MITRA Forum Question Details

First, shellac is a poor choice of surface coating, especially over painted surfaces, as it will inevitably darken. As to colors disappearing, this is either a function of them being masked by the darkened shellac or fading resulting from the use of non-lightfast pigmented paint. Colors exposed to the full spectrum of sunlight would need to be of the highest lightfastness to have any hope of survival.

Unless you made the Totem Pole yourself, we cannot recommend a restoration process or materials for such an endeavor. This should really be treated by a trained objects conservator. You can find one in your area by using the American Institute for the Conservation of Artistic and Historic Works “find a conservator tool.”

See below:

https://www.culturalheritage.org/about-conservation/find-a-conservator

​Okay, here I go about shellac again....I don't mean to berate the issue yet I remain confused about its properties and want to understand better.  Conservators have told me it darkens with age, don't use it.  However the woodworking industry says otherwise (i.e. see https://www.shellac.net/news/2018/12/21/7-questions-facts-myths-about-shellac-for-wood-finishing/) as does this article (https://www.sciencedirect.com/science/article/pii/B9780081002148000154); and my own 20+ sample of platina shellac has not darkened at all (I know, a relatively short amount of time, but there isn't any perceptible change).  Are there actual conservation studies on how a platina, bleached shellac ages, or is shellac's reputation anecdotal?  Or maybe changes are due to specific  considerations (i.e. darkening occurs only in colored or impure grades, or dependent on how shellac interacts with certain materials)?  Perhaps, given my lack of conservation training, I'm misinterpreting the studies that say shellac's not affected by UV light, and there are factors other than UV which cause it to darken that I'm not considering?

Thanks for your patience in explaining this to me.

Koo

​To clarify, shellac is not a resin but an exudation from an insect, the female lac bug (in a similar sense to how wax is produced and extruded from a bee). So it doesn't fit into the category of tree resins, most of which do discolor with age. 

Shellac is the insect exudate from the Keirra Lacca Kerr and other related insects. Basically, it is exuded to hold the insect’s eggs/larva to a branch until they are hatched. All raw shellac (meaning the raw exudate stuck to the host branch) has a deep color. This is a result of organic components in the host tree and, especially, inherent dyestuff in the resin itself. Truly raw shellac has a dyestuff that can be extracted in an alkaline aqueous solution to yield a deep crimson color. This was the principle use of lac until the 18th century. In fact, the term “lake” color was a corruption of the word lac. Lac was the source of one of the most important red dyestuffs used for the creation of red lakes. The various grades of shellac run from stick lac, which is the only grade that contains the full amount of lac dye, to relatively unrefined seed lac, to ruby, to amber, to lemon, to pale, to blonde, and finally bleached. However, there is no tree resin component in shellac (in the sense of added tree resin).

There is no doubt that shellac darkens. This has been observed for centuries. Bleached shellacs, especially modern methods of bleaching, are less quantified in terms of yellowing. I have paint outs of numerous contemporary bleached shellacs that have substantially yellowed in the past 7 years. These were applied quite thick. My initial reason for making these paint outs was to document autofluorescence under the influence of long-wave UV light. Incidentally, I observed a rather marked yellowing in that time. This seemed to correspond to all anecdotal and empirical observations about the resin. As I wrote to Koo more than a yar ago, the thickness of my paintouts likely exaggerated the perceived yellowing, but it does show the capacity of the material.

It seems to be true that modern bleached shellacs appear to empirically yellow to a minor degree in the short term (despite the results of my paintouts). There are other issues unrelating to these issues. Bleached shellacs, those that appear to yellow to a lesser degree, change greatly in their solubility overtime. This have been known for almost a century. Modern, dry, and bleached shellac will become insoluble in ethanol in a very short period of time. The dried film becomes even more so over time.

Soooo, at this point, I remain against the use of shellac for a transparent top coating for FINE ART that would be damaged by darkening over time. However, I am not an expert on everything. We have a query about this question to the conservation field at large to see if there is any new info to impart.

Lets also keep in mind that despite all of the dire predictions, there are numerous examples of copal/oil varnishes on 19th-century paintings that appear to have yellowed only minimally. Others using the exact same materials (we know because the Pre-Raphs were good at recording their materials) were train wrecks. Small additions of media or very thin applications of “theoretically” problematic materials may have no deleterious effect. In some instances, they may have been beneficial. I am only suggesting "best practice"

You are admirably patient with my questions, Brian - thank you for that.  My confusion arises not from your clearly worded replies; it's from the other voices on line, such as the study cited (I just pressed on that link and see it doesn't take you to the article, so I'll try again: pdf ).  I do believe your experience with shellac, as well as other conservators who affirm it darkens; yet I also read a seeming scientifc paper from the coating industry that says it doesn't darken...hence my confusion. I don't have the scientific background to understand the contradiction.  Valuable as anectodal evidence is (it's primarily what I rely on in my own understanding of materials), a conservation-based, scientific study on shellac could help to square or counter the opposing view that's put forth by woodworking & coating experts.  

Again, many thanks for your patience, and for looking into the subject further.  Koo