Im not sure about tufa but i glued POP with gorilla glue and seems very strong.

I don't like cracks, prefering to smash broken pieces and add tham slowly as aggregate to new projects. So, this is: for what it's worth advice: You could glue the grot and then put a layer of 1:1 portland and sand over the whole piece. with something that isn't going to hold water, it should work.

Let us know how it all turns out.

Mercedes

Depends on how bad the crack/break is. If it's just a clean break you can use "Liquid Nails". Paint it on the inside edge of the crack so that it doesn't show on the outside. You can also use mortar or the same hypertufa recipe. Soak your broken pieces in water then apply the mortar or hypertufa. Keep the mortar/hypertufa towards the back edge of the crack so it does show on the outside. If there are gaps in the break you can use any of the above items to glue the crack then fill the gaps with mortar or hypertufa then paint the whole thing with a slurry (runny mix) of mortar or hypertufa. Sometimes the "fix" takes longer than making a new one. If the break isn't too bad you can plant with something that will cascade out of the crack.

lilpeaches's, I usually remove the pot befor I start forming the facial features, that way,less chance of messing up. I repaired a crete and sand leaf with epoxy, and it worked out great, the last 2 pics in the leaf album will show the repair.
bonnie

Here is a link that might be useful: last 2 leaves