Talk to me about what forms on the top of the melt...

[Harry O-1]

For many years, I would melt the lead/tin, flux it, and cast a while. When the surface would get black (oxidize), I would flux again. When casting hot (which is most of the time since I cast a lot of hollow-base bullets), this could be as little as 10 minutes and certainly less than 15 minutes between fluxing. Then the mould and pin would have to get heated up again after fluxing, which lost several bullets before I got keepers again. While doing that, the surface would start getting black again. There was entirely too many rejects this way.

A few years ago, I decided to try boric acid on the surface. It works in protecting the surface from oxidizing. I could cast until the pot needed to be filled again, usually 5 or so pounds (half bullets and half sprue). I would scoop up what boric acid "glass" I could, put in the sprue and new lead/tin, melt it all down, flux it, and pour some more boric acid on. The problem with this is that the boric acid "glass" always got on the walls and built up, insulating them, until I had to take some coarse files to break it off. This was ugly work.

I decided to see what happened if I did not flux, did not use boric acid, and emptied the pot. What I would like to know from the people here is what is happening at each stage and what it means to the mix.

Here is what happens with my bottom pour pot. After fluxing, the surface starts to turn black as usual (from what I read, that is probably tin oxidizing on the surface). If ignored, the black will get darker and heavier. Eventually, it starts getting thicker (but still black). It starts getting a rough surface. Continuing on, it starts to get even thicker and starts lightening the color. It becomes a yellow-brown color and looks like a bunch of very small popped popcorn packed together. When in this state, it bridges the pot from wall to wall and stops moving. The lead/tin continues to get sucked down and there is a gap between the melt and the yellow-brown stuff. I can completely drain the pot. The yellow-brown stuff doesn't move once it bridges. When I break it up, it is about 3/16" to 1/4" thick and comes apart as a powder. It feels much, much lighter than lead or tin. It is easy to break up, clean off the walls, and thow away. What is it made from?

I have no doubt that there is tin in there, but I question how much since it feels so light. I don't seem to have problems with casting towards the end (this is with 2% to 3% tin mixtures in pure lead) like I would expect if I lost a lot of the tin. I have plenty of tin on hand. If replenishing it is all I have to do, this is a much easier way to cast than the first two ways I mentioned above. Can anyone here lead me through what is happening and how to quantify how much tin is being lost?

 

[JWFilips]

Interesting Question: For me ;
When I fill my bottom pour with my alloy ingots they have been really scrubbed in the smelting So I would hope they are clean. Once in my bottom pour I melt them and once up to temp, I reduce with Bees wax only to get any tin misplaced back in. Sometimes I will scoup off a bit a dusty grey matter other then that, the bee's wax forms a brown to black crust which I will ride all the way down to 1/3 pot...then I clean & i slowly add more clean ingots and start all over but will really scrape the sides and bottom then Reduce oonce with Bee's wax, clean the grey scrum, then drop another piece of wax to seal the top again in a black crust ...then ride that all down again to 1/3 pot. Sometimes I get some inclusions but it isn't dirt or oxide it it that bee's wax crust! ( I hope!)

 

[RicinYakima]

How clean is your pot? Have you emptied it and boiled soap and water in it to clean all the water soluble salts out of it? Black is not tin, tin oxide is yellow or golden depending upon temperature. It sounds like you have dirt/carbon in your pot, more than anything else.

Please be aware that the boric acid and Marvel-lux stuff liberates chlorine gas and tends to form hydrochloric acid. The concentrations are below your sense of smell and are harmless, BUT if the steel and iron in your casing area is cold, it will condense and rust everything is sight. Bill Ferguson and I had a long discussion about this on the old cast bullet forum about 15 years ago. We both live in deserts, so it may not be a problem if you live with high humidity all the time.

p.s. Nice to see you here, hope you have lots of posts, as I like 41 Colts, 30 WCF's, etc.

 

[Ian]

I'm no help, never have I had any black stuff form on top of my melt. When casting with soft scrap like roof flashing or boots and old drain pipe I will get little "flowers" of yellow and light brown stuff that resembles the mini-popcorn described, but only a few specks and a little sawdust and a stir and it's all reduced back to elemental lead.

