With my current plan to put together a travel-hone set up based on Jnats, there are a couple of challenges. One was to have a set of smaller Nagura to work with so in case I decided to hone a blade on the road I would have everything I needed. I could just toss a few full-size Nagura into a box, but I did want an even smaller/lighter option. Yes - OCD is at work here.
So - when I decided to cut a Nagura in half, I started with a Botan. I've mentioned before that Botan can have a sand-layer in it but I don't know if anyone has actually seen it. So - here it is. Both photos were taken after I popped it in half though. I should have tried to get an image of the black sand-line before hand I guess. At any rate, you can see pretty clearly what's going on. I soaked it overnight to get the sand to swell up so I could split it cleanly.
In use, one would not want to make a slurry using the narrow sides of this Nagura. Instead, using the stone with the sand-layer parallel to the Awasedo would be the right thing to do. When the Nagura was worn close to the line, it should be lapped clear of the sand and then the issue would be gone.
Not all Botan have sand inside of them like this, but it is not an uncommon situation.