The Chemistry Behind the Smell of Old Books

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Ever gone to a library and encountered a distinctive smell from old books which tends to linger. The smell of books that lingers in second-hand book stores is peculiar, but it is hard to know exactly what creates the nostalgic aroma. Somehow this aroma is desirable and encourages reading. Every reader is acquainted with the experience of being drawn to a book by its smell. A reader has this uncontrollable urge to read the book just because it smells old, just because it smells nice.

Now one British chemistry teacher has created an info graphic to explain how and why the scents of antique and new books differ. Old books have a sweet smell with notes of vanilla flowers and almonds, caused by the breakdown of chemical compounds in the paper, while new books smell like they do because of chemicals used in their manufacture.

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“E-book sales may be on the rise, but some readers remain faithful to their physical novels. Is it the act of turning a page or the prospect of stumbling upon new stories in bookstore aisles that keeps hardcover readers coming back for more? Most will tell you that e-readers lack the multi-sensory experience of physical books — the sight, the touch, and yes, the smell.”- Huffing Post

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Reading a favourite novel is equivalent to smelling it. The experience is similar to eating food at a distinctive place. It creates new memories and one remembers said instances by the smell of the pages. The ink and chemicals used in the production of a book reacts with heat, moisture and light, causing the organic materials to break down. This is especially true for books with high acidity, like those made during the 19th and 20th centuries. The lead scientist described the smell as ‘A combination of grassy notes with a tang of acids and a hint of vanilla over an underlying mustiness.'”

Lignin is what makes paper yellow with age because oxidation reactions cause it to break down into acids, which then help break down cellulose.

The smell of old books comes from chemical degradation and a number of compounds have been pinpointed. Benzaldehyde adds an almond-like scent, vanillin smells of vanilla and ethyl hexanol has a ‘slightly floral’ scent. Ethyl benzene and toluene, which are also produced, give off sweet odours.
And thus the books smell great!

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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.

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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 the extent of degradation of old books. Furfural is one of these compounds, shown below. It can also be used to determine the age and composition of books, with books published after the mid-1800s emitting more furfural, and its emission generally increasing with publication year relative to older books composed of cotton or linen paper.

Furfural

So, in conclusion, as with many aromas, we can’t point to one specific compound, or family of compounds, and categorically state that it’s the cause of the scent. However, we can identify potential contributors, and, particular in the case of old book smell, a number of compounds have been suggested. If anyone’s able to provide further information on ‘new book smell’ and its origins, it would be great to include some more specific details, but I suspect the large variations in the book-making process make this a tough ask.

Source: Compoundchem.com


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