Linseed oil, made from flax, is often used on furniture as a moisturising agent because it penetrates the upper surface of the wood and protects without sealing it. Unfortunately, it can be applied too thickly or too often and, over time, it can create a gummy, dark surface that traps dirt and never dries out completely. If this happens, you must first remove the residue, then the surface layer of the wood where linseed oil has penetrated.
Pour turpentine onto a soft cloth and rub it over the surface. Turpentine acts as a degreaser and will dissolve old layers of wax and linseed oil from the outer surface of the wood. Turn the cloth over and rub the surface again, repeating until no more residue comes up on the cloth.
Sand the surface with medium-grit sandpaper to remove the layer of wood where linseed oil has penetrated, working with the grain. When you've removed it all, the wood will be a lighter shade underneath. Wipe the surface with a damp cloth to remove wood particles and reveal any spots you may have missed.
Allow the surface to dry completely, then follow with a fine-grit sandpaper, working with the grain. This will sand down the areas where the grain is raised and remove any remaining areas of linseed oil penetration. Wipe the surface with a damp cloth and allow it to dry.
Run your hand across the dry surface, feeling for any grit, raised areas or unevenness. Sand again until you are confident that the surface is uniformly even and clean.
Use tung oil if you wish to restore the natural glow of the wood after removing the linseed oil. It doesn't take as long to dry and has many of the same moisturising and protective benefits for the wood.
Do not throw away linseed-oil-soaked rags in the trash. They can spontaneously combust and cause a fire. Treat the rags first by soaking them in water and sealing them in plastic bags before throwing them away.