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Brass tends to hold the residual heat from the molten lead longer than Aluminum.
May one ask how many casts do you normally do a minute…?
You have stated in your previous posting… that apparently you have tried to lower the heat on your lead pot, & most probably have gotten to the point that the lead is now beginning to cool before you can fill the mould completely…
This most probably is a potential cause of the apparent wrinkles that you are seeing when you go lower in temperature…
Thus I tend to believe that this is what is causing the wrinkles that you are seeing… at the lower temperature.
Too slow a fill rate even with the lead temperature very high will cause the same wrinkling effect…
That balance between fill rate, wrinkles, & lead frosting… is the point that I believe that you are looking for if shiny bullets is your ultimate goal.
A lot of casters tend to have more than one mould of the same type… so as to let one mould cool slightly in between fillings… to help prevent the mould from being too hot and thus producing the lead frosting…
Some go for more cavities vs the individual casting size so that the mould is not heated too quickly from a large cavity fill in a relatively small mould block.
Lacking a second mould of the same and general cavity size… you might try just slowing down a tiny bit to allow the mould to cool a bit in between casts to see what that gets you.
I also have a metal plate that I can set the mold down on for a moment in between casts if I start to see frosting beginning to appear… on my smaller mould blocks… because of a casting rate of 4 to 6 pours per minute…
Even 5 to 10 seconds sitting on that large metal plate is enough to do the trick for me…
That seems to help me in my case… though I realize that it may not be everyones meat and potatoes…
I know a lot of bullet casters do like Brass… Personally I prefer Aluminum for the reason that it cools off a tiny bit quicker, & seems to work better for me & my casting style…
Also Aluminum tends to weigh a whole lot less when doing an extended run… for a hour or two… with a 4 cavity mould… For me I get a better production rate, & far less rejects with Aluminum…
I’m not trying to start a discussion on the subject of Brass vs Aluminum… nor hand casting vs machine casting.
Each mould material & process as its assets and drawbacks…
We each have our own preferences …
Take Care & Stay Safe …
Mike B.