I have also made molds with silicone caulking & dish soap. And I have used the Naphtha + silicone caulking method. Both have a strong odor, but the latter is the worst of the two. Plus Naphtha is a flammable chemical, so you have to be pretty cautious with it generally. I have also used the cornstarch + silicone method. I think it was that one (cornstarch) that became brittle over time.
My recommendation if you try any these methods is to set up a work space in your garage or better yet, at a picnic table in the back yard. Even outdoors on a deck is preferable to smelling up your home. I've done it outside and as long as the weather is good, there the molds stayed to cure.
As for the odor, the caulking smell is bad enough on it's own, but with cornstarch it eventually wasn't so bad. The worst one was the Naphtha + silicone mold. The odor never seemed to go away, but we tossed it eventually anyway, so I can't say how long that one would have taken to dissipate.
But here's a video on that method if you are interested:
When I did it, I used more Naphtha than this guy, and in fact, painted the material onto to the surface, so that is probably part of the reason it was so awful smelling.
