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This paper presents a method and protocol for the study of varnishes and be protective layers applied throughout history to graphic works. The study is based on the characteristics and behaviour of varnishes when facing different types of deterioration. The methodology is founded on a series of physical–chemical tests that identify the characteristics of a varnish when applied to paper (surface and total drying time, degree of adherence, and penetration into the substrate) and in treatments, determining its resistance to adverse conditions of humidity, temperature, and light (resistance to humidity, immersion in water, and ageing by dry heat or in a climatic chamber). The effects of these tests are evaluated by taking into account a series of physical–chemical parameters characteristic of varnishes on graphic works: weight, dimensions, thickness, texture, optical features, and pH. Variations among these parameters serve to measure the resistance of varnishes to adverse conditions and natural ageing, whether they are applied as a conservation treatment or historically. All the information necessary to facilitate the transfer of this method to researchers and professionals is compiled in a protocol. This document comprises general information, descriptions of procedures and materials, and a series of annexes that serve simultaneously to collect and to study the data. The protocol is therefore designed to serve both as a tool when applying the method and as the germ of a database for the study of varnishes on graphic works. Shellac varnish was selected to test the method and the protocol. This resin has been used in the past to provide a waterproof varnish for paper-based maps as well as for images. Errors identified during the development of the protocol were ultimately corrected by incorporating new tests, items, and annexes. The protocol has proven to be a practical tool for the study of varnishes regarding their behaviour in the case of graphic works. [ABSTRACT FROM AUTHOR]