Marvellux, or "Marvacrap" as many of us here are fond of calling it, is actually a true flux and will adsorb and remove just about any metal oxides. It is NOT a sacrificial reducant, though, and will remove whatever it adsorbs when you skim. My recommendation is to simply put about 1/4" layer of sappy, coarse, yellow pine sawdust on top of your melt when you are ready to begin casting and leave it alone until you're done. It will smolder down to a layer of ash that seals the surface from further oxidation. When your pot gets low, throw in another dose of sawdust, light the smoke with a BBQ lighter, stir and scrape the sides of the pot while it's burning, skim everything when the fire dies out, add fresh lead, and repeat.

 

[Rick]

I've not seen what your describing either, makes me wonder what your alloy is. One thing is certain though, it's not tin.

Anyone tried bringing Marvacrap into my shop there would be a heckava fight. Heed Ian's advice and use sawdust for both fluxing and as a cover layer.

 

[Ian]

One caveat on the sawdust/ash layer: Don't throw sprues or culls back in while casting. They'll catch ash and drag it to the bottom of the pot where it gets trapped and will migrate to your spout, causing dross inclusions in your bullets.

Most of the time, using clean lead with only little tin in it and no protecting surface layer, a thin layer of dull gold/bronze colored oxide starts to form, eventually turning navy blue to purple as time goes on. It is just a skin, and doesn't seem to grow much in thickness once formed. That's what I've come to expect from casting with pure lead or alloys like 30 or 40:1.

 

[Rick]

Also you said you were casting hot, no need for that unless your using pure Pb and it sure doesn't sound like you are. Whatever is in your alloy the hotter you cast the more and the faster you'll get oxidation. As hot as I cast anything is 725 degrees and only that hot for small HP's, normally I'm at 690 to 700 degrees. Casting any hotter is tough on the alloy, slows down casting and makes it harder to get good bullets. Cranking up the heat is an old wives tale and no good comes from it.

 

[Brad]

I never get black stuff except little bits of wood chips that got under the melt. Those are rare now too as I changed my fluxing technique to eliminate them.
What I get is, over time, a lumpy dross on the surface. Not a bunch but over time it does accumulate more and more. It is more noticeable since I went to using a ladle. It tends to build up a bit on the ladle but a quick hit with some wax and it is gone from the ladle.

What is it? I think it is a mix similar to the melt itself. It is oxides that form a dross and the ladle casting makes it worse as I increase the air/melt terrace every time the ladle goes in and out of the melt. The lead running off the mould back into the pot also increase dross formation.

I have seen a reduction since I changed my temp on the PID from 720 to 700 F. Lower temps mean slower dross formation.

What I would love to be able to do is float a thin layer of paraffin on the melt surface to reduce the effects of oxidation and keep me ladle cleaner. What scares me is the chance of getting paraffin in the mould.

 

[Brad]

I may just fire up the pot and doing a little experimenting. Maybe skim the stuff off the top and have it analyzed along with a sample of the stuff in the melt. See if there is any significant difference in composition.
There is a guy on CB doing xrf tests for the cost of a pound of lead. Pretty low cost to get some better information.

 

[Ian]

I seem to remember some tests done on CB that indicated that reduced dross had about the same percentages of components as a sample of the alloy from which the dross was produced. Personally, I've done some simple specific gravity tests on reduced dross and found it always a little "lighter" (less mass per unit volume) than the base alloy, indicating to me a higher concentration of tin and antimony in the oxide scum. YMMV.

 

[James W. Miner]

