Frosting?

[Irascible]

What causes frosting only on localized spots? I'm casting with a 4 cavity H&G #51. I haven't used it for a few years and I don't remember this problem. It seems to be mostly at the bottom of the grease groove and is not deformed or sunken in. The alloy is the same as always, 10Lbs WW to 1 Lb of 50/50 solder.

[Le Loup Solitaire]

Hi, The frosting is not heat frosting, i.e. the melt being too hot, but more likely rather some zinc is present....probably just enough to cause it. This spot frosting is caused by interference with the surface tension. The source is probably in some of the WW that is in your mix. Getting rid of zinc interference/contamination is a real PITA. If it gets worse then you will have the sunken spot syndrome or the rounded band version. When it is not severe such as in your case some casters just ignore the frosting and cast thru the metal involved until the condition goes away. The frosting has no effect on accuracy. Others continue to flux, skim the dross untill eventually the zinc % drops to a lower level where bullets are acceptable. Other casters who are afflicted with this prob try diluting the zinc to a lower point by continually adding known clean metal until again the zinc level is knocked down to where the bullets are ok. Some folks simply throw the alloy away and start over, but unless the contamination is severe I would consider this as a poor choice since mild zinc presence can be dealt with. Best way to deal with this problem is in the preventative stage; if possible watch out for very light WW's or strange looking ones when smelting. Hope that this helps. LLS

[Irascible]

Well, I thought that could be a possibility and for 4 days in a row I remelted at 500 deg, fluxed and skimmed the top of the melt then refluxed. I was hoping if it was Zinc I was skimming and that I would get rid of it and not the other components of the alloy. I guess I'll do more of the same!
I, however did notice that when using the same smelted batch of WW to make a 50%WW/50% lead mix for hunting that It really cast well. That,I guess, adds credence to your idea of diluting the Zinc even more! Thanks.