Re: What causes the yellow boils in our ice rinks?

Hi Brad:

My apologies for the delay in answering you (your question stumped 4 of our experts!), the winter is almost over, but this is an interesting topic (to a Canadian like me, anyways). I remember that when I made ice rinks in my backyard, I would have exactly the same problem. It always bugged me because those discoloured areas were always bumpy and interfered with our hockey playing: they were a tripping hazard and messed up my puck handling.

As far as I can tell, nobody has ever studied these bumps and what causes them, but I’ll speculate on what they probably are. When water freezes into ice, it often rejects materials that are dissolved in it. For instance, when seawater freezes, most of the salt is rejected and forms a very salty brine, leaving behind pure water ice. This is partly true for organic material as well, though according to this site some may be retained. So here is my hypothesized reason for those yellow bumps:
1. When ice forms over grass or soil (instead of snow), some organic matter dissolves in the water prior to freezing.
2. As the ice forms, some of that organic matter is rejected (to the top and bottom surface of the ice), and some remains in the ice.
3. The organics in the ice will alter the crystal structure of the ice, which is what makes it appear “softer”, as well as adding colour. This page talks a bit about the effects of organics on ice crystals.
4. The colour of the organics will also tend to make that patch of ice absorb heat from sunlight a little better, so on a sunny day, it will tend to start melting a little sooner.
5. The combination of melting, refreezing, and rejection of organic (and inorganic, for that matter) material leads to the yellow patches becoming a little higher than the ice around it.

I asked my sister about this (she’s a better hockey player than me!), and she tells me that it is indeed important not to expose any grass or dirt on the first flood. Her suggestions for good ice include having a deep base of well-compacted snow, and being careful to apply a small amount of water during the first flood. It should only create a slush that is allowed to refreeze. I know that some people also put down plastic sheets before the first snowfall to ensure that they get a nice ice surface. You can find out more with a quick web search (there are some companies that sell special rakes for smoothing out ice surfaces). I came across this site in my searches, and it has lots of suggestions.

I hope that helps, and best of luck with your future rinks!
Rob Campbell, MAD Scientist