It is not uncommon when reading scholarly opinions or fatwas of learned Muslims that impurity is not transfered unless the medium if water or wet. So if your hand is dry and the impure substance say pig skin is also dry, and both come in contact then your hand has not become najis. However if the pig skin was wet and your hand came in contact then your hand becomes najis.
I haven’t researched all the hadith regarding this topic yet however what is the logic that water causes impurities to be transfered otherwise not. And if water is the medium which causes the impurity of one substance to be transfered to another then obviously something physical must have been transfered between the two substances. If this physical impurity was removed from the impure substance does it become impure.
I can understand if this conclusion was derived directly from ahadith but then why was it thought that wet mediums cause impurities to transfer back then. I cannot understand the scholarly logic here other then reiterating what is mentioned in some hadith somewhere.