Originally Posted by
prickett
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