1. A METHOD OF PREPARING A REFRACTORY INSULATING BLOCK WHICH COMPRISES SLURRYING 100 PARTS OF REFRACTORY MATERIALS CONSISTING ESSENTIALLY OF ROCK WOOL AND A SUGARAMMONIUM SULFATE BINDER, IN ABOUT 600 TO 1000 PARTS OF WATER, FILTERING THE LURRY AND SHAPING TO SOLID MATERIAL INTO BLOCK FORM AND BAKING THE SAME AT ABOUT 300*F. TO ABOUT 500*F., THE AMOUNT OF SUGAR-AMMONIUM SULFATE BINDER BEINS SUFFICIENT TO PROVIDE 0.5 TO 6 PARTS OF BINDER, DRY BASIS, IN THE FILTERED BLOCK, THE AMOUNT OF AMMONIUM SULFATE IN THE SUGAR-AMMONIUM SULFATE BINDER BEING 1 TO 15 PERCENT, DRY BASIS.