I think that the volume of the mold vs the volume of metal being poured is more of a factor than just weight. I will often pour less metal than the mold will hold because I am looking for a specific dimension on one edge of the ingot. The ingot will usually be irregular on the top edge, probably because there isn’t enough weight behind the melt to even it out. I don’t do centrifugal casting, but I know that the size of the spru vs the size of what you are casting can have an impact on the completeness of the casting. Weight may be a factor here too. Regarding Delft clay. I have had the casting fail because clay came loose in the casting process, but can’t say as I have ever had it get embedded in the casting causing problems when rolling. Delft clay is a lot of fun to work with. The little round frame that comes with the kit is good for small pieces, but you might want to make bigger frames. I make them out of PVC pipe and have even made a large one using PVC boards. Just don’t get the torch near the PVC. I should go to the local fabrication shop and buy pieces of steel pipe, but I just haven’t gotten around to it. Good luck…Rob