This is a followup on the original post featuring the Making a "Thing In a Bottle" tutorial. Since then I've made about three dozen more bottled specimens, progressively refining the technique as issues impacting the long-term stability of the projects pop up. I've previously written about ways to insure an air-tight seal. This post will cover the long term viability of a variety of materials I've used.
First off, I have some serious doubts about the survivability of polymer clay. By itself it's a brittle material that is likely to crack as repeated impacts of the sculpted critter against the interior of the glass bottle create stress fractures. This can be alleviated somewhat by using a wire armature as reinforcement, but fine detail work is always going to be extremely delicate. The *only* way to prevent specimens breaking apart is to coat them with a soft, yielding material that can absorb and disperse the force of impacts. Both latex, in the form of rubber cement or liquid latex adhesive, and silicone are ideal for this.
Speaking of latex, I've been shocked by how tough it is. I have specimens coated with hardened liquid latex floating in distilled water, isopropyl alcohol, and glycerine and none of them have shown the slightest signs of deterioration. This probably shouldn't have been a surprise, since exposure to air and sunlight are what causes the latex in masks to deteriorate. The environment inside a specimen bottle is naturally free of ultraviolet light (it's blocked by the glass) and the limited amount of oxygen present probably isn't enough to trigger any significant oxidation.