I have read about shellac becoming brittle with age and having a small shelf life.. This was also noticed by microscopists many years ago.
In preparing microscope slides the glass cover slip above the specimen is sometimes given a protective ring of shellac. This is painted on top of the sealing ring that holds the glass cover slip in place and seals any liquids around the specimen from leaking out.
The shellac can have dyes mixed in to give an attractive and practical hard coat to protect the sealing ring.
Microscopists noted that after long periods of time the shellac could become brittle and chip and flake off.
An English preparer of microscope slides named Ernie Ives recommended the addition of a few drops of Castor Oil be added to the mix of Methylated Spirits and Shellac. This acts as a plasticiser, allowing a little more flexibility to the mix.
I mixed up about quarter of a pint with roughly a third of a teaspoon of castor oil, six or seven years ago. It still works fine and the slides that I prepared then look fine.
I checked the contents of the jar a fortnight ago. Tested it by gluing glass, paper, leather and wood. All worked fine and held within the limitations to be expected from an adhesive like this.
So after going all around the houses, the message is mix a little castor oil into your shellac and meths varnish/glue and it could very well last a lot longer on the joint and on the shelf.
Sadly Ernie Ives who was also well known for his marquetry skills passed away a couple of years ago.