Not sure of the request to seal it after "harvest". It's been lying around in the woods for decades.
It will get stronger as it sometimes "shrinks"in its natural state.
Evaporate and not be as good? The stuff lays around in the woods for decades. In N GA and the upper Piedmont it comes from loblolly and shortleaf pine. The best stuff is longleaf and slash pine in South GA. Gets so hard you can break an axe handle easy and can spark when the axe head hits it.