Isaac Post wrote a book in 1852 titled "Voices From the Spirit World, Being Communications From Many Spirits by the Hand of Isaac Post, Medium" which might provide a clue to the type of mediumship that was going on between the Fox's sister (circa 1848) and Post's version of mediumship. In the book, Isaac Post refers to the prior mediums as "rapping mediums" whereas his mediumship took on a different appearance. He communicated with Spirit as a "writing medium" which we call today, Automatic Writing.
He channeled an assortment of people, some of them several times. He began writing down his channelings on July 4, 1851. One person he channeled on September 15, 1851 was George Fox. This Fox was not related to the Fox sisters/family. George Fox was the founder of the Quakers, a group that helped bring forth a lot of the concepts we have accepted that are attributed to Spiritualism. As previously mentioned, Isaac Post and his family had broken away from the Quakers in which the main issue revolved about slavery. Isaac Fox was an Abolitionist whereas the Quakers were neutral on the subject. When Isaac Post channeled George Fox on November 12, 1851, it was revealed that George Fox was not neutral as we had suspected but was involved in returning slaves to their 'rightful' owner.
The question as to when the Fox sisters and Mother became mediums seems to have been answered when they began talking back and forth to the Spirit in their home in which they communicated thru rappings.
Happiness is the result of an enlightened mind
whereas suffering is caused by a distorted mind.