http://www.aws.org/cgi-bin/mwf/topic_show.pl?id=1460 <---Here is a link to some discussion within this message board
focused on electrodes.

The specific reason that Thorium alloyed electrodes are problematic for use with Aluminum and Magnesium is that Alternating current (AC) is generally employed and the thoriated electrodes tend to decay if overheated during the reverse polarity side of the half cycle. This degradation of the electrode will manifest itself in two different ways. First the tip shape of the electrode will change mid weld and arc performance will suffer. Second and more important is the problem of tungsten itself transferring across the arc and into the weld pool and its byproduct of leaving unwanted inclusions. If your welding aluminum using DC straight polarity and helium than thorium alloyed electrodes may be an option. Other alloying elements, namely Zirconium and Cerium are blessed with performance properties which are more well suited for AC welding of aluminum and magnesium.

If you require *Sourced* Material, any of the following beginners GTAW manuals and standards have the necessary data

Modern Welding Technology by Howard B. Carey

Gas Tungsten Arc Welding by Lincoln Electric

Gas Tungsten Arc Welding by Miller Electric.

Guide for the Training and qualification of welding personnel, Level II Advanced welder.. By AWS.

Specification AWS A5.12