Hypothesis Testing, Water and Homogenization. Oh My! 

In my 20's I was a production (porcelain) potter - then spent 30 years in tech - now getting back into clay but I'm new to slip casting. Love your channel. I always used a sharp hole tool and turned it slowly so it slices through the leather hard clay - then cleaned up the hole with a wet round brush. Sharp drill bits by hand can work too (no drill, not enough control). You're right that the result is better if the clay is consistently leather hard. For example when making teapots (and other items with many parts) cover them all for a bit to even everything out before assembly.

Wetter clay will help. It's basically crumbling on the back where it breaks out, and wetter clay is less crumbly - or I should say more crumbly and less fracture-y. There'll be a tradeoff of too wet (cuts more easy with smaller breakouts, but the shape warps when you handle it) and too dry (bigger breakouts). I've had to make holes in slip casted christmas trees for the light pegs to go in, and I've got a method figured out - I wait until the tree is almost leather hard, then apply a drop of water and cut through slowly using the same tool you're using, working water into the cut so the cutting tool liquifies the clay as it cuts through it. Fully clean your punch after every hole. Pushing clay further up the punch takes force, and you'll be applying that force to the clay you're trying to cut through. I use a wooden skewer to push out the "core sample" and then a pipe cleaner in a bucket of water - It's annoying but I find it's necessary. Wife suggests supporting the clay from behind with a die like how a paper hole punch works. You could probably 3D print a weird looking set of hole punch tongs that'll do it. You can grind the holes with a Dremel, or extremely slowly with a hand drill, but clay dust goes everywhere and it's miserable. Can't recommend.

Interesting. So...if you put the object in a damp box or equivalent, overnight you think punching holes will not have the tear out problem. I can see that. You explain things so clearly!

I only drill when it is bone dry. Try experimenting with something from your scrap bucket. Rotating an exacto knife also works, but leaves a cone shaped hole.

I had a similar problem with candle holders. I wonder if there’s a way to amend the mold with something so that the holes simply form in situ

Maybe a handle awl better than using nail & drill, or using a shape hollow tube. BTW, higher moisture might provide higher plasticity of green clay object.