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I just read 'Kitchen Chem 101' and the home experiment with roux.  When 
experimenting with cornstarch, the results were--

"Cornstarch...It browned but was very lumpy.  I mashed it out and
added 1 cup water , boiled it to get rid of the lumps and it doesn't
not thicken.  But I could use it then thicken with something else
before serving."

I think the problem may have been the 'boiling to get rid of lumps'. 
Boiling may add to the lump problem.

I have found that cornstarch works very well.  It browns nicely, just like 
wheat flour.  If you add the liquid slowly, stirring with a whisk instead of 
a spoon, there should be no lumps.  Keep the heat on medium (barely simmer 
rather than boil the mixture)-- It may take a bit longer to thicken this 
way, but the result will be a smooth, thickened roux.  Stir constantly with 
the whisk until mixture thickens to prevent lumps and sticking on the bottom 
of the pan.

Connie 

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