The highest grade I have seen from Rio Tinto specs is NF, a pharmaceutical grade, which lists only heavy metals at <20 ppm. The difference between Rio Tinto Tech granular and powder is ONLY in the water insoluble particles, which are <400 ppm in the granular and none in the powder. The variance in measurement of percent borax decahydrate is in the water content, which will vary with storage conditions due to changes in humidity. the same variance can occur in any grade, including AR.
Originally Posted by Kirk Keyes
Your AR grade assay was for a particular sample of known hydration