Casting high temp lead?

[giz189]

If you don't have a copy of Lyman Cast Boolitt Handbook 3rd Edition, it is of much value in understanding of the alloy separating or not separating. There is also a section on casting at high temperatures and melting and fluxing of cast alloys. It is very informative.

[rob45]

Is there any problem with casting lead at high temperatures? I cast yesterday at between 900 and 1000 degress to try to get better mould fillout.

There are several disadvantages associated with the use of unnecessarily high casting temperatures. Aside from the issue of vapor pressure as discussed by others in this thread, wear of the equipment is unnecessarily increased. In addition, maintenance of the alloy constituents becomes increasingly more difficult.

Consider this: Pure lead melts at 621F, and, to the best of my knowledge, alloying anything else with that lead will lower the melting point. Nevertheless, all usable lead alloys for bullet casting that I have come across are going to melt at or below 621F and become solid at or above 464 F.

Taking into consideration that the highest melt temperature we're likely to come across is 621 F, we can obtain a general idea of how hot to keep the melt for practical casting.
Not only do we need to have a liquidus state (where all of it is molten), we need to keep the melt substantially above that temperature in order to compensate for the immediate heat loss as the lead is extracted from the pot; after all, it does us no good to have the lead start solidifying before the mold cavity is full. For most, practical experience has indicated that the best temperature range to keep our melt is somewhere around 150-250 F degrees above the actual melting temperature.
Using the highest melting temperature of pure lead at 621F and applying the commonly accepted increase just stated, one can easily see why 775-825F is the commonly accepted temperature to keep the melt in the pot (when casting pure lead). Use of the various lead alloys allows us to run the pot at still lower temperatures, and we should do so whenever possible.
Currently I can think of only two reasons (although there may be others) why one would ever want to run excessively high temperatures:
1. Casting in colder atmospheric conditions- Obviously people doing outdoor casting in colder weather may need to keep the melt hotter, but under those conditions the increased heat of the melt itself will begin to introduce problems concerning oxidation.
2. Excessively slow rate of pour- Individuals relatively new to casting are often amazed (and frustrated) that the lead can become solid so quickly; this is simply due to not yet establishing a suitably fast rate of pour (this is assuming that the mold itself is at a suitable operating temp). Also, those casting comparatively large/long bullets sometimes find that elevated temps (above "normal") make for easier casting.

Conclusion: Unless one is dealing with extraordinary circumstances, it is usually best to maintain the lowest temperature possible while maintaining acceptable casting results. The use of excessively high temperatures almost always adds complications.



I've read an occasional post stating that at high temps the tin is affected. Any truth to that?

The applicable use of tin has long been a topic of discussion among us (bullet casters). We each utilize tin for different purposes.
Irregardless of why we include tin in our alloys, we recognize that it is the most expensive component, plus the fact that it is desirable to maintain alloy content once we do introduce it.

So, do higher operating temperatures have an effect on tin content? Most certainly. In order to gain some understanding of what is happening, and why temperature has an effect, lets go back to the beginning.

From a mining perspective, tin is commonly available in the mineral known as cassiterite, or tin dioxide (SnO2). To obtain tin from the mineral, the chemical reaction that takes place is:

SnO2 + 2C -----> Sn + 2CO

For the less chemically astute among us (and that certainly includes me!), what this means is that if we chemically react cassiterite with carbon, we will obtain tin and carbon monoxide.
Anyone want to take a guess on how our forefathers got their tin? Since I wasn't there, and I'm not a metallurgist, nor a geologist, nor a chemist, I can only speculate. But upon analyzing the formula above, I can easily see where smelting cassiterite mineral with burning coal would produce the tin.
Note: To all the chem geeks out there, I am too computer-illiterate to know how to properly insert the subscript and proper arrow symbol in the formula. But that is of no significance to us, as this discussion simply pertains to boolit casters helping each other.
Interestingly, one can deduce from the equation above that taking oxides of tin (you know, that grey scum they tell us not to skim off) and combining them with carbon will again reduce the tin back into the alloy. Essentially, whenever we use a carbon-based flux, we are doing the same thing that was done to get the tin from the cassiterite mineral.

So now that we know that tin is subject to chemical reactions for the purpose of obtaining it, the question arises- what do we need to worry about concerning our casting temperatures?

Keeping in mind that a lead alloy is nothing more than a solution, the constituents of that alloy (especially tin in our discussion) are subject to chemical reactions, such as oxidation. Oxidation, by definition, occurs by combining something with oxygen. This reaction can take place without our help- for example, leave steel sitting unprotected and the result is ferric oxide, or rust. Leave lead sitting in an unsuitable environment (outdoors, etc.) and pretty soon it will start getting "crusty"- those are oxides.
Knowing that some chemical reactions can occur without aid, we also realize that there are some things that speed up a chemical reaction:
1. Increased concentrations of the reactants.
2. Use of a catalyst.
3. Increased temperature.

