Thats left over gland heads and stocks that are devoid of there goodness. If you iso wash it, you'll just have to filter them out again.
left over STALKS.. the heads are gone.. ok.. kief depending on quality.. ok.. so dry sift.. with 1 screen .. has the stalks and heads.. and plant matter.. the thing is.. to some people KIEF is just HEADS... to others.. kief is.. just a gold powder.. that consists of heads stalks and plant matter.. this is why.. u see people claiming they are getting 90% returns on bho.. and others getting 20-50%.. all kief isnt created equal.. with alot of the shit ive made.. ive noticed.. your best bet.. with KIEF.. is.. to do a Small NUG RUN.. breaking the 90% of the nugs up with a grinder.. then the mixing the kief in . with that.. packing it. the last 10% of nug.. pack FULL.. this helps filter any plant material... and then i use 3 unbleached coffee filters.. a stainless steal mesh screen .. and another coffee filter over the stainless .. and clamp it.. and spray.. ive noticed.. a much lighter product.. and sooooo stinky.. its great.. i thought at first.. o the more filters it might clog.. the thing with making bho is.. ur not just getting the trichome heads as a hole. you dissolve them so u cant clog your filter with keif.. the reason people have blow outs with it clogging.. isnt because of the screen its the kief itself.. it dissolves and creates EMPTY SPACE in you tube.. allowing for PRESSURE to be pushing the kief mud lol out like a potato gun.. instead of allowing the butane to pass threw the material.. .. thats why mixing with material is best.. and its best to do with nug.. if u wana do a straight kief run.. u wana do the soak method.. get your temps to were its not evaporating at all.. and soak.. and strain..
but i hope that helps u understand.. a Solvent dissolves the heads.. and a screen or bubble bags just collects them.. which .. a solvent doesnt create as stinky a product.. compared to a properly made dry sift ... the taste and smell are always reduced with a solvent .. but that doesnt mean u cannot make a nice tasty shatter or wax.. because u deff can.. but u get what you put in with a good drysift or bho.. so if ur just taking all your trim including fan leaves.. ur going to get a worse product then if u were only putting in crystally sugar trim.. or bud..
goodluck man.. bho is an art.. and only worth making with the right equipment. a vac and chamber.. and a heat source and laser thermometer .. a closed system is great for yield and saving butane.. but costs lots.. so its hard to get the investment cash ..
and if u take ur material out of the tube right after u blow it.. while its still like moist kinda.. and put on a plate to allow the butane to escape.. the tubes stay pretty clean.