Sumac is from the same, almost colourless, group of gallotannins like oak gall and tara. This makes it a great base for the 'galling' stage of cellulose mordanting, before mordanting with alum. 
 
Sumac was well known and used by the master dyers of the 18th century, and it is mentioned often in the old recipe books.
 
In our sumac powder the hydrolysable tannin content is up to 30%. Use around 40% WOF for a good tannin base on all textiles. Simmer powder for one hour with plenty of water, strain through a mesh and add textiles to the remaining tanning water for another 1.5 hours at around 60ºC and leave overnight. Stir occasionally. You can re-use the galling bath.