Perhaps you have a vacuum holding the mold to the piece. Drill a hole in the bottom of the mold, and prop up the mold edge. Then tap on the piece through the hole. Or, cut off the mold.
Wet the broken surface and apply wet mix to build up the desired surface. The repair will most likely be OK.
I often chip edges of planters on purpose to give a rugged, used look.

thanks, fredw10, that sounds like very helpful advice. I just broke the latest small bowl to smithereens. Guess I moved too soon. I left it a week-this is getting very frustrating

Its so disappointing when your pot breaks from handling it to soon or trying to get it out of the mold. If you used metal bowls they are very hard to unmold. Cement attaches itself to metal. Plastic bowls work well and the Dollar store has wonderful shapes and sizes for only a dollar a piece. If using metal line the bowl with a plastic garbage bag. They come out perfect and the marks it leaves adds a nice texture to the outside and use a wire brush to roughen the outside. I dont know what recipe you are using but if you add sand it makes the pot stronger Ive found. I love working with hypertufa and hope to make a fountain this year also using leaf castings for the waterfall part.

I use plastic molds as often as I can then if I have problem with releasing the item I place the mold in very warm water which makes the mold more flexible, have had lots of good luck using this method. I would think a hairdryer would work on a larger size mold. Also going around the top edges with a dental pick or something similar to remove the Crete from the edges of the mold really helps. We all experience those breaks, but I find to quickly make a new piece sooths those tears. Good luck with the next item. wendy

I have had trouble in the past but have repaired with a new mix of same tufa mix or crete which ever was used. Some have been great and some just usable. One pot I loved I put flower wire around the two big pieces of the pot and put another layer of tuffa over the entire pot and covered the wire. It still is holding fine with a plant in it. It is a pot about 15 inches by 12 inches. I have read where people keep putting layers and layers on projects so I cant imagine it wouldnt work to repair most things. I am not a pro at this but I think I read to be sure to wet your project you are adding to. Maybe someone can jump in and add to this.

I'm no pro either, but justadncr is correct, I have repaired items by roughen up the edges(important) of the break first then give it a good blast with a mist of water, then mix up just plain cement with a little wood glue added, no water or sand, then apply to both pieces, to hold tight put in a vice if possible. After it is set up (couple of days), you can now apply another layer of the mix you were using to the whole item.

Thanks for the tip elephantear. I use a little wood glue but I hadnt thought to mix dry cement with wood glue!!

My 2 cents:
I have stopped using peat moss in my projects. By definition my projects are not hypertufa and that is okay. Most of what I have read on various forums and message boards verifies my experiences with "tufa"; a very high failure rate, long unnecessary curing times, eventual collapse, and the adding of various additives necessary to strengthen the structure. Although many additives (aggregates) do little to increase the strength when peat moss ia part of the mix.

My research and hands on experiences suggest peat moss is a part of the hypertufa mixes for two main reasons. Weight, peat by volume reduces project weight by at least 50% depending on the mix. And surface appearance, peat will eventually dissolve, or decompose leaving a pock-marked surface simulating volcanic rock. However, tufa is not volcanic rock.

"(Tufa is the name for an unusual geological form of calcite rock. Tufa is a rough, thick, rock-like calcium carbonate deposit that forms by precipitation from bodies of water with a high dissolved calcium content. Tufa is not to be confused with tuff, which is volcanic.

Tuff (from the Italian "tufo") is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption.

Hypertufa is an anthropic rock made from various aggregates bonded together using Portland cement.
Hypertufa is intended as a manufactured substitute for natural tufa, which is a slowly precipitated limestone rock; being very porous, it is favorable for plant growth.

The basic recipe for hypertufa uses the classic proportions for mortar (1 part cement: 3 parts aggregate) hypertufa is composed of
3 parts Portland cement - Type I
4 parts peat
5 parts perlite
or 3 parts cement: 9 parts aggregate
Other recipes may call for sand or gravel as aggregates, but they add unwanted weight without necessarily adding strength. To increase structural strength and longevity, synthetic reinforcing fibers (plastics, not fiberglass) and liquid acrylic may be incorporated into the mixture. Powdered mineral colors (in small amounts) tint the hypertufa to resemble natural rock. ------ From Wikipedia, the free encyclopedia)"

My mixes are variations of Portland cement, sand, perlite, and vermiculite. Sometime I use all ingredients. Usually I use sand only in projects I do not intend to carve or manipulate the shape after or during construction. The sand will eventual destroy most files and blades.

Here are a few references I use:

Sculpting with Cement by Lynn Olson uses cement and steel wool to create "fiber" cement and a mix of sand/cement for sculpting.

Sculpture as Experience by Judith Peck covers the use of Portland cement and vermiculite (two parts vermiculite one part cement) as a sculpting medium.

Sculpture in Concrete by John Mills states "the sculptor usually achieves the tensile strength so important to him, by making freestanding forms hollow, working on the principle that, by its very nature, a hollow sphere has a greater strength than a solid sphere, and a tube has a greater tensile strength than a solid rod."

Creating with Concrete by Sherri Warner Hunter has 13 different mixes for cement sculpture use and three slurries to use in connecting concrete pieces.