It is monolithic. I carefully and slowly used DVA's diamond ultra thin purple butterfly disc to open the embrasures on the buccal and incisal. Then I used a "brillo pad" wheel(you recommended Patrick) to smooth the surface of the bridge. The stains were applied after that and the bridge was put under a drying lamp for 15-20 minutes. After than I went in the sintering oven overnight with our other zirconia crowns.
I know you probably do these on a regular basis and don't have issues, but two things in here I would change. The diamond disk for us is a big no-no. With any al ceramic restoration (pre or post sinter/crystallization/pressing) we want to stay away from causing a very shard stress point that a disk can cause. I use the side of a very long, thin, and tapered diamond bur for achieving this. That way the embrasure is more rounded at the depth of it, instead of a very fine slice. I don't think that is the problem here (though it might be) due to the direction of the crack.
The second thing ( and more likely in my mind) is your drying time. With a bridge we would recommend a minimum of 25 minutes at 140C or 270F. But with larger/thicker bridges, I would dry for an hour at those temps. With larger /thicker bridges like this, they soak up a lot of liquid and if not completely dry prior to going in the furnace, the liquid turns to steam very quickly as it heats and can cause damage.
I do have a 25mm LT0 disk and can mill/finish/color/sinter for you if you would like as well as send one to you in the pre-sintered state for you to finish. Just send me the coloring you need, along with the STL,and I'll try to copy your picture.
