That it comes off after being hit is fine by me, acting as a sacrificial coating is a good thing. As you can see it was made using iron rich clay, also horse dung and sand. The mixture used was made up by feel, rather than following any particular proportional recipe, with the key ideas that it had to be able to be smoothed onto the cucurbit, yet with sufficient sand and dung to hold together when drying.
Unsurprisingly, because the general recipe is the same as medieval bronze casting moulds, it looks very much like medieval mould material that I have examined, except that a lot of that often contained chaff or bits of straw as the organic material, rather than dung.
Here’s something else, a wide flat graphite crucible, meant more as a nice open bowl like a scorifier, and for a similar purpose, i.e. oxidising substances over a fire.
Unfortunately in this case I didn’t dry it for long enough in my oven. It had a wee while, but not enough, so when I put it on the fire to be properly fired, the bottom popped off due to residual internal moisture turning to steam and expanding.