This question comes at rather interesting moment for a number of reasons….it is also a question that requires a complicated answer so I will attempt to do my best. I very much hope we will hear from other conservators on this subject as well as multiple opinions can surely provide you with a comprehensive understanding on the subject of cleaning pictures. First I would like to direct you to a number of wonderful resources that have been written about this subject; some are more technical while others are much more broad in scope and cover the history of field, a topic that is integrally tied to cleaning practices and how they have evolved. The Getty has two publications that I highly recommend: “Historical and Philosophical Issues in the Conservation of Cultural Heritage” and “Issues in the Conservation of Paintings.” For some background on the history of the field in the US there is also “A Laboratory for Art: Harvard's Fogg Museum and the Emergence of Conservation in America, 1900-1950” by Francesca Bewer and “Changing Approaches in Art Conservation: 1925 to the Present” by Joyce Hill Stoner (in the book “Scientific Examination of Art: Modern Techniques in Conservation and Analysis. Arthur M. Sackler Colloquia, March 19–21, 2003, The National Academies Press, Washington, DC). I also wrote a short article that focuses on how training programs evolved in the US that can be accessed for free on my academia page but that is perhaps less relevant to the question.
It is important to note that the conservation field has evolved out of a craft-based profession and in some ways still is evolving. Some of the earliest accounts we have of conservation date very, very far back and typically involve artists being recruited for certain things, whether it be cleaning or “touching up” an artwork. It was not really until our field began to interact more closely with the scientific field (around the turn of the century) that things began to change. The book on the Fogg Museum covers this beautifully although institutions in Europe were also experiencing this shift during the early part of the 20th c. This brings us up to the present where many conservation graduate programs now require a significant amount of science credits (college level) for students to even be considered as potential applicants (in the US students are typically required to complete two semesters of organic chemistry). I believe such requirements have been effect since the mid to late 70s. Furthermore, graduate education training does not end after the 2-4 period (depending on the curriculum). Students often go on to participate in lengthy fellowships or post-graduate internships, which is in a sense continuing on with a mentoring process as they hone their skills to become more independent.
Why does this matter? It matters IMMENSELY when it comes to the “science of cleaning” which is what varnish removal is truly about. In the past restorers had to rely on what was available to them….this also includes the liturgical staff within a church who were often charged with tending to the artwork. In the Medieval and Renaissance times this might involve the use of terribly caustic solutions (e.g. lye) while later on restorers working in their studios began to have access to things such as alcohol, turpentine, and other solvents. Today we have a much wider array of materials for cleaning, thanks again in part to science. We consider pH, solubility parameters, surface chemistry, and occasionally employ sophisticated cleaning systems such as aqueous emulsions and solvent/aqueous gels. Whatever is chosen depends ENTIRELY on what is safest for the artwork in question. While much of what we have learned regarding cleaning has come from science, much has also been learned from our mistakes made as field long ago (or not so long ago as the case may be). And it is extremely important to consider the context of these “mistakes.” Some were performed in the solitary confinement of an artist’s studio, likely by an artist. Some of the most famous forgers, past and present, were/are in fact artists who also attempted to practice conservation. While other restoration practices were done in a more “public” manner and endorsed by the staff of a given museum, institution, or organization.
So it is a bit rash when some academics point fingers at the field of conservation as whole, laying blame on a profession that has since evolved and would no longer sanction certain practices. This becomes particularly problematic when academics (or non-academics) post non-color corrected photos taken to demonstrate the effects of “before and after” cleaning, often manipulating the angle of the light (e.g. raking light) so that the post-treatment photo appears completely distorted (online articles of ArtWatch are full of such images). Furthermore, archival black and white images of pictures that are sometimes used to “prove” that a paintings was over-cleaned may not be as reliable as we think; B&W photographs are objects that are subjected to changes over time (e.g. alterations in contrast) just as any other artwork and may have even been poorly developed to begin with.
Today I would state that things have been made much more clear by the incorporation of scientific principles in cleaning: we now know for example what NOT to do to an oil painting…scrubbing the surface with a caustic cleaning product (high pH) like lye for example falls within that category. Even with these wonderful advancements in educational training, we are still oftentimes associated with the stigma that we work alone, in secrecy, and practice a “craft.” This is likely because there are still many individuals out there that do practice in this manner. There is no law against them doing so and in the US such individuals are free to join the American Institute for Conservation (AIC), which is our primary national organization. Many countries have such organizations and there is even an international body, the International Institute for Conservation (IIC). But again ANYONE can join these groups, with or without graduate training. As there are no national or international guidelines on what makes a “good” conservator, there are no national or international guidelines on “best cleaning practices.” As each country is different, both in their educational stance but also what resources they have available, I suspect that such guidelines will initially manifest country by country….and indeed this has already happened in Canada and the UK who have both implemented certification programs to try and differentiate between those who have obtained adequate training and experience and those who have not. In the US we are farther behind on this. We have a “membership status” process for people to apply for “Professional Associate” status and later on the more prestigious “Fellow Status.” I applied to and was awarded PA status once I became eligible to do so. This then allows me to be placed in the searchable database on AIC’s website “Find a Conservator” where clients and institutions who are seeking restoration services can locate me along with other PA members that are on the list. The problem is that we have been finding that on occasion some PA and Fellow members have not been exercising “best conservation practices” and there is unfortunately no way to address this at the moment. AIC (as does many national groups) does have a Code of Ethics but it is perfectly legal for someone to start a business, join AIC, and simply post that they “abide by AIC’s code of ethics” without ever having gone through professional training. So this is the unfortunate situation at the moment. For now I still refer artists, clients, and museums to the database but with the caveat that their homework STARTS there….it does not end there. It is best to locate an individual using this database and then find out when and where they obtained their training….perhaps even following up with their alma mater. The graduate programs are also excellent resources to locate trained and responsible conservators. Devil’s advocate might state that there are also bad conservators out there with Masters Degrees as well….and that may well be true…but they are far outnumbered by self-proclaimed restorers who claim to be “professionals.” But the international dialogue today, given social media and other online resources, is encouraging. Younger people are more aware of these issues and more inclined to take them on…they tend to be more open and willing to engage with the public about all things to do with the profession, whether it be cleaning, retouching, varnishing, etc. This is how it should continue to be if we are to be respected and valued as a field. When it comes to works of art in public institutions, this is even more true. I leave you with the plea to be wary of attention-grabbing headlines regarding “botched cleanings” and “restoration disasters.” These articles are often written by journalists who have been ill-informed. If an institution has been open about the restoration of an object before, during, and after a restoration treatment then no harsh criticism unless it comes from an international committee of CONSERVATORS (not just art historians) that assembles voluntarily to question a treatment’s proposal or progress. But if an institution chooses to remain shrouded in secrecy when it comes to conservation they must unfortunately be prepared to deal with the consequences.