The term witch jar may be a variation of the classic witch bottle. These are folk magic spell bottles for protection against witchcraft and conjuring. The oldest recording that mentions the witch bottle is from the late 17th century. The bottles or jars are usually filled with the person the bottle is to protect or the victims' urine, hair, fingernails, or menstrual blood as well as herbs, red wine or vinegar, earth, snad, sea water, thorns, pins, and needles, rusty nails, pieces of broken glass or bone. They are then hidden away and keep out of sight by walling them up, burring them in the foundation of a building, or on the far corner of the property.
Touching human ashes with your bare hands after you used the ashes to call upon a demon is not the best idea. Not sure why you called upon a demon or what task you gave it to do but you may want to consider banishing it and then protecting yourself. You will need to cleanse yourself as well. If you feel cursed you are simply because of your thoughts. This is where the banishing of negative energy is important as well as the cleansing of your body and all personal space. You may also have to repeat this process a few times for it to be completely effective.