Notice the amazing similarity between Mount St. Helen's and Mount Pinatubo ashes. And they are from opposite sides of the planet! Yet a common commercial material in North America, Navajo Pumice, is completely different.
Materials that source Na2O, K2O, Li2O, CaO, MgO and other fluxes but are not feldspars or frits. Remember that materials can be flux sources but also perform many other roles. For example, talc is a flux in high temperature glazes, but a matting agent in low temperatures ones. It can also be a flux, a filler and an expansion increaser in bodies.