I know gum has different qualities (grades) but I didn't think that the stuff from arts supply was inferior. Well, you learn something every day.

Here is a link to some interesting (but not neccessarily related to artistic use of the gum) information: http://www.fao.org/docrep/w3735e/w3735e22.htm

I don't know where you are located in the Union, but here is one supplier: http://www.kremer-pigmente.com/en/index.html