Thoughts on Thymol

Great question. The quick answer is don't add Thymol to the 99% Isopropyl Alcohol to help prevent mold. In the recent past the use of Thymol was recommended by Book Conservators (as recently as 1986) to retard mold in books.  However, only two years later an article was published in the American Institute for Conservation's Book and Paper Group Annual Volume 7 by Leslie M. Kruth that was clearly against its use. She flat out says that using a thymol chamber it is not effective and just gives the mold more time to grow and spread. 

To be clear: Thymol is a toxic substance and can cause gastric pain, nausea, vomiting, central hyperactivity, convulsions, coma, cardiac and respiratory collapse. It is not worth the risk to your own health and it is ineffective to get rid of mold. 

I strongly recommend against using any anti-fungal/mold solutions such as thymol, clove oil, wintergreen, tea-tree or any kind of bleach applied directly to any part of a book. These are all substances that can cause damage to book materials such as paper, leather and book-cloth.

You could leave your book in an indoor sunbeam and have a better mold reduction effect. It is time consuming though to turn each page every few hours and of course sunlight can cause its own damage through heat and fading. 

When I first started learning book restoration with my teacher, he used thymol to keep the paste from going bad. He just added one or two drops to about two cups of paste. An alternative was clove oil which smelled way better but I always worried about the oil transferring to the pages. Now that I use Nori Paste instead of making it myself I don't have to worry about moldy paste, or oily or toxic mold inhibitors because the Nori Paste doesn't go bad like home-made paste did.

Unfortunately mold is insidious and once established (no matter what measures are taken) it will be present forever. Keep it dormant through good environmental factors like low humidity, good air circulation and stable temperatures. If it is possible to replace the book do it! Mold can spread and it can infect the other books in your library.