Sculpting a BJD with balancing in mind

Hi! Welcome to this part of the forum!

I think it's just a matter of practice and lots of small adjustments. It probably helps to make the pieces (especially the legs) as even and equal in weight as possible. If one leg is longer than the other, even slightly, it will throw off the balance - watch out for things like that.

Good luck! I hope you'll share your progress when you're ready!

 

Short answer: Friction and physics.

If the joints don't have enough friction to hold the weight of the doll, it won't matter how well balanced it is. This also ties in to how the joints are constructed.
Basic tricks like a low centre of gravity, an upright posture, larger feet etc. makes a doll easier to balance.

Everything beyond that it mostly trial and error and a matter of preferences and priorities.

Materials play a part as well. Since you mentioned an original sculpt in polymer clay and a porcelaine cast, the porcelaine (being hollow) may behave differently than you original sculpt did. It's the same when casting in resin. Don't be surprised if the first porototype cast don't behave exactly as the original did. That's what prototypes and "junk casts" are for, after all.

 

The center of gravity is where the balancing point is in the doll, if that makes sense (English is not my first language, so I may miss some of the proper terms). A normal human that stands straight is fairly simple to balance in that most of the mass and weight is in a mostly straight column, but if you are making a doll that is very hunched, has a bent legs, large wings or other fantasy parts och just anything with a bit of weight that is not close to that centre column, you are going to have to compensate for that in some way, or the doll will fall over. The further out from the center the extra wheight is, the larger the effect of of will be.
The higher up from the floor you get the larger the effect for added wheight will be. The smaller the surface of the dolls feet, the easier it has to fall over, the further the feet are from the center of gravity, the larger they need to be to compensate. Or rather, you can compenate by making the feet bigger, but it is possible to do it in other ways as well.

I don't know about intentionally putting more weight in the lower body. Since that will still likely mean weight off the ground it will still make the doll easier to tip over, just less so than it would if you put that weight in, say, a large head. But keeping the weight low in the top part of the doll is a good idea if you suspect that the balance will be a problem.

So if you, for example, are making a doll with tiny feet and a very large head, you will have to balance that doll very carefully to make sure the weight distribution is even around the center of gravity. If you on the other hand make a doll with huge feet and a tiny head you can get away with a much more uneven distribution.
In the end it's mostly up to you how you want to go about it. You can look at existing dolls at see how they have solved it, you can read the doll making threads here to see artist struggle with it and you can look at reviews from owners of the odder fantasy sculpts out there to see when, how and if dolls with unusual body configurations manage to stand and pose.