Why does lead nitrate powder on heating starts turning yellow from the top rather than from the bottom? Today while in school, I observed the thermal decomposition of lead nitrate. The peculiar thing I noticed was that the lead nitrate powder in the test tube starts turning yellow (due to the formation of lead oxide) from the top and then towards the bottom. This indicates that the certain decomposition temperature is first reached at the top and later at the bottom. 

I tried to apply a logic that as soon as some heat was provided to the bottom of the powder in the test tube, it was quicky transferred to the powder present above it and so on till the top. At this point, since the heat had nowhere to go (since air is a bad conductor is heat) it got accumulated at the top and hence it reached a higher temperature.

However I'm unsure of my logic because for the top powder to reach the specified temperature, the lower powder must first reach either the same or a higher temperature (otherwise the powder present above it would not get heated).
Lead oxide is a peculiar type of salt. It's brown when it's hot and turns yellow when it's cool. The lower most part remaining white could be due to two possible reasons:


The top-powder becoming yellow indicates the region is more cooler than the salt with is present at the lower part of the test tube. The lower most part is in direct contact with the flame and the  upper part is receiving heat due to conduction and losing heat to the atmosphere; many factors could be at play here.
Another possible deduction could be the paucity of atmospheric oxygen at the bottom of the test tube than at the top and hence the rate of conversion to lead oxide to the top would be faster than at the bottom.




In case you haven't done the experiment and want to see what the OP is trying to convey, here's a youtube video that demonstrates the whole lab-based thermal decomposition of lead nitrate (it starts at 1.32  incase the timed setting of the link fails):
http://www.youtube.com/watch?v=csEnz9ZsAFM&amp;t=1m32s