/three month posting hiatus
Here's a long overdue tutorial a few people on coscom requested ages ago...
^---on how to make molds for things like these. please note that for the super-tiny gems, I used the generic casting tray that is available any and everywhere, and for the medium sized ones I was able to use a paint tray as a ready-made mold. the five shapes needed for the waist cincher thing and the two sizes of larger gems I needed were unavailable, so I had to find another solution.
Okay, Umi obviously has a lot of different size gems. there are a total of 20. :/
I looked around on the internet for ages, but even if I had ordered semi-circle spheres that were kinda almost the right size and tailored my sewing and sizing around them, stiff plastic is pretty obviously a really, really bad idea for casting gems in (good luck getting them out when they've cured...). and I'd have still been at a loss for the corset-gems since those had to be of a very specific shape. So I had to find some way to make my own flexible molds. Luckily, smoothon plastics exist! Look here and at a Dick Blick's near you (if you have one in your town).
the first step was to figure out how large I wanted the gems to be. after doing a rough sizing of them in paper/tagboard, I find that cutting out chipboard for a base is a good way to proceed, as it provides a solid foundation for whatever type of clay you choose to put on top of it. the chipboard is fairly stiff so it takes a little bit of shaving to get it down to the perfect dimensions once again after transferring mediums. Numbering them can also be handy, if you have a lot of similar shapes floating around.
the next step is to sculpt your gems. I used wax paper to help keep the sculpey from sticking to anything.
Incidentally, spray finish also makes a good rolling pin in a pinch, if you're cooking averse like I am and are not domestic enough to already possess one amongst your kitchen utensils.
After I finished shaping and beveling my sculpey gems, I baked them and then sanded them with a lower grit all the way up to 600 grit before hitting them with a few coats of spray sealant so that they were nice and smooth and shiny. I probably would have sanded to a higher grit if I had known where to find it at the time (apparently automotive stores extend into a higher range). since at the end you'll be casting something that's clear and will reflect light, ANY sort of tiny imperfection or roughness that was left on your original will be EXTREMELY noticeable, so be sure to take your time at this stage to get everything absolutely perfect and smooth.
After thinking a while on the most efficient way to arrange the pieces so that a maximum amount of casting rubber would be conserved, I decided on this configuration and then walled it in with some oil-based clay on top of some wax paper.
I went through the same process with making the original pieces for the headband and front shoulder join gems as well. I just shoved them inside some random plastic container, since they were a more conformable shape than the corset pieces.
Then it was mixing time!
dun dun dun! dramatic shot! (you're supposed to pour both cups into a 3rd cup, so that the rubber left sticking to the cup in the form of residue is equally distributed in terms of loss, but as you can see I neglected that step. oops.)
I tried really hard to take a shot of the rubber while I was pouring it (balancing a camera while trying to pose flowing substances is difficult!).
this is actually a picture from a different batch, but it illustrates the color your mixture turns when it's completely integrated into one. incidentally, if you let your bottles sit around after opening for several months too long, the rubber will still cure, but just be really, really stiff, as in the above picture. in the end I had to go buy new rubber (what I was using in the rest of the pictures) since it was too viscous to flow properly around my originals and capture their shape, or to let the bubbles escape >:
pouring it on!
the corset patch being inundated in goopy liquid.
it should level out nice and flat.
after curing, just pop your originals out, mix up a batch of resin, and cast away! here we have nono, tear, and KOS-MOS keeping guard over some panickedly-cast resin that I mixed something like, 6AM the day I left for otakon...>_<
an example of the end result~
another shot that encompasses waist cincher gems.
there are plenty of other tutorials that cover general easycast resin casting, so I won't go into it here. just do note that while the rubber cleans up fairly easily, god help you if get the resin on anything at any stage....nasty stuff.
as for the backing of the gems, I originally went with painting the back and then attaching them via hot glue to the costume. the hot glue stuck, but the adhesion of the paint itself to the gem was somewhat less than desired. my currently recommended method is painting a backing piece of tagboard/paper with the sparkle paint of your choice, dumping a pile of hot glue on it, smashing the gem onto it, and then trimming away the excess paper. THEN glue the gem to your costume. works sooooooooooo much better. (I know some people have had luck with E6000 as well, although I personally wasn't able to get it to work for me.)
hopefully this is useful! sorry again for the wait. this was my first foray into the world of making molds and casting, so I'm far from perfect at it yet. perhaps it can still shed a little light on the process for anyone was previously mystified by the process, however. (I wasn't able to find anything that addressed making molds for easycast resin when I was researching things for the first time myself, alas.)