Admittedly, this has nothing to do with the skater's version of a half pipe, it's about taking a big PVC pipe and cutting it in half to drape clay over it or in it. If you're looking for the skate version, I can't help you, sorry. This also isn't really about how to make something from clay from beginning to end, it's aimed more at those who have some experience with handbuilding already. There are already plenty of instructional writings out there about handbuilding with clay that are better than what I'd write, so I won't get too much into that in this Instructable. Rather, this is about a creative way to make forms using a material you may not have considered previously.
The idea for using a large PVC pipe as a form for clay came to me because I'm an art teacher and maker and I have lots of random materials sitting around in my classrooms and storage room. One day I happened to look at a section of PVC pipe and realized that I could easily make a slump or hump mold out of it by cutting it in half. So I did. I've made several projects using these forms so far, the most impressive of which is the teapot (still unglazed) pictured here. I will attempt to give enough explanation that someone with some basic tools and clay skills can get started making some things using this method. Therefore, I will just mention some of the more basic steps in handbuilding and not go into great detail about them.
Step 1: Prep the Pipe
The pipe I used is about 16 cm in diameter to the outsides of the pipe wall. I'm not sure what that is in American measurements and that doesn't bother me. (Tangent/rant here. Skip to the next paragraph if you don't want to read it.) I never could keep all of those fractions and random units of measurement straight when I was building stuff back home in Ohio and now I don't have to because I teach at an international school in China and we use metric. It's infinitely easier than the antiquated system we use in the States, which is also used only by Liberia and Burma. Yeah. There's a great video on YouTube about this called Are Imperial Measurements outdated? | Number Hub with Matt Parker | Head Squeeze. Look it up, it's worth the watch. (I tried to toss the link in here but couldn't get it to work - user error, I'm sure, but you can just copy and paste the title of the video into a YouTube search and find it.)
Back to clay here (this is where those of you who skipped the rant should pick up). You can cut the pipe off to whatever length you think you'll need. This mostly depends on what the forms are you'll be making. Tall vases would require longer pieces than teapots or bottles or things of similar stature. If you have access to a chop saw that would be the easiest and quickest way to get a straight cut. Otherwise, you can take a long piece of straight paper and wrap it around the pipe, lining up the edge and overlapping the ends of the paper, then use a Sharpie or something to trace the edge of the paper onto the pipe. This should give you a line around the pipe that is perpendicular to an imaginary straight line down the length of the pipe, which ends up being sort of important if you want to make things easier for yourself down the road. Next, cut the pipe off along the drawn line with a hacksaw or jigsaw. I suppose you could also use a circular saw, but that would be a bit trickier, I think. If you want both ends to be straight (and it's not already straight), repeat this step on the other end of the now somewhat shorter pipe.
Next, you need to cut the pipe in half lengthways. An easy way to do this is to lay the pipe on a table that has a very straight edge that is also fairly sharp, not rounded. I don't have a picture of this step, unfortunately. Lay the pipe so that it runs parallel to the edge of the table. Roll it right up to the edge of the table so that you have to hold it up to keep it there and so that the whole length of the pipe is touching the edge of the table. Then use the edge of the table as a guide to mark the pipe with a marker or something that won't stay permanently on your table edge. Another option is lay the pipe down against a very straight board and use the board as a guide to mark it. That's probably easier than the table method. I'm sure there are many other ways to this, I just go for simple and quick.
At this point it would be a good idea to trace the end of your pipe onto card stock, poster board, or something else you can make a template from. Old overhead transparencies or plastic folders work well for patterns you're going to keep around for a long time, but I wanted to be able to easily fold this one so I opted for some poster board scraps I had. In the picture of the templates I made, you can see that the bottom left one is the diameter of the pipe. Fold it neatly in half then into quarters, then mark the lines you folded. If you lay that on the end of the pipe with one of the quarter marks on the line you drew down the side of the pipe you can quickly tell where to make a line on the opposite side of the pipe to cut it evenly in half down the length of it. Make a tick mark there then use one of the methods described above to make the line down the opposite side of the pipe from the first one. Now you're ready to cut the pipe in half down the length. So get to it. It's probably easiest to use a jigsaw for this but a hacksaw will also work. After you're done with all of your cutting make sure you clean up the edges a bit with sandpaper or a file or rough rock or something.
Step 2: Pipe + Clay = Cool Curves
Now you're ready to start working with the clay.
Roll out a slab in whatever way you prefer to whatever thickness you like for whatever you're building. Whatever. Again, this Instructable is aimed primarily at folks who have some previous experience with handbuilding, so I'm not going to go into detail about every step. You know that after you roll out a slab you should make like a frog and ribbit. (Say ribbit slowly and break up the syllables and you should get that.) I have a slab roller that makes life a bit easier when building with slabs. You can use your great-great-grandmother's heirloom rolling pin from the Old Country to roll out a slab if you want, it really doesn't matter to me. It might matter to your mother, though.
