sleek:
" Excessive hide glue on a bows back tends to crackle when drawn. I doubt it will be beneficial to add more hide glue than needed to bond sinew on the back"
The question was targeting the ideal % of glue in the glue/sinew mix itself, given the difference % of shrinkage for glue and sinew: Yes, when resting above the sinew surface, glue will sometimes crack when highly stressed. But high-quality glue can stretch 10%, so cracking glue on a wood bow is likely just the bow giving its opinion of the glue's quality.
Your idea of glue-filled grooves in the belly surface is worth experiment with. The interface would have the average or both materials. More tests....
I wonder if there's any way to cause hide glue to run up the pores of ring-porous wood. Hot wood would keep it liquid, capillary action should work to some extent, slight suction on the other end might bring the glue all the way through. Seems a challenge, but a glue/wood hybrid belly has possibilities.
GlisGlis:
" I read they say to add vinegar to hide glue to preserve from mold and microorganism"
Thanks. I'll read up on that.