I was actually thinking on a way to produce the spalling effect without significantly reducing the overall integrity of the structure.
Would intentionally re-tempering the mix and using it as an outer layer on an otherwise finished pot do this, or would you risk the entire layer peeling away over time, losing the effect?
It might just be easier to re-wet the outside of a finished pot part way through the cure, but I don't have enough experience with concrete to say for sure.
This is the point of hypertuffa, but it's not as durable, lasting maybe a decade the way I hear it. Is there any reason not to use perlite to make it lighter?
On that note, anyone know if you can permanently dye perlite different colors? Then you can polish down the outside of the pot when finished to reveal colored specks. Saw a video where they did this with colored glass.
Pouring the "structural" main part of the pot, allow it to begin setting up, then adding a layer that has been retempered separately would probably work, if you could make the logistics work. Concrete "sets" by chemical hydration, crystal lattice structures develop as the cement powder hydrates. Water is consumed in this process. Concrete will set under water. The hardening process is a crystal formation process, has nothing to do with drying out. Drying out will arrest, or stop the hydration, but the humidity in the air is often enough to keep the hydration process going, so drying concrete to stop it from setting is not a practical method.
The amount of water needed to complete the hydration reaction to change cement to concrete is so little that if you only added water the chemistry demanded the mix would still look like a dry powder, or at best a crumbly cookie dough. Excess water is added to make the mix plastic enough to place. The concrete then hydrates or "sets", usually in 24 hours you are at about 40% to 50 % strength, at 48 hours typically 75% strength, give or take and then compressive strength continues to climb for years. Slowly approaching, but never quite reaching 100% theoretical strength. (asymptotic curve).
Excess water not consumed in the hydration reactions becomes "bleed water" and will slowly weep out of the concrete through micro-channels. These micro-channels do become the weak points where water can re-enter the concrete and break it during freeze thaw cycling. This is the reason it is important to add no more water than necessary to make a mix with enough plasticity to work into the forms, molds and reinforcing constructions you have created for your project. It is better for the mix to be a bit stiff, than to be soft and runny.
One way to get the surface to "spall off", is the technique used to create exposed aggregate concrete. Create your mold or form to shape your piece. Coat the areas of the form where you want "spalling" or the aggregate to be exposed, coat these areas of the form with corn syrup or honey or a paste of powdered sugar. Sugar will retard the set of the cement paste it comes in contact with. Pour your pot. Allow to set until firm, at least 8 to 12 hours, long enough that it will be self supporting when turned out of the mold. Then use a stream of water directed at the areas where you had the sugar coating. The cement paste should be retarded to a depth of a 1/8th inch or so, wash away the paste at that location, the aggregate should be held in place by the cement paste behind it that was not retarded by the sugar. This trick works better in thicker walled pots. If your walls are very thin, the whole thing is likely to collapse and wash away on you. The original application was a patio would be laid, a colorful aggregate would be troweled into the surface, at the stage where the first bull float and smoothing trowel surface strike is done. Then a sugar solution is sprayed on the surface of the patio. Then a few hours later the cement paste is hosed off revealing the fancy stone aggregate (rose quartz, garnets, etc) that was troweled into the surface layer.
Sand blasting fully mature (90 day old or older) concrete is another way to expose aggregate and with hobby size or machine shop parts cleaning size sand blasting set ups, you can get very precise control of where aggregate is exposed and to what depth the aggregate is exposed.
As I think about it, sand blasting is probably the artistic tool that is far more useful, and easier to control than chemical retarders for modifying the surface traits of concrete. And many machine shops and even hobby shops have sand blasting set ups. Auto mechanics will have the air compressors. Sand blasting is not that hard to get a hold of. Auto restoration and machine shops are the two groups most likely to have sand blasting booths set up waiting for projects.