So it must be the decahydrate, as it should be if it says "Borax". Labels are strange things. Then, too, impurities can get into a product several places along the line. IMHO, borax is the thing we should worry about the least, as in most recipies + or - 10% won't make a noticable difference. The concentration of the sodium borate will affect the local rate of change of pH more than the equilibrium value. Old timers used to increase borax in D-76 to as much as 20 grams/l to boost the film speed.