Let me save you some time:
Mineral Spirits (also called "paint thinner", "solvent" and "Varsol")
will dissolve the glue holding MOST labels on jars. What I do is simply
scrape the impermeable surface paper off, and then apply mineral spirits
with an eye dropper to the porous paper beneath, put the jar with the
wet label in a plastic bag wrapped tightly around the jar (so the
mineral spirits doesn't evaporate) and allow the mineral spirits to
penetrate through the porous paper and dissolve the glue holding the
paper on to the glass. After 5 or 10 minutes, remove the jar and peel
the paper off with a razor blade or sharp knife. The paper will come
off easily, but you just need something sharp to get under it and lift
it off the glass.
On some jars, mineral spirits is very slow to work, and I expect that's
because they use a different glue because of the temperature of the
stuff originally put in the jars. Jam is jarred when it's hot because
it's too viscous to pump when it's cold, and so jam will usually be put
in glass jars that can stand the heat better.
In those cases, I use lacquer thinner or acetone. Lacquer thinner is
mostly toluene (it's typically 70 to 80 % toluene), so if you find
toluene for sale anywhere, you can use that instead of lacquer thinner.
Every paint store will sell lacquer thinner.
Acetone is the chemical found most commonly in women's nail polish
remover. Acetone is very fast to evaporate, so a similar solvent called
"amyl acetate" will often be added to the nail polish remover because
amyl acetate evaporates more slowly. Methyl Ethyl Ketone (MEK) is
acetone's closest chemical cousin, and it evaporates a little more
slowly than acetone, so if acetine works, then MEK should not only work,
but work better because of it's slower evaporation rate.
The advantage in using mineral spirits, lacquer thinner and acetone/MEK
over dish detergent and cooking oil is that every one of the former will
all evaporate completely without leaving any residue, whereas the latter
won't evaporate at all. That is, even if cooking oil dissolves the glue
holding the label on, you then still need to use something else to
remove the cooking oil. The benefit of dish washing detergent is that
it can be washed off with water, albeit plenty of water to remove all of
So, if it wuz me, I would save a step and just use a solvent that
evaporates completely, thereby preventing your having to use something
else to remove your label remover.