Hoover and Strong’s catalogue has a good article on reclaiming
scrap- though consider they are in the business of selling casting
grain and raw materials, the article is worth the few seconds to read
on master alloys, prepared alloys and scrap refining with new grain
and your scrap gold.
You will always get better results if you add at least 25% new gold
(or whatever the metal is that you are working with) to the pour.
With gold hitting 1000+ an ounce though the least possible to give
you good results in my opinion is 33. 3% new 24Kt gold with whatever
you are alloying to get whatever karat or colour you desire. Adding
no new gold is not recommended, neither is just adding new master
alloy to “clean up” and deoxidize the scrap you are using if your
pour is all scrap, sweeps, etc.
It is critical though that you remove all excess, or remaining
solder from your reclamations; just a little can cause splitting or
incomplete recrystallization in the pour. If you are making, say, a
blue- green gold alloy you would mix together:
1 gram of 24 kt yellow
0.532 grams of .999 silver
0.058 grams of Copper
0.1120 grams of Nickel
then multiply the resultant weight (1.710 grams) in this receipt
times the quantity you want to end up with for example you want 1
ounce of blu-green gold, then you would multiply the metals by about
15-16 parts of each (the troy ounce result you want being more or
less equal to 31.103. or 1 troy oz. )
If you use all scrap for the 15 grams of fine gold your result would
potentially split, or more likely pit when you rolled it out. Always
put the mass of scrap in a plastic box and run a magnet through it
to pull out any bits of ferrous containing scrap- that alone helps
eliminate many problems, then if you wish, add any master alloys you
like according to the manufacturers instructions as to karat and
colour desired and remember to account for the addition in any
formula you are using.
But most importantly to avoid crumbling, splitting or otherwise
spotted (from not mixing thoroughly) and streaked pours is to use as
clean ingredients as possible, and pre-heat the mould before the
pour. Failure to do any of these steps can give you streaks,
crumbling during rolling, pits on the ingot and/or in sheet rolled
from the ingot and holes in the ingot.
The mould should be neither too hot nor too cold- you should be able
to touch the mold without getting a burn. The best indicator though
is to coast the inside of your mould with wax or oil, when it smokes
it is at pouring temperature, the excess simply lubricates the mold
for easy release. I hope this helps.