My word for the stuff on top is just JUNK! I don't think there is much of value in it. Just oxidation and if you bottom pour, Ian has the best answer.
I ladle cast and set my Lee at 750 and for pure I go to 8 but never measured with my thermometer. Once the pot shuts off there is a lag time before it comes back on so it might change. The Lee with the remote thermostat is best. Seen Lyman and RCBS pots with so much lag, lead solidified before coming back on. Sticking a bi metal thermostat right on the pot will cause failure down the road.
I might flux less then anyone too. I just swirl with the ladle bottom and get lead in it from under oxidation. I never stick the nose of the ladle under lead and keep it clean with a cotton rag, I keep the hole clean by tapping the back of the ladle on my vise or the coffee can I use for skimming s.
I change alloys many times for testing and find a pile of junk in the pot bottom. For some crazy reason it does not float. The junk will migrate to the bottom spout and gave me fits so I never got a bottom pour to work right. I hated cleaning the spout and the mess.
I keep the ladle spout tight to the sprue plate, no leaks allowed and no mess. I fix a leak. I do not pour lead all over my molds.
You might reduce something from slag but I have not found it true. Some save slag and remelt it but I find nothing in mine but black dust. It keeps oxygen from the surface by itself.
I would not get anal over it.
I do not think tin or antimony is leaving solution to float on top. It is an alloy that takes special processes to separate.
I use wax to flux and have for well over 60 years, still forms carbon, just stir it deep. Maybe carbon from wood will suck up stuff but WHAT stuff? It is like a carbon water filter, holes to pull in much junk but is it your tin too?

 

[Rick]

Jim, I highly recommend Glen Fryxell's book chapter 4 on fluxing. Tin is reduced back to the melt with saw dust, not removed. No tin and antimony do not separate but they do oxidize on the surface of the melt when in contact with air (oxygen).

Ian, yes I did a test on what was removed during fluxing. I saved all the burnt sawdust from fluxing until I had a full 3 pound coffee can full, melted out of this a four pound ingot that tested very nearly the same as the base alloy.

 

[James W. Miner]

I have read it but my can of skim has nothing in it at all but dirt. What do you skim?
Pull the dirt to the edge and let lead run out. Why are you removing lead?
Tin and antimony will not float to the top to be removed with skimming. If true you would have tin or antimony inclusions in boolits. To take the metals apart is beyond what we do.
Bonds are not broken or we would breathe CO2 instead of oxygen.
I would love someone to tell me his skim is tin.. The alloy does Not part company.
if you scoop out lead when skimming and find it the same as what is in the pot. What is proven? You lost nothing!

 

[Harry O-1]

To answer some of the questions:

1) Yes, I have to cast hot. I cast mostly hollow-base bullets and they need a LOT of heat to get the skirt to form perfectly. Lower heat will invariably cause a "hairlip" which means a reject. When casting flat-base bullets without a lot of intricate grooves, I cast at a lower temperature. The only difference between the two is the length of time it takes for the black streaks on the surface to form.
2) I bought approx 1,200lbs of salvaged roofing sheet about two decades ago. I am down to about 700-800lbs now. It was reportedly pure lead. I had some analyzed a while back (by a friend back when I was still working) and it was essentially pure lead with trace amounts of several other metals (the usual suspects). Its Bhn is 4.5, which is lower than what most scrap lead is, but higher than chemically pure lead.
3) I use 2% to 3% of tin with the roofing sheet for hollow-base bullets. It has a Bhn of 7.5 to 8.0 when cast it into bullets. It does a great job. I have other mixes with WW, monotype, and X-ray lead to get any Bhn I need. Usually, those are at lower heats, since they are used only with flat-base bullets.
4) I live in a place that has high humidity. There is rust on the sides and bottom of the pot from the first time I heat it. There is no getting away from that.
5) I have a lot of beeswax and use it to flux, when I flux. It melts on the surface of the mix and smokes. Sometimes it self ignites. It it doesn't, I light it with a match. When it burns, it does not smoke. Smoke makes the wife unhappy. After it burns out, I run the spoon down the sides, and then stir the mix. I scoop out the junk that comes to the surface and collects in the center. The surface of the mix is now glossy silver in color. I am unsure how a layer of beeswax could protect the surface (as was mentioned above). My beeswax either smokes a lot or burns. Nothing inbetween.
6) I tried Marvelux many years ago and quickly dropped it. It did not protect the surface like boric acid, but it built up on the sides like boric acid. When I took a coarse file to the sides of the pot after using Marvelux a while, I uncovered a bunch of layers like tree rings. I still have most of a bottle of that stuff that I won't use.
7) I have never tried wood chips or anything else that would burn to lay on the surface of the melt. I get enough grief about the smoke when I flux with beeswax.
8) After fluxing, the surface is shiny silver. Then it gets dull silver. Then it gets duller and darker. Then it starts getting black. This has been since I first started casting approx 25 years ago. Eventually, the entire surface is black. It was a VERY thin surface coating. I always assumed it was tin and then fluxed at this point. Most of it would go back into solution and the surface would be shiny silver again. It was not until recently that I decided to see what would happen if I did not flux at that point. Interesting that no one else gets the same color. I have no explanation.
9) I now use a Lee 20lb pot. I don't have much problem with plugging in the bottom-pour spout. That might be because I am running it at a higher temperature than most here. Just a guess. I don't have much problem with leaks either, since I added a weight to the rod that raises and lowers.
10) Since using boric acid, I got out of the habit of dropping the sprue back in the pot. I wait until it is ready to be recharged and dump it in with whatever ingots I need to bring it up to 10lbs to 12lbs. The 10lb down to 5lb range seems to be (roughly) the best range for getting good bullets for me.
11) From what people are saying here, the yellow-blown color I am getting at the end might be from tin. I will weigh what I got from my last casting session and see what it came up to. I will also check G. Fryxells book.

