What Causes the Smell of New & Old Books?
Everyone’s familiar with the smell of old books, the weirdly intoxicating scent that haunts libraries and second-hand book stores. Similarly, who doesn’t enjoy riffling through the pages of a newly purchased book and breathing in the crisp aroma of new paper and freshly printed ink? As with all aromas, the origins can be traced back to a number of chemical constituents, so we can examine the processes and compounds that can contribute to both.
As far as the smell of new books goes, it’s actually quite difficult to pinpoint specific compounds, for a number of reasons. Firstly, there seems to be a scarcity of scientific research that’s been carried out on the subject – to be fair, it’s understandable why it might not exactly be high up on the priority list. Secondly, the variation in the chemicals used to manufacture books also means that it’s an aroma that will vary from book to book. Add to this the fact that there are literally hundreds of compounds involved, and it becomes clearer why it evades attribution to a small selection of chemicals.
It’s likely that the bulk of ‘new book smell’ can be put down to three main sources: the paper itself (and the chemicals used in its manufacture), the inks used to print the book, and the adhesives used in the book-binding process.
The manufacture of paper requires the use of chemicals at several stages. Large amounts of paper are made from wood pulp (though it can also be made from cotton and textiles) – chemicals such as sodium hydroxide, often referred to in this context as ‘caustic soda’, can be added to increase pH and cause fibres in the pulp to swell. The fibres are then bleached with a number of other chemicals, including hydrogen peroxide; then, they are mixed with large amounts of water. This water will contain additives to modify the properties of the the paper – for example, AKD (alkyl ketene dimer) is commonly used as a ‘sizing agent’ to improve the water-resistance of the paper.
Many other chemicals are also used – this is just a very rough overview. The upshot of this is that some of these chemicals can contribute, through their reactions or otherwise, to the release of volatile organic compounds (VOCs) into the air, the odours of which we can detect. The same is true of chemicals used in the inks, and the adhesives used in the books. A number of different adhesives are used for book-binding, many of which are based on organic ‘co-polymers’ – large numbers of smaller molecules chemically chained together.
As stated, differences in paper, adhesives, and inks used will influence the ‘new book smell’, so not all new books will smell the same – perhaps the reason why no research has yet attempted to definitively define the aroma.
An aroma that has had much more research carried out around it, however, is that of old books. There’s a reason for this, as it’s been investigated as a potential method for assessing the condition of old books, by monitoring the concentrations of different organic compounds that they give off. As a result, we can be a little more certain on some of the many compounds that contribute to the smell.
Generally, it is the chemical breakdown of compounds within paper that leads to the production of ‘old book smell’. Paper contains, amongst other chemicals, cellulose, and smaller amounts of lignin – much less in more modern books than in books from more than one hundred years ago. Both of these originate from the trees the paper is made from; finer papers will contain much less lignin than, for example, newsprint. In trees, lignin helps bind cellulose fibres together, keeping the wood stiff; it’s also responsible for old paper’s yellowing with age, as oxidation reactions cause it to break down into acids, which then help break down cellulose.
‘Old book smell’ is derived from this chemical degradation. Modern, high quality papers will undergo chemical processing to remove lignin, but breakdown of cellulose in the paper can still occur (albeit at a much slower rate) due to the presence of acids in the surroundings. These reactions, referred to generally as ‘acid hydrolysis’, produce a wide range of volatile organic compounds, many of which are likely to contribute to the smell of old books. A selected number of compounds have had their contributions pinpointed: benzaldehyde adds an almond-like scent; vanillin adds a vanilla-like scent; ethyl benzene and toluene impart sweet odours; and 2-ethyl hexanol has a ‘slightly floral’ contribution. Other aldehydes and alcohols produced by these reactions have low odour thresholds and also contribute.
Other compounds given off have been marked as useful for determining…