Do tubes "burn in"?

My concern was not that after burn in the tubes would sound the same. I was just seeing if I liked the 12SX7's at all. They sounded different than the 12SN7's, as expected, but I did not expect them to have such poor clarity, especially noticable with jazz, acoustic, and vocals. But, as stated, they need to warm up ... really warm up. Maybe obvious to most people, but it really was not to me. My stock equipment sounds pretty much the same whether warming up for 10 minutes or 1 hour, so I was not used to allowing for more warm up time. After warming up the 12SX7's for a complete hour, they sounded way better than before. Now I like them, although they are not as clear as the 12SN7's, after proper warm up their clarity is acceptable, which is good because I like their other attributes: better imaging; a little tubier in the upper and lower freqs. Jimi Hendrix and Rory Gallagher never sounded so good. Thanks guys for all of your explanations and insight.

They definitely settle and smooth out after maybe 10 hours.. Just probably something with the metal getting heated and or some kinda substance burning off the plates and emiting a stronger more consistent current = sound.. Only thing I have to support that there is a DEFINATE physical change in a tube one way or the other although rather minor is, Many times when I plug a brand new tube in, even for the maybe first 5 to 10 power ups, the Puff, Peak, Pop, And fizzle a little thru the system, and even with no speakers hooked up you can hear this crackling right through the Glass on the tube itself, then all of a sudden on power up and useage they are smooth as butter and dead silent, so something is adjusting due to the applied heat I would assume.

What is normally used to clean tube contacts?