I spent a lot of time doing research before building this sculpture, and there was a lot of guesswork involved, so I wanted to document the process for possible future use. And since I'm already documenting, why not share it?
There are three parts to this process: building a support frame, adding a fill material, and layering concrete on top of it.
Step 1: Sketching It Out
You can see I slowly start to settle on something I like, and start sketching in sizes. I wanted this sculpture to be just taller than me, about 6'2'' or so.
Step 2: Cut and Bend Metal
I then cut the metal with a chop saw and curved it, and laid it out on the ground, checking with a tape measure to make sure everything would fit. The pole in the middle only serves as a measurement tool.
I used a fancy metal roll-bender to curve this, another option would be creating a wood frame and then heating the metal to a forging temperature and bending it around the form.
Also during this time I cast the glass blocks that I would be using, but I thought that was a little too much for one instructable.
Step 3: Weld the Frame
I'm not a great welder (I practiced for about an hour before I made this) but I'm not sure my welds would have been great even if I'd had more experience - there are a lot of small, weird angles, and it was difficult to position everything for each of the thirty or so welds.
As you can see I've added some sheet metal to the bottom and added some weight to it. The glass I'm going to add will be about 60lbs, and I don't want this to fall over backwards.
Step 4: Expandy Foam!
Here I'm using expandy foam (aka "Great Stuff") to cover all of the metal. I think this took 5 or 6 cans total. Important: definitely use the brand name foam here. It's about the same price as generic, but sticks way better, which is important.
Unfortunately I don't have any pictures, but the foam won't stick vertically or upside down. To fix this, I sprayed a line of foam on a sheet of plastic wrap, and then wrapped the foam around the metal arms. The plastic wrap holds it in place - BUT it won't harden properly if you leave it there. So you have to attach it, and then as it's drying, remove the plastic wrap bit by bit. The dry parts will come free easily, but the plastic will still be stuck to the parts that haven't dried, so you simply pull as much as you can and leave it until more dries. Time consuming, but worth it, I think.
Also during this step I glued my glass in (with PC7), and sanded down (I actually used a rough wood saw) the expandy foam into the shape I wanted.
Step 5: First Layers of Concrete
Safety first! Use gloves when working with cement. It will burn your hands, but you won't notice that it's burning your hands until it gets through every layer of your skin - and then it's quite painful.
I started by making the base (mainly to add weight and smooth out the foam) with Quickrete Fast-Setting Concrete Mix. In my opinion, the best for the price for this job.
In a bucket, mix as much Quickrete as you can use before it sets (20 minutes, if I recall), with just enough water that you can work with it. It should be looser than clay but not so loose that it doesn't stick. This concrete won't easily stick vertically or upside down because of the aggregate inside it.
For the arms, I simply sifted the aggregate (pebbles) out of the Fast-Setting Concrete Mix and applied it with a clay scraper (any small flatish object would work). Removing the aggregate allows it to stick to the foam, and I was able to get it to apply vertically. For upside-down applications, I did just what I did with the expandy foam: I put a mound of cement on some plastic wrap and wrapped it into place until it dried. This is much easier to do - the cement will harden in plastic, so there's no weird process for removing it. This process also leaves a smooth, glassy surface on the cement, if you're into that.
Most (cheaper) cements need to stay moist to set properly. It's recommended that you cover it or spray it with water while it's setting. A process that can take days. I did both to play it safe.
Step 6: Final Layers of Concrete
The next layer should be smooth concrete. After a fair bit of research, I came up with this recipe: 1 part portland cement to 1 part silica flour (by weight). Mix them thoroughly dry (don't forget to wear a mask!), and then add water as before, just so that the cement is workable.
Again apply a layer of this cement. You can sand it down to get a very smooth finish, as you can see in the photos.
You'll need to cover this/keep it moist as well, so that it properly cures without cracking.
Step 7: Add Colored Cement
I used the same, smooth coat as before, but this time much more water, so that it was basically a slip coat. I applied just enough to cover the gray beneath, and then smoothed it out.
Then I sprayed it with concrete sealant and allowed it to dry. Sealant means you don't have to cover it or keep it moist.
Step 8: Final Touches
And I made a mini-gallery for it right where it was, because at this point it was almost impossible to move.
I did finally move it, so there's a shot of it outside.
I hope this was helpful to someone!