3 possibilities:
On a suitably discreet test area:
1) Rub with turpentine. If that takes off the shine, it is an oil finish.
If not...
2) Rub with alcohol/methylated spirits. If that takes off the shine, it is shellac.
If not...
3) It is some kind of varnish, and the jelly-type paint stripper is the best way to remove it. GOOD NEWS: as shown elsewhere (if you followed my links) after stripping and removing any stripper residue, you can go straight to linseed oilapplication.
Please note: old wood needs to be treated with a bit of respect. Follow the order above.
At NO TIME do you need water, caustic soda, oven cleaner or sandpaper!