Brass Patina Recipe?

[Mal Paso]

Ok, so here is a picture. These are the 2 worst cavities. Nowhere else in the cavities is there significant lead except the bottom of the bands at the block face. The grey in the middle, top band, of the right cavity is light from a window. Most of the guys having no trouble are running higher alloy temps. I will try that as well as seeing if the lead will wipe off with a cloth rag once the blocks are heated.

I shot bunch of 429421s yesterday and don't think it's quite the boolit the MP 432256 is so I've got to get this figured.

[longbow]

I gotta say, I am baffled!

I run "hot" when casting with brass moulds and I have not had any tinning at all even in a brand new unbroken in mould.

Having said that, I normally cast wheelweights or range scrap and I add little if any tin ~ maybe a bit of 50/50 solder but usually none.

The only times I have ever had any tinning trouble at all is with iron/steel moulds I have lapped, not re-blued or heat blued then overheated at pre-heat and gotten then a little past the alloy melting temp... oops! In an extreme case, I have actually had to melt out a boolit. Then, yes I have had severe tinning but I just re-heat and wipe it off then let the mould cool down well below casting temperature then cast until it is back up to casting temperature. No more problem. After heat bluing no more tinning.

Oddly, I have not had the problem with brass moulds! Even new shiny clean brass.

Are you running high tin addition? If so then maybe that is the difference but even then with your patina I wouldn't have thought you would get trouble like that.

It certainly isn't that I didn't believe you, I just have never experienced any sort of problem like you are having. A picture is worth a thousand words!

Yes, if you carefully and even heat the mould up to the melting point of the alloy, you should be able to wipe the lead off with a rag or paper towel then let it cool down and cast to bring it back up to temperature.

No other suggestions.

Longbow

PS: One more thought just occurred to me. I seldom prep moulds for casting other than wiping them off with a paper towel or rag... even brand new moulds. I do not scrub with dishwashing liquid, soak in mineral spirits, spray with brake cleaner, etc. I simply wipe them off then preheat until they smoke (which is below the melting point of lead) then start casting. I seldom have many rejects and don't get tinning even in new moulds. Maybe the oils oxidize and leave a "varnish". I also oil all my moulds ~ yes, even brass and aluminum because the sprue plates are steel ~ when I store them.

[Mal Paso]

I'm casting at 660F with alloy just under 3% Antimony and less than 2% Tin. Don't use anything but a little Sprue Lube on the top and pins. It sounds counter intuitive but I will try hotter alloy and see what happens.

You can see how the boolits would hang up rather than falling out of the mold.

[Iron Mike Golf]

At one time you could get acid core solder for plumbing (not sure it still exists on the market). Is it possible there is residue from the vinegar acting as a fluxing agent?

[Mal Paso]

The mold was thoroughly washed in Distilled Water before the heat treat and the mold cast 3,000 boolits before the problem was even noticeable. I can't imagine a trace of any chemical in the cavities.

[longbow]

Mal Paso:

Just after I posted here, I saw another thread with the same problem using a Mihec brass mould!

Yet another thought occurred to me ~ you might try using a graphite or other mould release which should coat the brass and not allow tinning at all.

It is worth a try.

I am not sure I would try casting at higher temperature though since tinning will probably get worse. Heating the mould to remove the tinning, yes, but not casting hotter if you are getting fillout now anyway. Cooler would be better at least to the point that you cannot get good boolit fillout.

There may be something to what Iron Mike Golf says too. A mould release should fix that too if it is a problem.

I would figure with the oxide layer you have that tinning would be next to impossible! Think of intentionally trying to solder something with a layer of oxide like that on it! That would be a no go.

Sorry, that's all I've got.

Longbow

[Mal Paso]

I'm running the Alloy just hot enough to get good fillout. Even, sharp bands all the way around. It may be that the temperatures of mold and alloy are too close together. I you look close the deposits are nodules not continuous and wetted to the brass. More like Sintered metal.

Higher Alloy Temp might get me temperature differential with the mold.

The 432640 isn't having problems but it has a better patina and runs at a higher temperature.