To help the cleaning process a piece of paraffin can be dropped into the pot. After it has burned, the dross is more easily skimmed off.
Another method of cleaning is to add to every ten pounds of metal, one teaspoonful of equal parts of powdered rosin, borax and flour of sulphur. Stir this compound in a little and when the lead is at the proper temperature there will be a flare, flash or a little burning. A sort of tinfoil popcorn effect will be noticed floating on top of the metal. Continue stirring until this melts down and then skim off the soft particles.
An easy way to test the temperature of the melted lead is to insert the end of a pine stick for a moment. When the wood blackens or burns, the lead is hot enough to pour.
Whenever the lead is in melted form and exposed to the air, as in the iron pot, there will be a slight oxidation take place which appears as a yellowish-red scum. Do not confuse this with the dross and dirt which is to be skimmed off, but if the scum gets thick, then skim it off before pouring any of the metal.
PREPARING THE MOULD
One of the greatest difficulties in making lead castings has been to get the metal to completely fill the mould. There are several reasons for this, but they can easily be overcome.
If the mould, which is generally made of iron, is very cold, it will chill the melted lead and cause it to become solid before it has reached all parts.
The surface of a cast iron mould is very porous, and the melted lead, partially entering the pores of the iron, is held back long enough to chill and set.
To remedy these faults, it is sometimes advisable to heat the mould in order to get perfectly shaped castings.
To close the pores of the iron moulds they are sometimes “smoked” with a flame like that of “Prest-O-Lite” gas or else dusted with graphite.