So, if increased temperatures aggravate the tendency for tin to oxidize, how hot do you want to run your pot?


Also, does the alloy separate into its constituent metals (e.g. does the tin and antimony separate)?
I am not qualified to give an accurate answer to this. I do know that tin and antimony combine to form the intermetallic compound SnSb, and I also realize that the constituent preference order of oxidation is:
1. Tin (most suspect to oxidation)
2. Antimony
3. Lead (least suspect)

As I understand it, all components of the alloy are subject to oxidation, and by now we realize that our best practice is to limit oxidation by:
a. Limiting temperatures to a low, yet practical, level.
b. Limiting available oxygen to the alloy constituents by use of a barrier; e.g., charcoal, kitty litter, sawdust, etc. Also, the initial layer of oxides themselves serve to reduce further oxidation.
c. Minimized disturbance of the melt- aggressive stirring introduces oxygen into the melt; this is also why the ladle caster needs to flux on a regular basis.


When removing the dross, might I really be removing the tin?
If you understood everything above, your question is already answered. If you did not, then the answer is yes, more than you think.

Thanks in advance

Hope this clears some things up.
And to the metallurgists and chemists out there, please take it easy on me regarding any inaccuracies; I have not had to think in this way for nearly 25 years.

Good Luck.

[prickett]

Hope this clears some things up.
And to the metallurgists and chemists out there, please take it easy on me regarding any inaccuracies; I have not had to think in this way for nearly 25 years.

Good Luck.

Wow! Thanks for all the great info everyone!

[fredj338]

I cast w/ several diff kinds of alloy & never have to resort to temps much more than 750. Your technique &/or mold are in question. I "pressure" cast w/ hand held molds. That is putting the pour spout into the sprue plate & filling the mold. When full, back off a tiny bit to form the sprue. THis always results in good mold fill-out, even w/ HP molds. Make sure your molds are clean. I find heating htem up a bit on the top of the pot, then hitting them w/ degreaser works to remove any oils.

[sagacious]

Gravity separation can and will happen when there is significant antimony without tin as a binder. You could never dip off pure antimony but definitely could dip off a very concentrated mixture. Even at 6% antimony tries to float some off till you get the tin up to about 2%. I'm not talking about dross from oxidation but the oatmeal looking stuff that you can make a hole in with a spoon and watch it fill with liquid lead.

This is not separation caused by gravity, or differences in specific gravity or density. It's a separation caused by antimony's fickle solubility in lead, especially-- as you note-- in the absence of tin. For example, molten Pb60/Sn40 solder will not 'gravity separate' into a tin-rich layer on top of a lead-rich layer. The reason is due to the miscibility/solubility of those two alloy components, even though the density of lead and tin is quite different.

"Gravity separation" is a myth that keeps getting repeated. Let's put it to rest:

It is a common misconception that because they are less dense than lead, antimony and tin may undergo gravity separation from the melt. Nothing could be further from the truth. In the absence of oxygen or oxidizing materials, melted lead alloys will remain stable and mixed virtually forever. And from Lyman, [3]Perhaps the single most significant error in all the bullet casting literature is the misconception that lead-tin-antimony alloy melts gravity segregate.

Beginner and advanced casters alike can read the whole article here: http://www.lasc.us/CastBulletAlloy.htm

Perhaps the single most significant error in all the bullet casting literature is the misconception that lead-tin-antimony alloy melts gravity segregate. Bullet casters have been lead to believe that unless they flux a melt on a regular basis, the less dense tin and antimony will separate from the lead and rise to the surface where they will no longer be available to harden the alloy. This is absolutely wrong, and in fact quite the opposite is true; tin and antimony, either as individual additives or in combination, literally dissolve in molten lead to form true, stable solutions, just as table salt or sugar will dissolve in water. And, with the exception of oxidation or an electrochemical potential, once the solutioning has occurred, there is no force, gravitational or otherwise that can separate the constituents.

I'm sure you know this, but I think it's important to note it here for those still learning.

[MtGun44]

Vapor pressure cannot be in units of "parts per million" it is in units of pressure.

I think the units you are looking are Pascals, a proper metric unit of pressure. You cannot
predict the parts per million unless you know air exchange rate in the room and the
surface area of the molten alloy and the partial pressure, etc. It is not something that
you could "look up", since it is a concentration in the air not a basic property of a
material like vapor pressure vs temperature is.

