I've heard of the use of epsom salts as well which got me thinking.. I've had some bad (what I would call weak stuff) where you could specificially see shards of glass, as well as another salt or powder (which I presume to be epsom salts)..
Now, given previous knowledge of epsom salts, and how it'll hit your stomach and just dump basically everything (*cough* give you the shits *cough*), I've found after said substance above that I got much the same effect, suggesting the prominent use of epsom salts.