I know its an old thread, i just wnat to confirm the dirty oven supicion, it took me 3 times to get it right...
The easiest way was just to use one of those foil bakeing tins and cover it with foil to "seal" it... don't use the foiled paper lids you sometimes get with the tins.. (that was failure no2), the paper inside starts to smoke and somehow that draws into the cooking epsoms salts. The foil sealed bake worked perfect, providing me with nearly crystal clear acetone (from the exact same epsom salts box's contents, I made yellow acetone from before).