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I pasted my old hanging strop with white chalk paste on the linen and CrOx on the leather. Its hanging in my shave den so I can use it mid shave if I want to, works really well!

Was it a green crayon? If it was, it sounds like it was chromium oxide, which is an abrasive paste used for touching up or finishing a razors edge.

There is also a white TI paste which is chalk...