Purity - Testing cocaine quality on spoon??

[qwerty22]

Hello..swim reading the thread`making crack with bicarb soda method`,and using the spoon method to test quality of the coke..Now wondered if you could help me.
Swim has heard people say that they are doin this method to test they coke to see how good it is and it came back at 40% pure??now swim wanted to know how this method works,ie how they work out the % of the coke..swim assumes you weigh before you cook and then weigh after and work out from there??is swim right??if you could shed some light and how you do it swim would be extremely grateful....thanks in advance.

swim had also used the search function and up2 now cannoit find the answer he`s looking for that why swim posted this