I've made several similar pots using the same mold for all. It's 24" square at the top and 10" square at the bottom for the outer mold (I don't know how to describe the shape other than a pyramid with the topp cut off, but upside down). For the outer mold, I used 1/4" plywood, with 2x4" reinforement at the edges, screwed together so as to easily dissassemle and remove the mold. For the inner mold, I use a similarly tapered bov made out of melamine covered 1/2" particleboard with reinforcement of 2x2" at the corners. The outer mold has 3 coats of polyurethane and I coat with oil, and the inner I just coat the melamine with oil. I've had no problems with durability with either tufa or concrete.

I also used a thick plastic cat litter container for the inner mold previously, with the only issue being it had straight sides, which made it much more difficult to remove.

What I'm trying to say is that durability is not an issue, and a plastic mold would be preferrable to a metal one. Even better would be a mold that has a slight taperering to a narrower circumference towards the bottom of the mold. With straight sides, you're going to have a difficult time removing the mold without destroying it or the pot or both, and you'd have to do it right as the tufa is set up enough. I also got a lot of crumbling around the edges.

Also, no matter what you use, if you're trying to save the inner mold, you have to drill a few holes in the bottom of the mold to let air in as you remove it, or you'll have a vacuum under it which won't let you pull it out.

Ken

Hi Mr Miagi,

Wow, for a newbie you're certainly starting with a big project.
I haven't made anything quite that large but I agree with Ken that a metal inner mold is apt to give you trouble. If you can find a suitable sized plastic inner mold you could fill it with sand and that would support the weight.

Jan

Also, it helps not to mix the tufa too soupy wet. A drier mixture won't want to press out on the mold so much, and will make for a stronger result product after cucring.

Thank you Jan and Ken for the info and advice. Jan, the fill the inner-mold with sand tip was one of those, 'Now why didn't i think of that' things.That will work. Thanks. Ken. using an inner mold with taper sides makes alot of sense. I'm still going to use the barrel for my current project but it will stay in-place as part of the piece. Thinking of tileing the inside with small black river stones and then grouting them in place. If this project is successful i'll post some pic's. In the process of removing some cedar fence posts from a natural pond in an oriental garden i'm designing, i learned how strong the bottom suction can be when trying to pull an object out of a hole. Good tip on drilling some holes in the bottom of a inner mold. Will keep you guys posted on my progress. I'm starting to be concerned on the weight of this thing. My wooden mold is up to about 80lbs. already. LOL

My pots of about the same dimensions (with a thick wall as well, being between 2.5-5") are quie heavy. Each one takes a 90 lb sack of portland, plus the peat and vermiculite, and probably ends up around 130-140lbs when dry. I built a platform to put under them, so I could pick it out of the mold bottom, and put it on the platform, then use a dolly to wheel the platform and pot to someplace shady to finish curing, which takes 4-6 weeks to reach final color and hardness. I've moved it out of the mold myself, but I highly recommend getting a friend to help if at all possible.

Thanks again for the advice Ken. Have another question. I wanted the pot to be black or at least to be a dark grey. I purchased some Quikrete liquid cement color (charcoal) to add to the mix but read in some other threads that coloring tends to weaken the cement. Did you color your pots, and if so, what did you use? I'm purchasing one of those small electric cement-mixers from HD today to help with all my cement projects. Last year i hand-mixed 35 bags of ready-mix for my pond. What a chore. Probably will pick-up a back-brace too. LOL John

John,
Just want to add to the information above. If using a metal insert, start with a flat piece of flashing and roll it to the inner diamter that you are trying to creat a void with. run plain masking tape on the outer portion, and place a couple of pieces of duct tape on the inner portion of the metal insert. Coat the outer portion of the flashing (the side that will sit next to the mix) with a release oil, agent.
Once the pot is cast, relax for about four hours and you can undue the pieces of duct tape inside and begin to roll your metal flashing inward. The cast should be set but still flexible to allow for this.
If using the sand or water in a container method to create that inner void, wrap the container in a couple layers of newspaper, and secure the paper with masking tape slightly. The paper provides a waste holder that will allow for your inner container to release much more easily.
Look forward to seeing what you come up with.
Be creative,

Dena

Dena. Thank you for the info. Much appreciated.
John