When I think of a collection, I think of some unifying thing--process, style, color, etc. It's generally a body of work, consisting of 3 pieces or more. Think of people who have a "signature" style: Jean Stark, Harold O'Connor, Steve Midgett. For example, Jean Stark has a "signature" style of granulation. Her "collections" range from granulated miniature Christmas ornaments, granulated rings/necklaces/earrings and woven chains. She also does very precise articulations and terminations, again part of her "signature" style. Steve Midgett's "signature" style is mokume gane, but he has "collections" of wedding rings, tea posts, sculpture, etc. using his mokume gane.