Behkol was what I used to use. It has since been purchased by Mohawk (I think??). Mohawk sells “Shellac Reducer” which I believe is the old Behkol denatured alcohol. You can sometimes find it on the shelf at Woodcraft. If they don’t have it, they can order it. The MSDS lists 75 to 100% ethanol and then butanol and isopropanol as denaturants. One of my teachers taught me to dissolve the shellac with Behkol, but then thin with “the cheaper stuff.” If you want to do more experiments, you might be interested in looking for testing protocols that involve applying shellac to glass. Don’t act in reliance upon the info I gave for the MSDS. The MSDS can change and maybe what I’m looking at doesn’t match what you buy….always look it up yourself.