The Best Conservation Plan – Less is More
Thomas Davies, Plymouth Dock, 1796. Watercolour over graphite on cream wove paper, 34.8 x 51.9 cm. Gift of Dr. Alan Wilkinson, England, 2019, in memory of Dr. R.H. Hubbard, Chief Curator of the National Gallery of Canada (1954–78). National Gallery of Canada, Ottawa. Photo: NGC
As part of the preparation for any exhibition at the National Gallery of Canada, including the recently presented Gathered Leaves: Discoveries from the Drawings Vault which is currently on view at the Audain Art Museum, the Gallery's conservators assess the condition of each work of art and plan a conservation treatment if deemed necessary by conservators and curators. One of the works identified as a candidate for treatment prior to going on view in the Gathered Leaves exhibition, was Plymouth Dock (1796) by the military painter Thomas Davies.
The paper of this 18th-century watercolour has yellowed with age. This yellowing has been caused primarily by a chemical process called photo-oxidation, that has occurred due to cumulative exposure to ultraviolet radiation prior to being acquired by the Gallery. Ultraviolet radiation often comes from exposure to daylight, but also from some artificial lights such as halogen and fluorescent lamps. A common issue for works on paper, photo-oxidation not only causes aesthetic changes, but also chemical and physical changes that result in a weakening of the paper. Paper conservators have a range of tools and techniques at their disposal to treat such cases, with the aim of improving not only the aesthetics of the work by reducing its distracting yellowed appearance, but also improving the chemical and physical properties of the paper support in order to help its long-term preservation.
Analysis of the watercolour View of the Great Falls on the Ottawa River, Lower Canada (1791) by Thomas Davies in the Restauration and Conservation Lab at the National Gallery of Canada, 2025. Photo: NGC
The first step before implementing any treatment is always a thorough examination and testing of the work of art, to ensure that the treatment plan is appropriate for that particular work. In the case of Plymouth Dock, closer visual examination led the Prints and Drawings conservation team to notice localized discolouration or browning in certain areas, which was especially visible on the reverse of the work. This hinted at an issue caused by specific pigments. Watercolour paint is made from finely ground pigments of a chosen colour, combined with a binder (usually gum arabic) and additives to modify its properties. For example, honey is often used as a humectant (it helps the paint retain moisture) and natural preservative, while glycerin is used to make the paint more flexible and less prone to cracking, while also serving as a humectant.
Certain historically used pigments are known to develop condition issues with age. A common example is lead white, which will darken with cumulated contact with pollutants. For Davies' watercolour, we decided to use ultraviolet imaging – a tool often used by conservators to help with the identification of specific materials. And indeed, under ultraviolet light, it was clear that the affected areas had a distinctive dark appearance, which suggested the presence of copper-based pigments. Our suspicions were subsequently confirmed through copper-detection test strips.
Ultraviolet image of Plymouth Dock (left) and verso of the watercolour under normal light (right), showing areas affected by copper-based pigments. Photo: NGC / RCL
Historically, copper-based pigments were widely used to achieve green and blue colours, verdigris and azurite being two common examples. These pigments degrade over time, especially with exposure to light, heat and humidity, causing the green or blue colours to gradually shift to brown. This, in turn, leads to a degradation of the paper on which the work is painted, which also becomes brown and more brittle. Since this degradation process is exacerbated by humidity, yet water must be used to reduce discolouration, it created a problem for a treatment plan. Conservation research has shown that aqueous treatments will have a negative effect on copper-based pigments in the long term, even if initial results appear positive. Indeed, the discolouration or browning of the paper might be reduced immediately following the treatment; however, contact with water will exacerbate the pigment degradation, which will lead to further loss of their original colour, further browning, and a greater weakening of the paper over time.
Localized treatment is sometimes used to avoid problematic media. Specialized tools can allow conservators to limit the application of an aqueous treatment to specific areas of a work of art, while avoiding other areas that should not be put in contact with moisture. However, in Plymouth Dock, the localization of the copper-based pigments, notably along the skyline, rendered the treatment too risky, since the sky was the area which would have benefitted the most from treatment. For this reason, the best decision in this case was to avoid treating the work of art at this time, and re-consider treatment in the future, when research has advanced further, and new tools are at our disposal. In the meantime, proper storage and exhibition in controlled museum environments will ensure its long-term preservation.
Art conservation is an ever-evolving field. In accordance with their professional code of ethics, conservators have a responsibility to stay abreast of developments in the field. They make decisions every day to ensure the long-term preservation of cultural property, based on current scientific knowledge. And sometimes, the best plan is not to treat … for now.
Gathered Leaves: Discoveries from the Drawings Vault is on view at the Audain Art Museum until October 13, 2025. For the catalogue, please visit the Boutique web page. Share this article and subscribe to our newsletters to stay up-to-date on the latest articles, Gallery exhibitions, news and events, and to learn more about art in Canada.
