Crudos,
What the wax does is it helps the 'slag' to rise to the top of the molten lead to be scraped off. You can use a chunk of paraffin or candle wax; it doesn't have to be bees wax. I just use bees wax as I have allot on hand.
Be careful when you throw the wax into the molten lead as it will ignite. If it doesn't my Lyman book suggests taking a match and igniting the fumes. I have never had to do that though
Slag = impurities
On another note: I weigh each bullet after being cast because their might be air voids. Then I put them into three categories. The categories are:
1) Average weight,
2) Two grains under average, and
3) Two grains over average.
This process results in 25 - 50% of the bullets or balls going back into a labeled bag for the next time I cast.
My brother tells me I am a bit anal concerning the weights of the bullets and balls I cast
you can see why I cast a 'shit-ton' of all calibers at one time.
Once I am done for the day, I spray the molds used with WD40 and put them into a relatively 'air tight' ammo can.