Pepitas, the inside of pumpkin seeds, fluoresce under UV light! It is a stunning coral orange color. This photo doesn’t quite do it justice. The compound causing the fluorescence is protochlorophillide, a precursor to chlorophyll. (Chlorophyll is also fluorescent under UV light, but it glows a deep red.)
The seeds themselves have a slight glow if you shine a black light on them but in my picture at the top, they are crushed with isopropanol (rubbing alcohol), which solubilizes the pigment. We found this fluorescence by mistake actually. Several things in your pantry fluoresce under UV light (like honey, canola oil, tonic water, and peanut butter) and my daughter and I were scanning our shelves for other surprises. Sure enough we saw a faint glow on some of the hulled pumpkin seeds. I did a little research online and found out about protochlorophyllide. We also saw a similar glow from brown rice that was slightly green on the edge and I wonder if it’s the same molecule!
In one bowl, dissolve 1/2 tsp of borax into 1/2 cup of hot water.
In the other bowl, mix 1/4 cup of glue with 1/4 cup water.
A tsp at a time, mix 4-6 tsp of activated charcoal into the glue/water mixture. It won’t mix in at first, but just keep stirring, it will eventually mix in! Just a minute or so of stirring. Stop adding when it’s black enough for you. Mix until thoroughly combined.
Add 12 tsp borax solution to the glue mixture a teaspoon at a time while stirring. This slow addition of the borax ensures a super smooth slime without the need for lots of kneading. You will know when to stop adding when all of the black glue mixture is in the ball of slime and there is none left in a puddle at the bottom of the bowl.
Pick up the glob and fold and squish a few times. You’re done! After you’re finished playing with it, store it in an air tight container. If it feels gooey after a few days, add a little more borax solution till you get the consistency you like. We think this gradual goo-ing might be due to the activated charcoal absorbing some of the borax (see below for what borax does!).
The glue contains a long molecule called polyvinyl alcohol (PVA). It is a polymer, which is a molecule that contains repeats of a subunit molecule (for example, “A” is a subunit and “AAAAAAAAAAAAAAA” is a polymer of A). Borax (sodium tetraborate) is a small molecule that can stick to parts of the PVA through hydrogen bonds. One side of the borate molecule can stick to one strand of PVA, while the other side of the borate can also stick to a different strand of PVA, creating a bridge between the two PVA strands. This is called crosslinking.
Having many crosslinking sites usually makes a polymer more rigid, but the interesting thing about borate/PVA crosslinking is that the bond is transient, meaning it can easily break and reform somewhere else. This causes the slime to act kind of like a liquid and kind of like a solid. If given time, the PVA can ‘flow’ as gravity pulls and breaks the borate/PVA crosslinks. It acts like a slime instead of a true liquid because as the PVA molecules pass by more borate, they can momentarily bond to borate and another strand of PVA, slowing down the flow. If you pull the slime fast, you break all those bonds quickly, allowing the slime to act like a solid momentarily.