I think the key is the way that the hash is processed, with outdoors drying being important as it also removes any residual chlorophyll. What is the difference between hash and "scissors hash" made from trimming dried buds? It is much like the hash of old and doesn't melt when you burn it. I think it is a combination of the pressing process and maybe something with the moisture content when it is pressed. If it is slightly damp when pressed, it would harden up quite a bit after drying out a little, much like scissor hash hardens up after sitting a little. I don't think it has anything to do with sand of other contaminants, as you would have a pile of unburned sand sitting in your pipe after smoking it, since sand and dirt doesn't burn too well.
Thats what I was thinking. Scissor hash burns when lit from the corner and keeps burning like hash i used to buy in the 80’s.
It definitely has plant matter in it. Maybe pure extraction forms make full melt and hash had a lot of plant matter in it?