A few years ago I made my first Thing in a Bottle out of a glass apothecary jar and a dollar store toy "grows in water" starfish. For my second Thing in a Bottle, I used an apothecary jar with a smaller opening and I used a toy snake instead of a starfish, hoping it would give me a better look once it was in the bottle.
The first step was to soak the snake in water for a few days, clean it off and let it dry out and shrink again. I don't know why, but for some reason, the first time you grow these things they end up creating a thick coating of slime all over themselves. That needed to be cleaned off before I could paint it.
After cleaning off the slime and allowing the snake to dry out and re-shrink, I cut off its tail. The whole snake would have been far too large to fit into the jar once expanded, so I only used a small piece of the tail. Then I used an exacto knife to cut the stump into four tentacles, to make the new "head" look like a tentacle faced worm. As crude as my effort was, it still looks better than a cut off stump.
Next I painted the thing with acrylic paints mixed with latex rubber. I made sure that I left the paint job spotty, with thin areas and small areas that had no paint. This would ensure that the latex would stretch unevenly when the thing grew, causing the surface color and texture of the original snake to show through in places. It makes for a very interesting skin texture. I also used permanent markers to color some areas, like the new "mouth".
Then I put the specimen into the jar and filled it with water. After a few days it will soak up the water and grow to several times its original size. I will have to top off the water as it grows.
After it was grown, I topped off the water level and added a little red UV reactive dye. I normally go for yellows and greens in these types of things, but I thought I would try something different.
I just love how the latex stretches and gives me that unique mottled skin appearance.
Finally, it was capped with a cork and the cork was tied on with hemp twine. I like this look better than the wax seal most people use. In order to prevent the water from evaporating through the cork, I first coated the cork with a little clear silicone sealant. I can't remember for certain, but I think I picked up that trick from Propnomicon.
That's pretty much where it stayed for a little while. This prop was originally made for use in the Aethernauts LARP at Origins, so I didn't do any labeling or antiquing to it, as it was meant to be a specimen captured by the ship's doctor just recently.
However, after the LARP was finished, I did add a Miskatonic University label to the jar as well as some antiquing to the bottle.
The label was printed on paper and antiqued with tea and/or walnut crystals. It was written on with a quill pen and sepia ink. Then it was white glued to the bottle, and after the glue was dry, I sanded the edges of the label to simulate years of wear.
The bottle was antiqued by lightly misting it from above with spray on adhesive. Then it was gently sprinkled with black grout mix powder.
Et voila. A specimen of an unidentified parasite fit for a fictional university collection.