I'm pretty much stumped on this one. SWIM has a batch of spice that is very harsh and sorta hurts to smoke. SWIM has cleaned it three times, and dried it thoroughly with a fan each time. SWIM has also ground it to a smear between a glass pie plate and a cube-shaped bottle, repeatedly. Right after grinding, it will smoke smooth; but if it then sits for awhile, it is harsh again.

What could this be?

I depends what you smoke it with ? I use GVG and I do not feel anything. If your spice is clean as you say than it is most definitely your smoking device or technique or both.

How did you "clean it" ? What is your method of smoking?

Recrystallize, then redissolve in non-denatured ethanol and acetone, then re-evap (to get rid of naphtha traces). Then get a GVG and a torch lighter

I think it is your technique. Some expert members told me this and it changed everything. I realized I was BURNING the spice. You have gotta vape it, so be sure you hold the lighter a little bit away from the spice. If you put the flame right on it, more than likely it will burn. Once I started hitting it better, I was rewarded with a clean hit.