You take alum. to alum. or copp. to alum. and put together like regular junction, except you must only use a purple wire nut. It might be filled with the substance you mentioned....But you cannot make one of these yourself! you must use ul listed nuts. Just remember to downsize your wire. A 12ga alum wire should tie to a 14-2 copp. and a 10ga alum. to a 12ga copp., as alum. conductors do not have the ampacity copper does.