Zinc why is it bad?

[steif]

Hi,
I've been casting on and off for years and am positive I have had some zinc off and on over the years, what does it do to the boolits if it's contaminated? I've got 1000 lb or more of various ingots, scraps, etc.. how can I tell if it's contaminated or not?
I've never had many problems casting other than the frosted or wrinkled not filled out etc.. but once the mold heated up and got to the right temp they smoothed out.. cast a few hundred once and I remember and some were frosted like a matted finish and they seemed to work fine in the sizer and shooting them, so what would I look for to tell if there's zinc in them thar ingots??

[docone31]

I have been wondering this myself.
I have melted wheel weights and molded for years. Mostly for pistol.
From my readings on contaminents, zinc makes the bullets hard. This interferes with rifling.
I really never noticed it with my pistols. I also never had any reliable way to measure it. I shot IMHSA. Hit the steel, down it goes.
I really turn up the heat in my pot, so a lot of contaminents float. At least what is not stable to the alloy. I have run some real junk through my barrels. I did not notice excessive wear and I shot quite a few loads.
I will be interested in following this thread.

[trooperdan]

If your melt looks like silver oatmeal, it has zinc in it! It just forms kind of "mush" that just won't act right!

[Buckshot]

................Zinc radicaly increases the surface tension so the alloy becomes sludgy, ergo poor fillout and other problems. Tin on the other hand is like detergent in water. Makes it wetter. However you can't add tin to a zinc contaminated batch of alloy to fix it. It only takes a very small percentage of zinc to screw things up bad.

...............Buckshot

[DLCTEX]

I recently tested zinc contamination and reported on it in this section under "Wanted: Contaminated Lead". The zinc does not incorporate into the lead easily and wants to stay on top of the melt, but if fluxed and stirred in it will rise to the top and can mostly be removed. It will then cast good boolits, but must be run hotter and seems to clog the nozzle on bottom pours unless kept really hot. I think for you guys who ladle, it would present more problems as the zinc just keeps rising. With my bottom pour, I just let it float and dump the last of the pot. The boolits are bright and showed frosting in spots that were cosmetic and rubbed off easily. The boolits were not brittle and mashed uniformally when hammered. It would be best to keep zinc out, which is not hard as it melts at a higher temp and floats long after lead has melted, but does not seem to be an end all event for at least the bottom caster. DALE

[Mumblypeg]

So in other words, as it forms on top just skim it off as best you can and go on casting? I've been casting for years and never knew what it was untill I got on this site but as best I can remember I always pulled off the weights that didn't melt right away as I thought something wasn't right. It all makes sense now...

[Ricochet]

Yes, you can skim it off as long as you keep the alloy temp well below the melting point of zinc (787°F, for pure Zn) and let it stand. Along with the high surface tension, the zinc crystallizes out in the molten lead at temperatures below that, but well before the lead alloy solidifies. So it can clog pour spouts, and in the mould cavity it will crystallize and start to float up toward the sprue plate, leading to unbalanced boolits.

We've been misled that stuff floating up toward the top is tin separating out and needs to be blended back in by fluxing and increasing temperature if necessary. Tin doesn't separate gravitationally from lead, it's all liquid and as fully miscible as alcohol and water. (Tin and antimony can oxidize out of the alloy, which is why printers had to refresh their type metals periodically.)

Not everything that floats and looks metallic is zinc, though. Antimony melts in the pure form at 1170°F, and in high-antimony type metals (higher Sb content than linotype) there's a mixed phase as it heats up where antimony crystals form a sludge in the liquid metal. Wouldn't want to skim that off.

[Mumblypeg]

Thanks Rickochet

[leftiye]

Kinda in line with what Rick said - If you get the "oatmeal effect" - zinc mush on the top of the melt, you can actually set the mush on one side of a pan (skim it off) with the bottom of the pan tilted, and the mush stuff on the high end of the pan, and useable alloy will separate and run out of the mush. More if you Keep it about maybe 600 degrees.

