PaulB, quote: "I think there is little doubt that to achieve any particular feature in the blade surface depends to some extent on the raw material. However if you consider neighbouring schools such as Bizen and Bitchu, they were producing very different products with raw material originating from the same source. While ko-Bizen and ko-Aoe had many common features by the time the Fukuoka Ichimonji and chu-Aoe schools were producing the characteristics were markedly different. If the raw material was the same the differences seen must be a result of technique."
Now I am not an expert, but it may be possible to refine this a little more. Aoe was beside the Takahashi River which runs from Bitchu Matsuyama/Takahashi down to Tamashima. Traditionally there was trading by ship from the mouth of the Takahashi River with Osaka and the Kansai, ie Yamato/Yamashina area. I know iron was found at the foot of KinoJo Yama, west of Okayama, and there is archaeological evidence of the very oldest iron smelting in Japan, producing weapons and armour.
Were not their (Aoe's, Bitchu's) sources of iron very different from the Bizen iron sands of the Yoshii River, beyond the central Asahi River, thus two major rivers over to the east, which passed down through Bizen/Yoshii/Fukuoka and Saidaiji?