. Langmuir, 18, 8198 - 8203 , (2002). | Article | ISI | ChemPort |