Another part of your problem may be that the microwave energy generated by (almost) all commercial microwave ovens has very little effect on paraffin.  They are designed to heat water, with the peak wavelength of the klystron emission designed at a frequency absorbed by water.  This is partly why they do such a poor job of defrosting; ice does not absorb at the same wavelength.

You can make a delicious "reverse baked alaska" in the microwave:  Form ice-cream into a 2-3 inch ball, hollow it out, and fill with jam.  Re-cover the ball with ice-cream, and microwave it.  You will want to experiment with this, as it is easy to overheat the jam, causing either a mess in the microwave, or a burnt tongue when eating it. The icecream takes a long time to melt.

I suspect that where you had melting of your paraffin, it was because there was a little residual water on the slide, enough to absorb the energy, and transfer it by conduction to the slide, and then to the paraffin.

Good luck with your experiments,

Adrian Leek.
 

Melissa Jensen wrote:

  We have used our microwave to melt paraffin from tissue slides..to speed up the turn around time for recuts.3-4 mins with a 700 watt...Our old microwave bit the dust. We have a new one 1000 watt..I have set the power to 70% and used the 3-4 min thing...No luck..Tried it at 3-4-5- mins..also at full power..I cant find a consistent setting...any advise! Its either fried or paraffin gloss.   Thanks