By Viviana Molinari

When treating a leather binding, one of the main challenges is preventing colour change or darkening, especially if adhesives are involved. A promising solution I’ve found is using cyclomethicone D5, suggested by my colleague Alessandro Sidoti, head of the conservation workshop at the Biblioteca Nazionale di Firenze, who learnt about it from Gabriele Coccolini (Opificio delle Pietre Dure di Firenze). 

I first tried out cyclomethicone D5 several years ago while working on eighteenth-century book bindings made with very delicate, light-colored leather that readily absorbs moisture. Contact with wheat starch paste would have led to noticeable darkening along the edges, but cyclomethicone D5 turned out to be a real game-changer.

What is cyclomethicone D5?

It’s a silicone-based product used in the cosmetics industry. In conservation, it’s useful mainly due to its ability to temporarily waterproof a material. Silicone-based solvents are volatile – they are thought to evaporate completely from surfaces, leaving no residues. They repel water, but their polarity is so dramatically low that they fall completely outside the Teas solubility triangle and cannot dissolve fats or oils. In fact, they are sometimes referred to as non-solvents. Silicone-based solvents are already used for safely cleaning acrylic paint films and for cleaning natural history specimens, such as feathers and bones, where the silicone serves to protect surfaces from absorbing chemicals during cleaning. The same properties make them ideal for use on leather, as they don’t alter its natural composition.

I have used cyclomethicone on many types of leather from various centuries. In my experience, cyclomethicone does not change the texture or colour of the leather. Just a few brushstrokes are sufficient, and the material becomes waterproof almost immediately. It is easy to use and does not need dilution or special preparation. The results are very promising, and I hope to see more research into the uses, properties and limitations of this product in the future.

Practical tips for use

(The photographs illustrating the following procedure were taken during a recent treatment on two volumes with a full leather binding belonging to the Biblioteca San Francesco della Vigna di Venezia. The two volumes, both from the fifteenth century, had very similar bindings: full goatskin, with wooden boards and blind decorations. They also had remnants of clasps.)

First, always test a small hidden area at least one day before the actual treatment to ensure compatibility (Figs. 1, 2). In my experience, the leather always returns to its original colour by the next day, but always test to be safe.

Figs. 1 & 2 – Testing cyclomethicone on the leather before treatment (left) and after the evaporation (right). Photography by Viviana Molinari; courtesy of the Biblioteca San Francesco della Vigna di Venezia.

Once confirmed safe, apply the product directly to the areas that need to be adhered or treated. You can use any type of brush.

If possible, apply on both the grain (Fig. 3) and the flesh side (Fig. 4). However, I have had treatments where I could not lift the leather too much; in these cases, it was enough to make several passes from the grain side. 

To achieve good waterproofing, the material should be soaked through with cyclomethicone, as shown in the images.

Figs. 3 & 4 – Application of the cyclomethicone on the grain side of the leather (left) and application on the flesh side (right). Photography by Viviana Molinari; courtesy of the Biblioteca San Francesco della Vigna di Venezia.

To take full advantage of the waterproofing effect, the area should be pasted out (Fig. 5) and adhered immediately after the application. Cyclomethicone evaporates at room temperature; the higher the temperature, the faster it evaporates. It seems to evaporate at the same rate regardless of the type of adhesive used; however, I have found it most useful with wheat starch paste. Without cyclomethicone, wheat starch paste causes the leather to darken more than the other adhesives because of its higher water content. I am not aware of any safety concerns related to fumes from the evaporating solvent (cyclomethicone is widely used in cosmetic products). However, this could be a good area for further research.

Keep in mind that if the volume requires bandaging (Fig. 6), for example to secure the adhesion of the spine, it is best to remove the bandages after a few hours instead of keeping them on overnight. This is to allow the solvent to evaporate properly. The evaporation of the solvent also slows down whenever the leather is covered (for example, with blotters). If the leather is kept covered overnight, it will still be permeated with the solvent and will look very dark when uncovered the next day. However, it will still return to its original colour after some additional evaporation time (about 24 hours). Over the years, I have used cyclomethicone on many types and colours of leather, and I have never yet seen a permanent colour change.

Figs. 5 & 6 – Spreading the wheat starch paste (left) and bandaging the spine (right). Photography by Viviana Molinari; courtesy of the Biblioteca San Francesco della Vigna di Venezia.

Concluding thoughts

Using cyclomethicone D5 is simple – almost trivial – but it yields excellent results (Figs. 7, 8). It helps maintain the leather’s integrity, keeping it clean, stable, and protected throughout the conservation process.

Based on my experience and that of other colleagues who use it, there is no visible change in the colour or texture of the leather after treatment. We did not notice any change even when revisiting the treated volumes several years later. Existing studies on conservation treatments of various materials¹ suggest that cyclomethicone evaporates completely and leaves no residues. However, more research in that area would be welcome.

Figs. 7 & 8 – The spine is still saturated with cyclomethicone (left) and the spine after the treatment (right). Photography by Viviana Molinari; courtesy of the Biblioteca San Francesco della Vigna di Venezia.

Footnotes

¹ Proprietà ed esempi di utilizzo di materiali siliconici nel restauro di manufatti artistici. Vol. 1, a cura di P. Cremonesi, Ed. Il Prato, Collana Arte, 2016.

Biography

A book and paper conservator, Viviana Molinari runs her own studio and has been working as a freelancer for Italian libraries, archives and private clients for 25 years. She has graduated from the Botticino Restoration School in Italy and has gained valuable internship experience in France at the National Library and National Archives in Paris, as well as in the United States at the Folger Shakespeare Library in Washington, D.C.