Sprinkle yeast, sugar and salt over water in large bowl; let stand until dissolved. Stir to blend. Let stand until foamy and proofed, abut 10 minutes.
Add 5 cups flour to yeast mixture and stir to incorporate. Stir in another 1 cup flour. Turn dough out onto generously floured surface. Knead in remaining 1 cup flour. Continue kneading until dough is very smooth and elastic, about 10 minutes.
Butter large bowl. Add dough, turning to coat entire surface. Cover and let rise in warm draft-free area until doubled in volume, about 1 hour.
Punch dough down. Let stand 15 minutes at room temperature.
Generously butter four 18-inch baguette pans. Knead dough 3 to 4 times. Divide evenly into 4 pieces. Roll 1 piece into 7x15-inch oblong. Roll up as for jelly roll. Set seam side down in 1 prepared pan. Repeat with remaining pieces. Using razor blade, make three to four 3/8-inch-deep slashes atop each loaf. Brush each with glaze. Let stand in warm draft-free area until dough rises to top of pans, about 1 hour.
Preheat oven to 450 F. Bake loaves 15 minutes. Reduce oven temperature to 350 F. Continue baking until bread sounds hollow when tapped on bottom, about 30 minutes. Remove from pans. Serve immediately. (Can be prepared 3 months ahead. Cool completely. Wrap in foil and freeze. Rewarm in 350 F oven for about 20 minutes. )