A fresh or slightly wilted plant vinegar extract will also preserve many of the medicinal properties. This would preserve the lawsone compound that removes urushiol oil for a few months, if stored in the fridge.
A lot of herbalists let jewelweed wilt for a few hours to reduce water content and then make an infused oil. That oil can be used as is or made into a salve or ointment. This is where experience helps because if the water content is too high, it will cause spoilage of the ointment. The targeted compound, lawsone, should last 6-12 months if the oil/ointment is stored in the fridge.
Blending leaves, flowers, and stems in a little water and freezing them in ice cubes is another way of preserving them for later use. You can rub a cube right on the exposed area where you want to remove the urushiol oil or apply it to the rash.
For internal use, it can be dried for teas, but it loses a lot of the properties one would want for poison ivy prevention and treatment. The water content in the stems is so high that it can spoil during the drying process.
Lawsone is the chemical compound that binds with the urushiol oil and oxidizes it. This keeps urushiol from penetrating the skin. It is believed that the combination of lawsone and the saponins present in jewelweed work together as a poison ivy, oak, and sumac rash preventative. Again, it works best if used right after exposure.
Henna also contains lawsone but not enough saponins. If you don't mind staining your skin, you can still use the lawsone henna to deactivate urushiol if you combine it with a plant that has saponins (soapwort, chicory, pokeweed root, etc).
