Unlike most other finishes, shellac in liquid form deteriorates as it ages. It doesn’t dry as rapidly or as hard and it watermarks easier. The deterioration goes faster when the shellac is stored in warm conditions and slower in cool conditions.
Generally speaking, if the shellac is older than 6 months-to-a-year, you should test it for drying and watermarking before using it on critical surfaces such as tabletops.
Shellac also deteriorates in solid flake form, but this usually takes much longer and you will know there is a problem because the shellac doesn’t dissolve completely in denatured alcohol. Clear or blonde shellac deteriorates much faster than orange, garnet or button shellac. Again, the deterioration is faster when the shellac flakes are stored in warm or hot conditions.