Modvat credit on sulphuric acid clarified: restriction to duty on acid actually consumed upheld under classification rules. The Board reaffirmed that Modvat Credit on sulphuric acid for detergent manufacture is to be restricted to the duty element on the acid actually consumed in sulphonation, derived by deducting the duty-equivalent of spent acid from duty paid on the concentrated acid or oleum. The Chief Chemist opined that spent acid remains the same product and, aligned with tariff conference views, is classifiable under Heading 2807; nevertheless the Board confirmed the earlier instruction needs no review.