Piecing together the chemistry behind conservation

Posted on 11/04/2010

Free Radical Chemistry Centre researchers working with the Centre for Cultural Materials Conservation in Melbourne are providing insight into new preservation techniques for culturally significant materials such as artworks and historical objects.

“If you asked an art conservator about the role of free radicals in art conservation, they would probably mention the free radical-based reaction involved in the polymerisation of materials such as the oils and acrylics used in paint formulations; the promotion of free radical formation in association with  titanium dioxide which is commonly used as an additive to paint formulations and the need to understand the mechanisms of radical based deterioration of materials and their effect on the longevity of cultural materials,” says Caroline Kyi, PhD student with the Free Radical Chemistry Centre and art conservator.

Free radicals present in the original materials from which objects and artworks are made can potentially, due to their unstable nature, react with free radicals surrounding us in our natural environment. Free radical associated damage can occur in many forms such as the deterioration of paint and varnish layers leading to colour fade, cracking, powdering and loss.

Outdoor monuments present unique and complex conservation problems, often as a direct result of the unregulated environment in which they exist. Not only are they susceptible to damage from free radicals in UV light and pollution, degradation in the form of biodeterioration associated with bacterial and fungal growth can occur in conditions of high humidity. Initiation of bacterial growth can lead to the formation of biofilms. Biofilms are a co-ordinated aggregate of microorganisms in which cells act collectively to benefit the community. They are more destructive to artworks than individual bacteria because the wastes released and also their resistance to chemical and physical stresses are greatly increased.

“In my PhD research I am looking at ways to control biofilm growth on culturally significant materials where there are limitations to the treatment approaches available for use. Because biofilms are often more resistant to chemicals treatments such as the application of biocides they are difficult to remove without causing additional damage to the artwork or without a repeat treatment. As an alternative I am investigating ways to manipulate the environment within the biofilm to encourage dispersal rather than infection facilitating aggregation of bacteria. We are hoping that it will offer a minimally invasive option in the treatment of biodeterioration.” says Caroline.

Thinking carefully about the feasibility of any new treatment requires consideration of the minimal intervention philosophy that dictates many of the final treatment approaches adopted in art conservation. Caroline has decided to undertake extensive research into the use of nitric oxide (NO) to encourage biofilm dispersal.  NO has been known for some time to cause biofilm dispersal either directly or through the formation of reactive nitrogen species that are toxic to cells. Build-up of these cytotoxins ensure that conditions become unfavourable for continued biofilm growth.

“The use of NO is not without risk. It is a free radical which has the potential itself to cause damage if applied inappropriately. Therefore, my research will examine application best practice and the long term effects of NO on treated art. Most importantly it will investigate the effectiveness of NO in achieving biofilm dispersal on a variety of materials and provide an understanding of how it does so."

Caroline is conducting her research using sample pieces of stone, paper and canvas art, and using a model organism called Pseudomonas aeruginosa, a common species of bacteria found in soil and on many artworks. Much is known about the organism’s growth patterns and nutrient requirements, and it readily forms biofilms, of a blue-green colour. Advantageously it can also undergo anaerobic respiration in which nitrate is reduced to the free radical of interest nitric oxide.

“Previous researchers have investigated the ability of nitric oxide donor molecules to induce biofilm dispersal. These can be costly and may have the potential to compromise an artwork. Therefore using Pseudomonas aeruginosa as a model I am inducing bacteria in biofilms to produce their own NO and therefore initiate dispersal. The key to this research is finding ways to regulate bacterial NO production allowing conservators to have full control over biofilm growth whilst limiting the treatment with incompatible or harmful materials.”

“The treatment approach that develops from this is likely to be biphasic with the controlled biofilm dispersal in the first phase and the eradication of the dispersed bacteria in the second phase. Removing bacteria as single cells should be easier and require lower concentrations of chemicals,” adds Caroline.

Caroline has already shown that by cultivating the bacteria present on samples of cultural materials in an anaerobic environment and giving them a source of nitrogen in the form of potassium nitrate, they will produce NO.

Working with Associate Professor James Ziogas from the Department of Pharmacology at the University of Melbourne, she is now using a fluorescent probe to visualise the cells producing NO under a confocal microscope. Using this technique, Caroline is collecting information about the behaviour of cells producing NO and the timeframe NO takes to cause mobility and finally dispersal. This research will provide a detailed picture of NO mediated dispersal and allow Caroline to discover whether this is an effective preventative for biofilm growth in the context of art conservation.

“In the long-term I hope to make suggestions about how best to initiate NO influenced dispersal whether potassium nitrate or another precursor of NO should be incorporated into treatments, how it should be applied and the risks associated with its application. The most important consideration is that the treatment is effective in achieving dispersal whilst ensuring the ongoing preservation of culturally significant materials.”