Dave Anderson replies: Shellac is an evaporative finish and cures by releasing the alcohol it is dissolved in and turning back into a solid. As shellac ages it undergoes a process known as esterification which is a chemical change that prevents a full hardening of the material. I can’t tell you for sure about the color change, but I would test it by placing a few drops on a scrap of wood and waiting a few hours to see if it hardens. If it still remains tacky and/or soft, get rid of it. A safer thing is to just discard it and buy a new can or mix new flakes. Depending on what you bought, a batch or can has a shelf life of 2-3 years. It is not worth taking a chance of ruining or having to strip and refinish a project to save a few bucks.
Bob Oswald replies: I wouldn’t but I don’t have the reasoning to back it up. When natural products change color it usually means they’ve changed in some way that makes them not fit the original specifications.