Re: d'uVA Chroma Coal Heat Fixable Pastels
The couple I tried were fairly hard and fragile.
The color are rather acid and quite transparent (no white is available in the small color line).
Moreover, the color shades and surface quality changes when you heat set it.
IOW: you can pretty much forget a traditional chalk pastel handling or look to these. Though they may be useful for drawings, stenciling, layering, etc.
Sad But True: Teenyboppers overheard at a bookstore Art Section, "Look at that book, just like the toothpaste...REMBRANDT!"