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I have bought it here.https://www.ebay.de/itm/9-78-kg-Alaun-Kalialaun-Kaliumaluminiumsulfat-Kristalle-Knete-Hortensien-900g/263538342180?hash=item3d5c1bf924:g:fj0AAOSwUUdaoPue
I managed to get a small quantity of Alum and the bearing cap is currently sitting in a strong solution. There are some signs of corrosion but I expect it will take a couple of weeks.....