I like the cast iron bowls as well and that is what I have used so far but I wanted to see if I could figure this out. What would you use to sand the bowl and finish it? I think I might have used too much water in my first attempt. I'll post some pics of the mix if I can figure out how to add that to my post...

Marilyn,

Yes, in a heavier section such as the hemisphere you describe, you could use the concrete mix and get away with using it almost that same day. I would still suggest sifting out any aggregate over 1/2" mesh, just to get a smoother fill into whatever mold you're using. Lots and lots of vibrating or joggling too, to get a smooth surface to ride in the support ring.

I was describing a method to make what "firefly" asked about - a bowl, or hollow form with relatively thin cross section. That's a different kettle of fish, so to speak, from a solid form.

Rich

This is an interesting take on the idea. Sounds easier.

The cement was a smooth mix so perhaps it wasn't what I had said. After it had set for a bit, we pushed pencils or fingers about an inch into the top. After it hardened, we removed it from the container and piled the pitch on top. That is why we made the depression. they keep the pitch on the hemisphere.

marilyn

If you use straight Portland cement without any sand, aggregate or fibers, it is guaranteed to crack every time. There is simply too much shrinkage upon setting. Cement does not dry, it sets hydraulically through a chemical reaction between the cement and the water. After setting, it still retains a significant amount of free water that then must dry out of it - this causes the shrinkage. ONce it is completely dry, it still has water in it, but it is chemically bound to the cement. Firing it above the calcining point will release that water and turn the cement back into a powder form, essentially.

As I mentioned in my first post, you want the 1-2-3 mix of Portland, sand and gravel to be stiff - like say, cookie dough or just a bit softer. Then you won't have the problems of manipulating into the mold and it won't crack up when setting/drying. The fibers make a huge difference on this as well.

The basketball is a pretty good idea for the mold, but it will be too floppy to let you really work the comcrete into shape, unless you back it up. Place the basketball half in a box or barrel dep enough to hold and big enough to allow a full inch or more all around it, then slowly pour dry sand around the thing. Once you get sand up to about the level of the rim, you can dampen the sand with a water mist from the hose to "set" it. Sort of like building a sand castle at the beach, if you've ever done that. Now you have a stable mold to work your concrete into.

Never try to speed up the curing of concrete! Doing so will result in cracking, spalling and general loss of strength. Concrete actually develops its highest strength when cured under water, so keeping it moist for the first three to seven days is recommended. This is particularly true in thin-section work such as your bowl project.

This is great info, thank you!

Hi, I use a lot of concrete with my steel work, It's a basic mix of 2 parts sand 1 part pea gravel and 1 part cement. When casting you need to have your mix fairly wet. Take a look at www.revelstonecreations.com.au