Electrifying Butter

So to preface this, I'm going to be very clear that what I'm writing is not based on any real knowledge I have about how microwaves and chemistry works, and it is 99% based on information I read on the internet from sometimes sketchy websites (the 1% was based on conversations I had with Jeff while he was playing Halo).

So here's the story:
When we were in Michigan we were sitting down enjoying dinner when my sister and brother-in-law told us about a butter sparking problem they were having with their microwave. About a year ago (I think) they were trying to soften some butter in the microwave in a Denby (microwave safe) dish. After about 2 seconds there was a spark that shot across the microwave. They took it out and it didn't happen again until several weeks ago when it started happening regularly, that is, every time they tried to microwave butter.

So the facts are:
- only butter sparks
- they used the dish in the microwave with many different foods without any sparks
- the butter is unsalted (salt is an electrolyte)
- it happens well well before the butter melts, generally after 2 seconds or so
- they didn't leave the butter in long enough to know if it would continue to happen after the butter was soft/melted
- they had probably done it with two different types of butter
- I don't know what kind of microwave they have

We sat at the dinner table and peppered them with questions for the rest of the evening, not able to find anything obvious (we had 4 young well educated people: chemistry, engineering and biology majors sitting at the table, so hopefully our minds were at least asking the right questions). No physics majors though.

So we left without any answers, but I promised to blog about it if my sister showed me. So we went back to their house and stuck a piece of butter in. After about 1 second there was a PPPSSSHHHT! noise but no spark, but sister immediately stopped the microwave, although she was sure it would have sparked if she would have left it going.

So, after a bit of research on the web, here's my own personal crazy theory.
Two things are fairly well documented on the web (life lessons according to the interweb are often dangerous, but keeps things exciting, right?):
1. Microwaves spark on a more regular basis than you might imagine. Usually when this happens it is because it is dirty. It sounds like there is some side panel which is what gets dirty and the oil/food on there burns and sparks. I think that can be cleaned or replaced to solve the general sparking problem. But their problem only happened with butter.
2. Others have reported a butter sparking problem or other one-food related sparking problem. To explain this issue, a little background. Microwaves work by heating up the water in food (more specifically, it's a factor of the polarity of the water - at least according to some website). Microwaves are well absorbed by water, but not well absorbed by ice.

So here's my theory, assuming they don't have a dirty panel (which could cause sparking generally), I think when they start the microwave one bit of the cold butter starts to melt, and the liquid absorbs way way way more microwaves than the rest of the "pat" of butter (this was documented, that it can happen within a second). This large absorption of microwaves in relation to the rest of the butter causes the spark or at least the PPPPPPSSSSHHHHT! noise we heard. In addition, butter itself is a dipole and can get very very very hot in the microwave, so it seems reasonable to assume that in one or two seconds one little bit could melt and start absorbing more waves than the surrounding butter.

How's that for an uneducated guess at butter sparking?

There is also the well-documented experiment of grapes sparking in the microwave (actually, flaming), which is due to some very long complicated explanation which I didn't have the patience to read. So it is also possible that whatever causes grapes to flame might also cause their butter to spark. I'm not sure though. I think the mystery still continues.

But if anyone knows why this might be happening, let me know!