Try this do it outside as your going to make more smoke than your wife will like.
Heat the lead up to between 750 and 800 F. now stir in about a pea size piece of bees wax or carnauba wax and stir it in until it burns to ash scrape off the top. From a pet store get some activated charcoal and poar it on top of the lead. let it stand for ten minutes or so as the charcoal holds moisture and you want that to cook off. Now after it is dry stir it in completely you will be amazed at the crud that will come to the surface with the charcoal. Scrape off the top again and add a little but more bees wax stir it in and try casting.

40 Rod

Sand and grit, being much lighter than lead, will (or should) float to the top, not sink to the bottom.   For something to sink to the bottom it has to be heavier than lead.    I think the yellowish cast on top may be cadmium contamination.

Rather than getting activated charcoal, you can do the same thing with sawdust.   Just make sure it is dry and stir it into the lead alloy.   It will smoke a lot and when done you will have "activated" charcoal on top of your lead.   Works great with bottom draw as you just leave on the surface and it will prevent further oxidation of the lead.

George

George Babits wrote on Jan 25^th, 2022 at 3:27pm:

I guess the "sand" WAS heavier, because the the bottom pour bullets were unusable. Dipping off the top was better, but the yellow stuff just kept getting worse. It wasn't just a yellow cast, or color, but an actual powder. The flux product was about 50% the usual grey stuff and 50% yellow powder. Jeez, I hope it isn't cadmium. That has a worse health reputation than mercury. As to using sawdust, I used to haunt several craft channels and they use sawdust for both lead and brass fluxing. I usually use candles to flux, but I have several pounds of pure yellow beeswax and that's what I was using here. Hmmm.....Thanks much for the comments and help.

Yes I have had a yellow film form and eventually turn to blue.
It indicates you have impurities like zinc in your mix.
Watch as the lead first starts to melt and scoop off the sludge before it gets too hot to melt.
The impurities have a higher melting temperature.
Therefore as soon as the lower melting lead and tin have melted you scoop off the impurities before they get hot enough to melt.

Yes, I've had the same troubles - Green / yellow powder (lots of it) rising to the top and "dirt" that seemed to come out the bottom pour and migrate to the surface of cast bullets, making them look "dirty". Never did quite figure it out, though I "think" at least part of the problem was trying to "smelt" the raw lead in a too small (20 lb) pot and also having it too hot. The Lee bottom pour pots can / will get way too hot.
My problems went away with changing to another lead supply, built a PID temp controller and ladle pouring. Quite happy with my bullets now.
Still have 250 lbs of the under water cable that I've pre-smelted in a big (propane bottle) pot and poured into muffin tins - they all have quite a blue "hue" to them. Maybe Copper ? I don't know.
Link to the discussion I started on Cast Boolits, no real answers though. (You need to Login or Register to view media files and links)

About using sawdust for fluxing: It works, but the burnt sawdust is very fine and more difficult to remove. I have been using wood chips from a router or planer, much larger, less dust, easier to skim off and works just as good as the finer sawdust. 
Joe S

Can you see the “sand and grit” inclusions in the lead you bought?  If so, you ran across a vendor who is unusually uncaring about his product and customers’ satisfaction.

I empty my pot after casting into an ingot mould and generally it’s only the last partial ingot that has any impurities in it.  Even then, most of the crud is a gray-to-black powder that falls out of the pot after the lead is out.

Just at a guess from your description, I would say that you are casting too hot.  I don’t know how accurate the Lyman thermometer I use is, but the range I cast at shows 700-750 degrees.  This temperature gives me a shiny surface after fluxing, with an increasing amount of silver-gray oatmeal on the top as casting progresses.  I push this aside when I fill the dipper, and generally that’s all I have to do when casting large bullets until the lead level is too low for the dipper.  I have little experience with a bottom-pour pot, and that mostly casting small or pistol bullets in multicavity moulds.

My old Organic Chemistry professor told us that whenever you melt something, there is decomposition.  In the case of lead, the decomposition is oxidation.  Lead oxides will keep taking up oxygen, forming a yellow monoxide (litharge) a red dioxide (red lead, or minium) and a number of increasingly oxidized species from lead peroxide through more highly oxidized (and more poorly characterized) oxides, that go in color from brown to gray to black.

The stirring of the pot, both in fluxing and ladling, with the heat in the walls of the pot, will ensure that there is a deposit of these oxides on the pot below the surface, as well as on top of the melt.  Depending on temperature and casting style, people will see “rust” forming in their pots, though without any associated pitting.  This is likely the red-lead oxide, perhaps stayed at this point by whatever fluxing technique is used.  The yellow litharge color can also be a temporary or more persistent impurity.  I’ve seen all these colors, but I’ve found that proper fluxing at the temperatures I cast at always results in a shiny surface and a couple tablespoons worth of charcoal-gray-to-black powder, which are the oxides produced by that particular melt.

So I would recommend preheating your mould, running at a lower temperature, scraping the sides and bottom of the pot when you flux, skimming off the black dust on top, and seeing what happens then.