No air needed. MOISTURE is needed however if you want the casting to be strong.
After the first day you can throw the entire casting in a bucket of water for a week or two and it will be all the stronger for it.:)

Really?? HMmmm.
So i can make my hypertufa slurry and pour it into a latex mould (with a plaster mothermould for support around it) and let it set for a day, then remove the mothermould and toss the latex mould with the hypertufa still in it, in a big container of water? I would think that would break down the material instead of helping it- why is this?
And do you think the hypertufa is strong enough at this point to withstand a bit of pressure from trying to get the latex mould off?

Im not sure what the structural integrity of the material is like, so im not sure if i should make my object just all once peice, and wrestle with the latex when i try and unmould it, or make it in seperate peices that fit together. More work, but probably less risk. Also- Can you use hypertufa to adhere one peice to another? or will they not bond very strongly?

Yep you can throw mould and all into a bucket of water.

The rest of your question depends entirely on your tufa recipe and there are a gazillion of those.:)
A tufa slurry could be very weak and take several days to cure and even then be weak. Admixes like Acrylic latex can help a great deal here.

You need to know exactly what type of finish you want to accomplish. Maybe experiment with recipes in freeform to see if they have the required strength and finish.
Tufa gains most of it's strength after a week or so but continues to get stronger for about a month.
So to do what you wish a little practice and patience are probably in order. It's the waiting a week or ten days to get your result that drives me crazy. It's the worst part of this hobby.
What is even worse, your project could still turn to goo a year from now depending again on the recipe.:)

Well i wanted an aged and worn look to the outside of my objects- im assuming this would require more pete moss and such, verses more cement mixture- but would also sacrifice some of the structural integrity.

I guess i wanted the look of old stone- and im really not concerend about drying times. I will have to make hundreds of these objects, so i know my finished result isnt anywhere in the near future. The objects will be hanging, and wont really have any stresses or weathering on them, so they dont need to be extremely durable.

Would perhapse a better solution be a different mixture of concrete and additives? I have lots of concrete mixture around the house anyways- what if you added straight pete moss to this and water?

I just dont know of any other way to get a 'weathered stone' look with castable materials

Shmooey,

Your mention of using a slurry and pouring it into a mould makes me think it must be far too wet & that it won't work at all - the casting will be very weak and may even fall apart no matter how long you leave it to cure..

I have several latex molds, with either plaster or concrete mothermoulds for support round them, but when I fill them I make sure the hypertufa mix isn't pourable at all - it has to be packed in.

To separate the casting from the mold you need to have a mold with no overhangs. It isn't that the latex won't peel off the casting easily - it does. What matters is that there must be no overhangs in the plaster or concrete mothermold.

I've not tried using hypertufa to adhere one piece to another, but on I prefer not to try bonding pieces together, whether with mortar or anything else, unless I'm quite sure that the joint isn't going to show.

Yes i know that pouring it would really be feasable. i guess i used the wrong wording. I would take my latex mould first and pack everything in, before putting it all in the motherbould.
Ive done lots of moulds and castings before, so i know how overhangs work- and already have diagrams and measurements for where my peices will line up together.

I am also worried about the joints showing- so thats why im trying to do lots of reaserch before i try anything. Because i am making so many objects, i wouldnt want to do a one piece or one time mould.

In the mean time i thought i would expirament with different ratios of sands and moss and such- ive got some shapley lumps out drying in the garage.

Shmooey perhaps you could use Google images search and find something that is similar to what you want to make.
I'm assuming you are trying to imitate small carvings of the type found in archeological digs around the world.

A picture would help me to get an idea of what you are trying to do.

Nope, no little carvings :)

Birds (outstretched wings) and spoons. Not worried at all about making the spoon, but its the bird that has me stumped.

Shmooey, if you have a need to "pour" your mixture in, you might want to stay with a more traditional concrete mixture with the rock aggregate to maintain the strength. You could still switchout "some" of the aggregate with perlite/vermiculite for weight and probably even some peat. You could add an admixture for more strength--acrylic fortifier and/or fibers. To get a further aged pitted look, you could use Tango's suggestion of sprinkling the mold with baking soda which leaves small crevices. I think Tango had some samples of doing this in his photo album--check his "My Page" for the album. It was a mold of a star figure, I believe.

I would imagine that you would end up with some weaker, stressed areas in the wing area in which you would have to be careful with--can you manage to put some type of steel bar reinforcement in that area?. I guess experimenting will be your best indicator here.

Regarding the hypertufa recipe, I've heard that some people have had success filling both halves of the mold and slapping it together while it's still wet. If you desire the aged look anyway and you're using a steel file on it after it's unmolded to rough it up, you might find that the seams will blend in and/or you can concentrate on it at that time to blend them in. You can also use a cement slurry to touch it up somewhat.

I totally submerse my stuff in a water bath which I change out every couple of days. The longer it stays in there, the better (within a 28 day range). You can submerse it in the water as soon as it's "set" which is somewhere around a few hours. Also watch the temperature in which you cure it. No colder than 50 degrees and I believe the hot point is somewhere around 85-90 degrees, otherwise it stops curing.

Especially with a mold that has a weaker stress point on it, such as the wings, you'll want to avoid moving the piece too much because it can cause microfractures which weakens it. I guess you can be overly cautious with the stuff and give it it's best chance or find a happy medium that works for you and the application you're making which still will give you an acceptable outcome. Experiment, experiment, experiment.

Here is a link that might be useful: Baking Soda trick

What Howiedoin said about the baking soda for the aged look. You piece will be stronger if you just use portland and sand. If you don't need it lighter stick with that, the peat,perlite or what ever you use will make the piece weaker and you can loose some of your detail.
My 1.5 cents worth
Paws

HowieDoin, I don't think there is a hot point for curing. If there is, it is probably much hotter. Remember, someone was talking about using steam to decrease the curing time for concrete in pipes?
Shmooey, Sounds like an interesting project. If you use perlite, you might want to put it in a blender so you would have a nice smooth casting. Your mould marks can be sander and slurried out, I am sure. Just don't make your mix to wet, Tufa will hold your pieces together, but be sure to wet the cured pieces really well before sticking them together, If you have trouble, use a good concrete adhesive, then slurry your joints. Be sure to send us pictures.

Jo

THank you all for the input. I think i may just try a straight cement mixture, and go with the baking soda trick. The cement can set up, as well, within the mould itself?

I am thinking that the best way to avoid having to struggle with working the latex off of the wings- will be within the placement of the pour opening in the latex. Or as Jo said- i could really just make two halves of the bird, and not have to worry about an entirely sealed object - and pop the two together while they are still wet. I will be making some drawings within the next day and posting them, hopefully it will help me clarify what i am doing- and maybe you will have some suggestions for me :) THank you everyone already for all the help! Will post pictures soon :)

Jo, I might be thinking about the "setting" temperature. It sets up too fast above 90 without something to retard it. Maybe I was thinking the 90 degree curing was too high for me, not the concrete--too damn hot to be doing it above 90 degrees!

Howie, know what ya mean....was hot today and my tufa I made today is hard as a rock already. I just love it setting so quick, but too bad I have to cure it. Is there a magical way to cure it quicker?? I wish!!!

Shmoo, yes it will set up nicely in the mould. Not to worry.

Jo