Interior residue in mold?

[bslim]

I'm looking for some help with my 38 cal molds. After casting 5,000 boolits, I noticed a interior build-up along the joint line between the two mold halves. At first I thought it was just a lead build-up, but when I tried to remove it, no luck. Reheated the mold to casting temp. and used a Q tip, wouldn't touch it, then a soft cotton cloth, that didn't work either. Then i tried a hand held propane torch to give it a little bit of extra localized heat and even that didn't soften it up enough to wipe off. On a last ditch effort, I took a dental tool and gently scraped it off in small layers. Didn't scratch the mold! But, just a plain lead build-up would have come off much easier. My melt temp is at 720 degrees and from what I understand you would have to have the mold at over 800 degreed to be able to Tin it ? This happened on both my brass molds and my iron molds. Any ideas?

[DLCTEX]

Maybe someone lubed the mould with bullet lube and it has carbonized?

[shotman]

bet you are ladel pour and its the flux. it dont look like it gets in the pour but it does

[bslim]

When I scraped it off, ( the residue) had a nice shine to it. I'm casting from a Magma 40# bottom pour pot. The molds were new, cleaned with dish soap and hot water, rinsed and dried in my oven. That's why I'm a bit stumped?

[Calamity Jake]

Has to be something in your alloy. Empty your pot and clean it good then remelt alloy and flux real well, when you think it's clean flux again.

[dragonrider]

Do you lube the pins, could be lube that has migrated from there.

[MikeS]

Is it possible that you got a small amount of zinc in your alloy, and that buildup was zinc?

[shotman]

If its a steel mold oven cleaner will work . If its AL mold use and scrub with old tooth brush and wash soon.
Next as you know KROIL

[cbrick]

Has to be something in your alloy. Empty your pot and clean it good then remelt alloy and flux real well, when you think it's clean flux again.

Flux with sawdust. Waxes, oils & such cannot remove any contaminants, sawdust will.

Rick

[bslim]

I regularly flux with sawdust. Works just fine. After initially cleaning the build-up off of the mold, I have cast another 2,000 38's. Taking an old hair dryer, which was set on low speed/cold, I added a 3 second cooling cycle in between castings. The bullets coming out seem to be a little bit brighter, the fill is very good, and I do not have any further build-up. There is still a good chance, there was some contamination in my casting material, which has since been used up or fluxed out, but the problem seems to have gone away.
Thanks for the input guys.