This video demonstrates how to make your own silicone moulds using silicone sealant (caulking) and corn starch.  I had to try it out for myself and see how it worked for polymer clay.

I worked at a somewhat smaller scale.  Partly for reasons of space, partly because I didn’t want to dish out the money for one of those caulking guns, and partly because I didn’t know if those guns could be sealed off to keep the rest of it from setting.  And for doing a few experiments this was more than enough.

One caution, though: working with a smaller bowl like this?  You get cornstarch everywhere.  On my work surface, on the floor, on me—everything.  If your space allows it might be worth using a larger bowl, even for making small moulds.

Also, it had been a while since I’d actually watched the video, and I forgot that she was wearing gloves.  You can do this with your bare hands, but it does mean you’ll be picking bits of the silicone-and-cornstarch mix off your fingers and out from under your fingernails.

Overall I’m pretty pleased.  I made the mistake a few times of not mixing in enough cornstarch, and if you do that it will stick.  Bits of the mould will want to come off on your object.

At this size and thickness you don’t get the flexibility that she demonstrated in the video.  But they do bend enough to release the clay from the mould.  However, when using clay it’s a good idea to dust the mould with a bit of cornstarch or some other release agent first.  I had a few instances where the clay wanted to stick and got distorted.
So far they’ve held up pretty well. I’ve used this mould several times, and haven’t noticed any drop in detail nor seen any cracks in the mould.

To compare, here’s the original mother and a moulded piece:

As you can see it’s not a perfect replica, but it’s good enough for most purposes.

I still don’t know how these moulds will react to baking.  The silicone should be fine, but the cornstarch is a bit of a wildcard.  Polymer clay is baked at a pretty low temperature, so I suspect (hope) it will be fine, but what I don’t know is what, if anything, it will do to the clay that’s in the mould.  That’ll be my next experiment in the realm of DIY mouldmaking.

(Just as a side note, these moulds darken over time.  I didn’t realize it until I made a second batch and noticed the first batch was now a darker yellow than the new moulds.  I assume this is the cornstarch oxidizing or something and is probably benign, but time will tell!)