yes the bubbles have to be gone.
you need to make sure when you get the cube you wash the mats with a brush and soap to remove the releasing agent and remove the bubbles.
In the field spraying the mats like the other member explained works about he best with a bit of soapy water.
a piece of equipment that is fun to use and removes the gold from the cons is a spiral wheel panning machine. I have one and it really takes the pain out of removing fine gold from cons. the ones with adjustable speed gives me the best control.
when It gets down to the smallest stuff like -100 or -300 a miller table is the best in my opinion only. works for me. I have the one from Royal Manufacturing as I feel it was the best set up for the price and I love mine. works great and I clean up all the black sand each year I collect and am surprised at how much gold I would have left behind if I did not save the for the miller table later.
the old fashion way always works but just takes some time, fist dry the black sand by placing in the sun in a large 14" gold pan or a chaffing dish and let it dry out completely, take a magnet and run it around and remove the dry black sand. the remaining material will be mostly gold and some non magnetics like Hematite, leverite -lol, micas and other stuff I cannot begin to identify. but at this point the gold will be the heaviest and place that dry material in a gold pan and pan it easily. or you can use a paint brush and take the dry material and separate it with the paint brush.
anyway hope I helped out in some way and best of luck you to you. "Now go out there and get some gold!"