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It's perfect stuff. But sidewalk chalk, aka calcium sulfate, aka plaster of Paris, aka gypsum (it's all the same) works just as good, as long as you make sure to use white, non-colored quality. The trick is to paint the strop with lather, and rub it with the chalk until the pores are completely clogged. (work it in with your fingers). Dry with a hairdryer. Make your fingers greasy with strop dressing, or with mineral oil, and massage the surface of the strop. Allow to dry overnight. Brush lightly to remove excess dust. This will give you a great surface to strop on, that behaves the same like a linen strop that was filled with a tube of Dovo white (Dovo white Is to be used abundant, the idea is to clog the pores of the fabric).
Gypsum has a hardness of 2 on the Mohs scale. Diamond has 10, garnet has 7, and hardened steel 5-6. The Mohs scale doesn't have a mathematical function, but is purely an ordinal scale. However, in it's lower end it is more linear and in the higher end it become more logarithmical, meaning that the difference between the harder numbers is much greater than the differences at the softer numbers.
Something with a hardness of 2 will not be able to scratch something with a hardness of 3 and more. That is in fact how Mohs determined the hardnesses of this scale. Talcum is mark 1. Talcum can't scratch gypsum, and gypsum can't scratch calcite, hence gypsum is 2 pand calcite is 3. That's how Mohs arranged a number of well known minerals to which all other minerals can be related.
Gypsum merely has a buffing action on steel. That's why it must not be considered an abrasive, such as Chromiom Oxide, Iron Oxide, Diamond pastes and sprays, etc.
I have spend some time comparing clean fabric strips of various fibers with strips loaded with Dovo white paste, which is most likely chalk based, and the effect of plain chalk crayons. One of the things I particularly like is a fabric strop with clogged pores. It just like the feel on the razor. Dovo's V-pattern linen on their Juchten. leather strop, comes completely clogged with Dovo white. It's a very efficient strop. I also own a Tony Miller linen, of which I have one side clogged with the sidewalk chalk as explained in my first post. I like it better that the unconditioned side, it feels softer in use. As far as results a concerned, I don't know. The conditioned strops seem slightly more functional, but that could easily be my imagination. They won't make a razor shave better than an unconditioned strop, but they might just reach a tad deeper when it comes to dealing with the very first signs of edge deterioration.
I have never experienced any noticeable rounding of the edge, as happens with abrasive pastes. Gypsum is basically not harder than most of the component of natural fibers. Cellulose, which is a major component in natural fibers, is also known as "starch" in it's purest form. It has a hardness of 3.5 on the Mohs scale.