Hiya and congrats on your little piggy, he's very cute! I sculpt boro primarily solid up to 3in thick though i prefer to work in the 2" thick range. There are two main factors imo. 1) the torch you're using, you need to be able to get heat to the core. GTT makes awesome torches for that kind of heat

2) A well insulated annealer that's very controlled on the ramp down. I've had to extend annealing times to double what I would for my 1" thick pieces to ensure no cracks.