I take all the marbles (let's say 3 of them) and warm them up to 970 degrees F. Let them soak there at this temperature (and hope they don't thermal shock during the process...lol) and pick out one at a time and fix any spots where dings, dents or folds were before I put them in the kiln to begin with. If I don't do that, I won't know if the mixing with the other marbles will be a problem as if it cracks now, it surely will crack later. I'll do this with all 3 of the marbles. Once I am convinced that there won't ba a problem later (I hope) I'll join these 3 forever in making the one large one.
As far as art goes, I decide after rounding the 3 together IF I need to gently assist the marble - making a decision about the twists, swirls, solid color patches, etc. I change the punty (the rod that holds the marble when it's under construction) at least 5 times making sure none of the punty rod's glass is part of the original 3 marble's glass as it certainly will have COE issues.
Finally, I use what's called a "cold punty" to make the final rounding and burn that punty mark totally smooth so you can't see where it was one bit. As far as repairing old German swirls, I can duplicate the original makers punty mark if I so choose to do that. I now have over 600 fixes, rements under my belt since March this year because so many of the vintage collectors have contacted me and so far (knock on wood) they are happy with my work - or should I say my hobby...lol.
Does this help?????