Kiss My Knits!

So before I opened up a big container of plaster of Paris for a Brownie project with a bunch of second-graders, I thought I’d better give it a go first. And I’m glad I did!

Step one. Assemble materials. It was at this point I met the first obstacle. Do you see something in the picture in the right that could be potentially disastrous if you wanted to pour something semi-liquid into these eggs? Yep. Holes. I didn’t notice them when I was buying the eggs.

After some thought as to whether I should return them and try to get my money back, make some sort of plug with my hot glue gun, etc., I opted to try taping over the holes. I used one piece of tape on the inside of one end and one on the outside of the other to see if there was a difference in effectiveness. There wasn’t. Scotch tape works just fine, and it has the added benefit of being something my girls can do themselves as part of the project.


Step two. Mix up the plaster. This stuff set up much faster than I was expecting. I mixed up the blue, filled half of an egg, then added some yellow to try to get a green. We’re going to have to add a lot more color. I’m thinking about food coloring, but that can stain. The tempera paints won’t. By the time I added the green to the second half of the egg, it was grainy and I was able to pack it in with my fingers without getting any stuck to myself.

ETA: Looking at the original tutorial, I used a greater ratio of plaster to water than skiptomylou did. She used 1/2 cup plaster to 1/4 cup water. I used 3/4 c : 1/4 c.


Step three: Close ‘er up! Per the instructions, I closed the egg and gave it a good tap to settle the contents against each other. I opened it and scraped a little out, then closed it again. It still smushed out again. I didn’t think about the results of having fast-hardening plaster cementing my egg closed. Oops.


I sealed it shut with packing tape and put it in a Ziploc bag, reasoning that since the girls have to take it home to dry overnight, it should be safe and secure for the drive home. I don’t think their parents would thank me for having plaster dust all over the backseats of their cars.

But something cool happened that I did not expect! My egg felt warm. Then it felt HOT! Not like “spontaneous combustion” hot, more like “hot water bottle” hot. It’s not going to hurt the girls, but it’ll definitely be noticeable. Why did it get hot? A chemical between the calcium carbonate and the water generated an exothermic reaction, producing heat! HOW COOL IS THAT??

I’m sorry, I get really torqued by science. I remember hearing about how when they were making the Hoover Dam, there was so much chemical energy from the concrete that they had to run coolant rods through the the concrete to dissipate the heat. I quote Wikipedia: “During the concrete-pouring and curing portion of construction, it was necessary to pipe refrigerated water through tubes in the wet concrete. This was to remove the heat generated by the chemical reactions that solidify the concrete. (Otherwise, the setting and curing of the mass of concrete was calculated to take about 125 years!)” Other sources here, here, and here. I didn’t anticipate this occuring on such a small scale with my little egg!

Anyway, I’m trying to figure out how to explain this to a bunch of seven-year-olds. Maybe talk about how the plaster and the water combine and release a lot of energy, and that energy becomes heat? Maybe I can relate it to how microwave ovens make heat by making the water molecules vibrate. Or maybe I can explain how all that energy has to go somewhere, like when you’re feeling really energetic and you have to run around to let some of it off. I’m not sure yet! Anyway, it stayed hot for a good half hour before it started to cool back down again.

Moving on!

Step four: Open ‘er up! Technically, you’re supposed to wait overnight, but like all kids and children-at-heart, I was impatient. When I opened it, it still felt just a little damp.

The opening process was my downfall. I did grease the inside of the egg with petroleum jelly before glopping in the plaster. Unfortunately, I didn’t think about the plaster that had coated the place where the halves come together. Or maybe I didn’t coat it all the way to the top with the Vaseline. Or maybe the exothermic reaction generated enough heat to cause the material to expand. Whatever the reason, neither Emily nor I could get this open on our own. Much banging of the egg on things and grunting ensued. It didn’t help. I finally had to get a knife and run it around the inside of the egg near the middle. That worked. I had to do it for both sides.

ETA: Going back to the original tutorial, she did in fact mention that you’d need a knife to open it. I hadn’t noticed that when I first read.



However, I have to say, it did work. And now I’ll know not to fill it up all the way, and to use more water in the plaster. I think this will be a fun little project, if a bit messy. The whole thing came in at under $15. If I hadn’t bought tempera paints, it would have been even less. I’m still debating returning them (I used my daughter’s for the sample, didn’t open the new ones) and using food coloring. But our cookie sale was good enough that I don’t really have to. There are bound to be other opportunities for us to use poster paint.

Sandra (my co-leader) and I will also be giving out badges. And a couple of days ago, I also asked the liaison for the church whether we could use the playground. She hasn’t answered. What’s the old saying, “easier to ask forgiveness than permission?” So I think it’ll be a generally fun meeting. I’m actually looking forward to tomorrow!

ETA: We just got playground permission. Huzzah!