[dakotashooter2]

I think one can also be fooled into thinking that the mush is zink contamination. I don't believe this to always be the case. Inconsistent heating of the pot can also cause this. With a large (wide) pot on the burner the top edges of the pot may not maintain heat as well as the center section of the pot and often a "sludge' will form along the edges. I have on occasion held the thermometer allong the side of the pot then again in the middle and found significant temperature variances. I have also experienced this when my lee pot emptied to a certain level (possibly due to a bad element?). Mixing this "slulsh" from the side of the pot into the center of the pot and melting has yet to cause any fillout, hardness or brittleness of my boolits. The stuff that remains on top I do skim off.

[docone31]

I can now tell what zinc does in a Lee 20lb pot.
I accidently mixed in some zinc, I believe. At first it poured well. I like to keep the pot hot. As the molding session went along, mind you I was also breaking in a new mold also, the spout slowed to a drip, then went off.
The spout was slowly getting clogged with zinc oxidation! Yellow oxide came out of the spout when I probed it.
On top of the pot, the lead melt was turning Harley Purple, the colour of chromed headpipes. Then it started turning yellow. As I stirred, thinking it was just a small amount, I got a congealed mess.
I still have the mess on top of the pot, I ran the temp up real high and fluxed it several times. I plan on tossing in some borax on start up next time. The borax does not melt, it just loosens up and the oxides seem to stick to it. At 7 on the rheostat, the top layer in the pot seemed to be thick. Zinc. Just like someone said here on the forum. Like oatmeal. That is how I recognised it.
I am counting on the remaining zinc in the pot, raising to the top on next heating, then congealing on the surface.
Live and learn.

[steif]

Thanks!
That explains a lot, I don't recall ever seeing a oatmeal like slag on top of my pot, so maybe I've just been lucky. I still use the plumbers melt pot for big jobs and melt them down to ingot size for the electric pot I've got.. I'm thinking on firing it up for a meltfest in the next few weeks, I've collected a bunch of scraps here and there and the strangest thing I've got is a coffee can that was poured full of lead for some reason, I think the guy said he used it for a weight on his tent when he was camping... I'm planning on just inverting the can and melting the lead out of it. I've cast a few in the lee pot and it does have some issues...lol.. the purtyest I've ever cast was from that big propane pot.

[chrisx1]

Here is a picture of the ....oatmeal....


http://i264.photobucket.com/albums/ii174/chrisx1/DSC02433.jpg

[bushka]

Great subject!
How does the Tin element tend to seperate?
I would think it never does as long as there is melted Lead because Tin melts first?

I slagged off some of that oatmeal with the dark grey crumbs last session,and it is very hard.Fluxed with marvelux.
I had to shake the dipper to get it out of the nozzle.

[Ricochet]

Tin doesn't separate. It can oxidize faster than lead, though.

[vernm]

I have had oatmeal before and was able to get rid of it by raising the temperature and fluxing three or four times with a heaping table spoon of fresh pine sawdust, wet with resin.

While melting a 50lb block of stereotype (high antimony), I noticed the sluge Rickochet was talking about. Not oatmeal, but thick, not runny. I was using a cast iron pot on a plumber's furnace. As the temp came up, the melt turned silver and runny. Scared me a little. But, all turned out well after a little stirring and heat build up.

[badgeredd]

I sure appreciate you guys describing the appearance of the zinc contamination.
I have some lead ingots that I smelted up years and years ago. I melted a bunch of scrap lead up over a wood fire coal bed just to get it done while I was watching a brush pile burn up. Obviously I was just melting and had no control over the temperature I was getting. Really didn't know anything about contaminates then either. Werll I started the lead pot up last weekend and noticed some yellow/ gold colored skim on the melt. So being aware that it didn't look quite right and matched what I'd read on here, I turned the post down to 625 degree range. Lo and behold an oat meal slush started appearing on top. I let it sit for about an hour and skimmed off all the yukky stuff. Fluxed it well and stirred the skimmed pot and let it sit for another hour. Skimmed of some more slush but not nearly what I had the first time. Then I ran the remainder into ingots (which I marker VERY well and put them aside. I have about 50 pounds of the stuff to go through and then I'll use it for fishing weights and such. Thanks to you all for the descriptions so I knew what was going on.

On a side note, I also found out that the green copper oxidation from wire will combine quite readilly with lead at high temperature. Don't ask me how I know! But at least I am aware now so I don't ruin a melt of any size.

[carpetman]

I zinc it's bad because it aint tin with the other stuff.