Your question: Are you saying Venice matted its beads, before they were shipped to Africa?

Answer: Yes. This is documented in the book "Ercole Moretti" by Gianni Moretti. Probably in other places as well.

Another detail noted in this book is that the ends of many cylindrical beads were ground at an angle so that the beads would nest together and not pinch the skin. Up until I read this book, I had erroneously assumed the ends had been ground to shape in Africa.