JohnW, 1% impurity from boric acid you see here. http://www.chemicalland21.com/indust...ric%20acid.htm
(SALES SPECIFICATION).

BORIC ACID

APPEARANCE white cystals
BH3O3 99.0% min
PHOSPHATES 0.001% max
SULFATES 0.005% max
INSOLUBLES 0.005% (in methanol)
Ca 50ppm max
Cl 10ppm max
As 1ppm max
HEAVY METALS (as Pb) 10ppm max

George