To keep the clay from sticking to the PVC you can either line it with newspaper, as I've done here, or dust it with corn starch or something. I've tried both and prefer the newspaper, but that's just my personal preference. You do what works for you. As you can see in the photos, you can use the half pipes for either slump molds or hump molds. Again, which one you choose is up to you, just be aware that if you mix the two (hump and slump) the curve from the hump mold will be a bit wider than the curve from the slump mold because it's on the outside of the pipe and the curve from the slump mold is on the inside, which reduces the diameter. Hold the clay slab by the side edges, holding the edges up higher than the middle, and settle it into the half pipe so that the middle of the slab touches down first. This will help you get a smooth continuous curve. You can also gently press down on the slab to settle it in. Make like a frog again and ribbit, if needed. Old plastic cards work well for this if you don't have a store-bought rib. I do, but I usually grab a card anyway.
You can probably figure out how to put the clay over the hump mold so I won't describe that. I'll refrain from making any snarky remarks here about not being able to pour water from a bucket that has the instructions printed on the bottom of the bucket.
As you can see in the photos, if you're using the pipe as a hump mold the edges of the clay might rest on the work surface and cause the clay to bulge out from the side of the pipe. An easy way to fix this is to simply rest the pipe on a small box or something so the edges of the clay can hang down past the edges of the pipe. Make sure you have some slab pieces that you leave flat if you're going to add ends to bottles or other forms. After this you sit back and watch funny cat videos while the clay stiffens up to a workable handbuilding consistency. Or take a nap. Naps are good.
Step 3: Some Random Tips on Handbuilding Clay Forms
A picture of a paracord necklace?! What's that have to do with anything? I'll tell you. I have found that when I handbuild with clay I get annoyed that my wedding ring keeps making little dents in my clay. I don't want to just dump it in a dish or put it in my pocket, I'd rather still wear it, so I made a paracord necklace to wear it on when I'm doing work like this. This is actually the second one I've made for this purpose, and it's a bit long so I need to take one end apart and shorten it some. This has been a free tip, no extra charge for it.
After your clay has stiffened up to a workable consistency you can trim the edges. I like to just follow the edge of the pipe, as illustrated in one of the above photos. You can also push the whole slab down to the end of the pipe so that it hangs over slightly, then cut off the excess to make your slab have a nice, straight edge (this is assuming that you cut the end of the pipe off in a straight, even manner.) In the accompanying photos you can see that I further divided this particular slab in half down the middle because I wanted to make a smaller form, a bottle/flask, and that I left what is now the top edge of the slab uneven. The top can be made straight using a ruler and the cutting utensil of your choice later, once you decide how tall you want your form to be. Or you could do it now, whatever you prefer.
To determine whether or not I liked the shape and size of two quarter-width slabs joined together to make a bottle, I simply held two of my poster board pipe templates up to the light so that I could see the shape that would make. I liked it, so I proceeded to the next part, cutting the half pipe slab into quarter pipe slabs. To make a straight cut down the middle of the slab, I used my circle template divided into quarters to find the middle (see photo) and pressed a metal square against the clay, then cut it along the ruler part of the square.
After cutting out the pieces of your form you can start joining them together. To create beveled edges, I like to use a cheese cutter that I picked up at a thrift store for next to nothing, but you use whatever you're comfortable with. Don't forget to slip and score. My preferred scoring tool is one that I made from the barrel of a dried-out marker. I cut the tube into strips then cut teeth into one end. If the edges of your clay are still soft enough that you don't really need slip but can just add a little water before you score, this homemade tool works well because you can just dip it in water with the curved side down so that you're scooping out a little water, then score the edge of your slab with it. The water runs onto the clay as you're scoring and gets into it fairly well.
Step 4: You Know the Rest
This is where leaving some slabs flat comes into play. I believe you should be able to figure out the rest of building your form if you've done any building with slabs in the past. As I've stated elsewhere, this Instructable assumes you've done some slab building before and know how to make and add a bottom or handle to your form, as in the start of the bottle and the vase pictured here. I will add, however, that the texture on the vase was added just after I rolled the slabs out but before I put it in the slump mold. Also, the teapot was a bit more complex and required more thought and planning. In the colorful photo of it, the pink indicates slabs that were made using the half pipe mold and the bluish areas are those that were cut from flat slabs and added in. With the teapot, I was going for an art deco look and I wanted the curves of the handle, spout, and lid to reflect those of the body, so I used parts of circles for them. I even used little partial circles for the feet, which are not visible in this picture. In case you're wondering about the patterns all laid out in an earlier photo, not all of them were for this project.
Variations on using the PVC half pipes as molds could include using smaller but complete pipes with elbows, T's, or what have you and slumping the clay over whatever interesting thing you come up with. For example, if you use four elbows and some pipe to make a PVC square or rectangle, you could slump a slab over it to make an interesting platter or bowl. I would also like to make a larger version of the PVC square and slice it in half to make two square PVC troughs, then slump slabs in them and add a bowl or something to the middle to make a square chips and salsa feeding trough for parties. I'm not sure if I explained that enough for you to imagine it, but it's clear in my head. I'll add a photo of that if I ever do it. If you make something using a PVC mold, please post a picture of it so everyone can see it and get ideas!