IIRC, the vapor pressure is very, very low at any remotely resonable melt temperature,
like a fraction of 1 mm of mercury (a pressure measurement; 760 mm of Hg represents
normal sea level air pressure of 14.7 psi)

Using very large fonts is the online equivalent of screaming and is not appreciated in
the polite discussions that we have on this site. Occasional caps is resonable for emphasis,
but we need to be polite and respectful.

Bill

[lwknight]

I at least agree entirely that tin in lead is like soap in water. Once you mix it , you cannot unmix it without fractionalizations or other chemical precipitant process.

Antimony in lead is more like sand in water. Except in reverse. There will be a lead rich layer in the bottom and an antimony rich layer at the top. I do concede that it will not totally separate but, will partially segregate.

[AriM]

Hope this clears some things up.
And to the metallurgists and chemists out there, please take it easy on me regarding any inaccuracies; I have not had to think in this way for nearly 25 years.

Good Luck.

Golden....Your excellent post, simply re-enforces, what I already believed to be true. Makes me feel better about my current state of knowledge. You, sir, are a gem among the slag.

[Spudgunr]

Vapor pressure cannot be in units of "parts per million" it is in units of pressure.

I think the units you are looking are Pascals, a proper metric unit of pressure. You cannot
predict the parts per million unless you know air exchange rate in the room and the
surface area of the molten alloy and the partial pressure, etc. Bill

I am well familiar with Pascals. In fact, if you double check, I gave the partial pressure IN pascals. I then converted that to parts per million (which is easy to do since we know that we are usually close to 1 atmosphere of pressure), and I think I even gave the disclaimer that yes, that is at the molten leads surface and that it will take some time for it to disperse around the room.

1000 - .01 PA (.1 ppm)

See, .01Pa (ok, I capitalized the A when I shouldn't have, my bad).

To be fair, the vapor pressure concentration is at the surface of the molten lead. The vapor will slowly permeate the room, but would take a long time for the entire room to reach the above levels

And again, my disclaimer saying that the concentrations would be at the surface of the molten lead.

IIRC, the vapor pressure is very, very low at any remotely resonable melt temperature,
like a fraction of 1 mm of mercury (a pressure measurement; 760 mm of Hg represents
normal sea level air pressure of 14.7 psi)

And even a fraction of 1mm of mercury is a whole lot of parts per million. Just because the number is small doesn't mean it is benign.

Using very large fonts is the online equivalent of screaming and is not appreciated in
the polite discussions that we have on this site. Occasional caps is resonable for emphasis,
but we need to be polite and respectful.

I was TRYING to be screaming when I mentioned a number that is 60 times OSHA's PEL, that was previously stated to have no lead vapors, indicating that there is no harm in it. While I will be respectful, I will NOT be polite when it comes to matters that are clearly dangerous. Safety must be emphasized. If I were in the reloading area and somebody stated that you could just swap components and keep the same charge I would likewise be "yelling" (emphasizing with bold and large font).

Would anybody else now like to de-emphasize the point that lead DOES make vapors at a considerable level not much above our working temperatures? Again, I've made the personal decision to cast indoors, but not at super high temperatures, because I can see by the math once you start getting up above 800 degrees the lead vapors go up quick.

[Spudgunr]

And to address the tin thing. I'm not a metallurgist, I'm a chemical engineer. I cannot address the separation component. However, you can and will deplete your tin still. Your tin will oxidize on the top surface of your lead. If you skim that, you will now expose the metal again to oxygen, oxidizing your lead and tin. But, tin will oxidize faster. Skim and repeat until your tin concentration is significantly depleted. That is what fluxing fixes, it reduces the tin oxide back to tin, then you can stir it back in to the solution.

So, no, it won't gravity separate, but it will deplete over time, worse at high temperatures.

[lwknight]

Botton line is that there is just no sense in casting at 1000 degrees. If you cant get the job done at less than 800 degrees , something else is WRONG!

[MtGun44]

Spudgunr,

I appreciate and agree with your final point, and heartily recommend not heating lead
alloys to unecessarily hot temperatures.

I think the ppm calc is invalid as far as having any useful relationship about how much
lead is in the room air when casting at too high a temp. Thousands of casters have regularly
had their blood lead checked. The cases of lead levels higher than safe are always
traceable to ingestion rather than breathing vapors.

We need to take lead safety very seriously, but we need to focus on the things that are
really likely to make someone sick.

So - you are 100% correct - it is not smart to heat lead alloys above 800F or so. However,
the real world situation is that very few people do this and the real and serious risk that we
need to educate people about is eating, drinking or smoking around the casting or reloading area.

Bill