"How do you clean an Oil-Stone?"

 The photograph of the scanned page will answer your immediate question. 'Cleanliness' is a priority!
Just apply liberal amounts of 'lighter-fluid' to the surface of the stone and let that liquid settle in & on the stone. After a few moments, let the your strong magnet glide over the oil-stone..and "viola!"
 You can see the resulting action so much of the steel shards are now being lifted up from the stone. I'd do this a few more times, even with the liquid. You just gotta get the oil-stone near perfectly clean.
 The metal on the stone came from your gravers leaving little subtle amounts of steel firmly entrenched or embedded into the pores of the stone..A real dirty YUK job!