I use 2% thoriated on everything. You shouldn't use pure on an advanced square wave machine like a miller dynasty. For these blocks I was on a synchro wave 300. Sharp pointed 1/8" tungsten, #8 cup, 200 amps, balance was set to about 8 on the balance/dig control I think. The pointed electrode allows less arc wander than a balled tungsten and is less prone to tungsten spitting. You want to adjust the balance to where you have just enough cleaning to get the job done. I coulda turned the cleaning down on these a little but I didn't clean them before I welded them. Turning it more towards dig will cause the AC cycle to spend more time at the negative portion of the cycle and will shrink the white cleaning action closer to the bead. The cleaning action should be fairly close to the toelines of the weld. I'd say anywhere from 1/8"-1/4" or so.

A little note about AC aluminum welding. Aluminum has an oxide layer on it that is almost as hard as diamond and has a higher melting point than the metal below it. That's why we use AC.
AC is essentially dc - then dc+ repeated over and over.
Dc - focuses the heat towards the base metal. (penetrates)
Dc+ focuses the heat towards the tungsten. (cleans)
Balance control changes the time spent at each cycle.
Too much cleaning results in lack of penetration and a melted tungsten.
Too much penetration (dig) results in a sink hole type puddle that is not clearly defined.
The trick is the puddle will turn a shiny chrome color (think terminator 2 bad guy)
When the puddle turns that color only then add filler. If you try before that then the filler will melt and not join to anything ( think solder dropping on the ground)

That's all I'm typing for now as I'm laying in bed and have been up since 3:45 am. I'll add more tomorrow. 
One last thing, you should see me weld with two good hands.