It is well established that shellac dissolved in alcohol ('liquid shellac') has a limited shelf life. The actual shelf life is highly variable, depending on the source, manufacture, storage. But after sufficient time, a batch of liquid shellac will no longer dry hard. A good test is putting a little shellac on a piece of glass. Fresh liquid shellac will dry hard in about 15 minutes. Sufficiently old shellac will stay tacky/gummy indefinitely.

What is the mechanism behind this change in properties?

  • $\begingroup$ I think you should give us a sense of what level answer you expect. Shellac is complex. I suspect that many degradation routes are possible. For instance esterification can occurs. Or the water content might get, although slowly, over the threshold for hardening (alcohol absorbs water..). And so on. $\endgroup$
    – Alchimista
    Sep 15 at 10:23
  • $\begingroup$ I guess my real question is, "Why is shellac stable by itself, but degrades when dissolved in alcohol?" And the level of detail I'm looking for is something like, "<these> classes of processes result in <these> classes of changes. Those changes prevent hardening because <reasons>". I'm used to thinking of solutions as stable, and reactions (from college chemistry) as things that happen within minutes or hours and result in significantly different chemicals. $\endgroup$ Sep 16 at 18:38
  • $\begingroup$ that is correct. But as I said, and having no specific idea, I believe that even specialists of that material are still investigating. At least not because interested on shellac itself, but as working horse for techniques and methods. For instance, if the water content rises, it can reach a point where hardening does not take place. Esterification reaction can occur internally, or the material can degrade (kind of molecular weight reduction) so that smaller fragment do not hard. Etc. I am convinced that multiple path are involved depending on the batch origin and treatment. $\endgroup$
    – Alchimista
    Sep 16 at 19:26
  • $\begingroup$ Reaction can be also very slow. Among the immense number of synthetic and natural molecules know, relatively few are indeed both energetically and kinetically stable to avoid alterations along days, years or decades. Shellac might be one of that, but apparently not in hydroalcoholic solutions. $\endgroup$
    – Alchimista
    Sep 16 at 19:30
  • 1
    $\begingroup$ First. I am just saying what could make sense in a broad context. And what you said above is about the same line of thinking. Another possibility is partial cross linking. Even obtaining bigger chains can interfere whit proper hardening at the right moment. Water can be trapped inside kind of cage. And so on. For details, I suspect you have to refer to a real search, Google might be not enough. With luck, someone who indeed worked on shellac chemistry would perhaps respond. But I doubt that we have such an user here. $\endgroup$
    – Alchimista
    Sep 16 at 19:58

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