New mold blocks usually have some residual residue of cutting fluid used in their machining as received. Getting good castings requires this residue be either burned or washed off. Most people just start casting away after pre-heating the mold blocks, perhaps smoking the cavities to help matters along. I used to do this too until I was taught a better way which seems virtually foolproof.
When I take new aluminum mold blocks from Accurate or NOE out of the box, I use a horsehair acid brush from Brownells to coat and lightly scrub the cavities and all surfaces of the mold blocks with Palmolive dishwashing detergent. I then separate the mold block halves and place them cavities up on the bottom of a coffee can, rinse them lightly in hot water from the tap, as I fill the coffee can about half full, with more than adequate water to cover all surfaces and allow extra for boil off.
Bring the water up to a full rolling boil, then after 5 minutes turn the burner down to low heat, and maintain a slow simmer for at least 30 minutes. Afterwards remove the can from the stove, with pliers, place it in the sink and run hot tap water through it until all the suds and detergent are rinsed out.
Remove the rinsed blocks from the can, assemble them onto handles and pre-heat on your electric hot plate, using a 350 degree Tempilstik crayon to regulate the temperature while your lead pot heats up. The detergent boil leaves the blocks chemically clean and a neutral gray color. Once your lead melt is up to temperature, about 700 degs. F. and the Tempilstik mark has indicated the blocks are adequately pre-heated, you can cast good bullets immediately with no smoking.
73 de KE4SKY In Home Mix We Trust From the Home of Ed's Red in "Almost Heaven" West Virginia