Making a keel bulb mold from concrete

Is there anything special about a concrete mix for making a multi pour mold?

I know to reinforce it with wire cloth and rebar.

I know to cure it wet for 10 days and then dry cure it for 10 more days.

I warmed the last one I built before pouring the lead into it. It looks pretty worse for wear just after 2 pours.

Thanks in advance for any input.

Sam

 

Concrete has a bad habit of containing some water in. When heated (like in lead pouring) it (water) expands and pretty much destroys the concrete. Drying concrete is a PITA..

 

I'll second that Teddy, in that an uncoated concrete mold scares the hell out of me.

This said, it could be coated to lock down the moisture (which will remain for decades in concrete) with something like the stuff they use on the inside of pottery kilns. This will not burn off from the pour and isolate the moisture in the concrete from the molten lead.

 

When I did the earlier pour I used water glass to coat the interior of the mold.

I didn't have any problems, but I think I over heated the mold with a propane torch. (Lead continued to 'gas' off after the pour into the mold.) (I know not to breath this stuff and neither should anyone else.)

Sam