I had a large window curtain satisfactorily dry cleaned. It's made of
coarse, heavy cotton.

Later, I tried to clean a small stain on it with cold water and mild hand
soap. That stain was gone, but replaced by a much larger stain of the
wetted area. Plain soap and water doesn't stain cotton, I thought, so I
tried again with barely tepid water and a very little washing-up liquid.
That has always worked well elsewhere, but here it just enlarged the
stain to the new, bigger wetted area. It seems that the soap and water
has reacted with the residue of the dry cleaning solvent.

Anyone know how to remove this stain short of sending it back to the
dry cleaner?