I checked the book last night and here was the passage in relation to sodium bicarbonate:
This passage comes from "A Textbook of Fire Assaying" by Edward Bugbee, on pg. 8.
"Sodium bicarbonate (NaHCO3): Is still used by some assayers as an alkaline flux, mainly because of its cheapness and purity. It is decomposed, however, when heated to 276C, forming the anhydrous normal carbonate with the liberation of water vapor and carbon dioxide. The large volume of water vapor and carbon dioxide released, passing up through the charge before it has softened, is bound to carry off more or less of the fine ore and thus contributes to the so-called 'dusting' loss.
The bicarbonate contains but 63.1 per cent of Na2CO3 (sodium carbonate), and therefore when it is used as a substitute for the normal carbonate 158 parts are required for each 100 parts of normal carbonate. Because of the above serious disadvantages the use of the anhydrous normal carbonate is recommended in all cases. The only advantage witch can now be claimed for the bicarbonate is that it does not deliquesce."
Hope this helps.