 

[fiver]

I think it may just be something simple like the wax.

I rarely flux or reduce and I gave up on putting stuff on top of the pot long ago. [tried kitty litter etc]
if the oxides just sit there, they prevent further oxides from forming anyway so why bother?
I just keep the pot full [which keeps the temperatures level anyway] and occasionally scrape the whole pot down, mash everything against the side of the pot [light some wax on fire in the mix] and toss the leftover junk on top into a bucket to be melted down later [when I melt down a new batch of lead/ww's or whatever]

 

[Rick]

I have read it but my can of skim has nothing in it at all but dirt. What do you skim?
Pull the dirt to the edge and let lead run out. Why are you removing lead?
Tin and antimony will not float to the top to be removed with skimming. If true you would have tin or antimony inclusions in boolits. To take the metals apart is beyond what we do.
Bonds are not broken or we would breathe CO2 instead of oxygen.
I would love someone to tell me his skim is tin.. The alloy does Not part company.
if you scoop out lead when skimming and find it the same as what is in the pot. What is proven? You lost nothing!

Jim, your not paying attention. NOBODY has said the alloy separates. NOBODY! Separation as your talking about does not occur. However . . . Separation and oxidation are two completely different things and Oxidation DOES occur.

 

[fiver]

adding carbon to the alloy starts a process called carborization, this is why most recommend that you use some type of wood and char it down while stirring it in the mix.
this adds carbon to the alloy which acts as a 'binding agent' to the alloy's constituents.
the trick is getting the carbon back out of the alloy so it doesn't cause inclusions.
mostly it will ride thermal heat currents to the surface and will then float, fine particles of dirt and brake dust do the same thing eventually.
this is why I turn the heat up when actually doing a good fluxing and oxide return on a batch of alloy.

 

[KHornet]

To add my two cents to this thread. A few years back I read (someplace) That sturing with a soft dowel will add all the carbon necessary. So I tried it ILO sawdust and sturrring, and got the same results with just sturing with the dowel. That's what I have been doing for a number of years now, and am satisfied with the results. I sturr when the pot gets half gone as well.

 

[Rick]

If you stir with a wood stick how do you prevent pieces of charred wood from flaking off and becoming trapped below the surface?

 

[Ian]

If you stir with a wood stick how do you prevent pieces of charred wood from flaking off and becoming trapped below the surface?

Don't ever poke the stick deeper than halfway to the bottom. Use an SS teaspoon for scratching around below that.

Dumping culls, ingots, and sprues back into the pot causes inclusions too. The "skin" of the alloy bits is composed of oxides just like the scum that forms on the surface of the melt. That skin won't melt back in, it has to be chemically reduced at the surface. Since the solids sink to the bottom of the melt, the skins that remain after the solids re-melt are often trapped below the surface tension of the alloy on the bottom of the pot. Then the oxidized stuff migrates to the spout and forms inclusions in the bullets. People are often mystified how dross keeps forming on the bottom of the pot, that's one way, and why we need to scrape the bottom really well after dumping a bunch of alloy back into the pot.

Ladle-pourers don't have to worry much about stuff on the bottom, obviously. One more thing: If you feed only molten alloy to your casting pot from another pot, you'll never get dross on the bottom.