For a medium like clay getting rigid angles and perfect curves is a nightmare. However 3-D printing and making a mold opens up a whole new world of possible shapes and forms.
This mold will be for a bowl, I know it looks like a big hunk of plastic right now
Made at TechShop Chandler
Step 1: Setting Up
I made my mold boards out of a piece of melamine and a few plywood scraps screwed on. This makes it easy to clamp a rectangle in any size so they can be reused. I then placed my 3-D printed object inside. The top of the bowl will be where I pour my slip (casting clay) in so I don't want to make a plaster wall of that. I made a small clay slab to block that part off and to attach it to the table. I then sealed up all of the seams with clay so I don't make a mess (spoiler I made a huge mess)
Step 2: Mixing Plaster
To make ceramic molds you need to use pottery plaster, not plaster of paris. This can be bought at a ceramics store or online, it's fairly cheap. 30 bucks for 50 pounds is what I paid.
Fill your bucket up with cold water, then slowly add the plaster powder. You'll keep adding plaster until it floats on the surface for about 5 seconds like in the first picture. Once it is at that point mix it gently with your hand/whatever for around 10-15 minutes try not to get any air bubbles in it. The consistency should be a little thicker than milk, not as thick as peanut butter, like that drinkable yogurt... That stuff is weird.
After it's at the right consistency pour it into your mold until it completely covers the object your are molding and then plus some. You want at least an inch around all walls.
Step 3: Part 2
Once the plaster dries around your object which takes around 15 minutes you can remove your boards and take a look. If you are like me you'll realize that you didn't plan a way to remove your object from the plaster before hand and now you'll have to be super careful about how you remove it.
Most people make molds of whole objects, since 3-D printing has a limited bed size and takes a long time I thought I would print half of the bowl since it was symmetrical to save myself time. Unfortunately it made it hard to remove from the plaster.
Anyway, once I removed it I flipped it over and inserted a little bit inside my first half of the mold (about an inch) to prevent the next batch of plaster from leaking in. I also made registration holes by rotating a dime around in the still soft plaster.
Set up just like you did before, seal the seams and so on, mix the plaster. But before you pour the plaster brush everything the new batch of plaster with dish soap. This will act as a mold release and prevent the 2 parts of the mold from sticking to each other
Step 4: All Done!
Once that plaster dries you will be ready to see your mold. Unfortunately like a real dingus I was a little too rough removing my part and I chipped the bottom of my mold. If I wanted to I could patch it using more plaster. However I am just going to redo it right. Once you have your mold it will need to dry for a week before you cast anything in it.
The second 2 pictures are of a mold I made the exact same way for a cup. You slip in the top and since the plaster absorbs moisture it drys the clay only around the edges. Once the wall thickness is where it needs to be your pour the remaining slip back into your bucket, wait 20 minutes and you have a ceramic part to use. More instructables on casting and making slip to come.