Yes, from animal hides, almost any animal but primarily from cattle hides. This is simply because of the abundance of cattle hides due to the enormous world wide consumption of beef and subsequent tanning of hides for leather. The trimmed hide pieces (too small or irregular shapes to provide useable leather) are shipped to the hide glue plant.
The glue manufacturing process is basically the following:
wash to remove dirt
soak in lime water for 60-90 days
wash to remove hair and lime
neutralize with acid, drain, wash & drain
add water, heat to 110-120 deg. F for 2-4 hours (called an extraction)
drain off the dilute glue solution, evaporate, chill, dry, grind
repeat last 2 steps 3-4 times to extract all of the glue with the temperature being increased 20-25 deg. F each time.
The process can lend itself to "home brewing" but it is messy and the aroma is found by some to be less than exciting! Your best bet is to buy the finished product from a reliable supplier.
Hide glue is a protein derived from the simple hydrolysis of collagen which is a principal protein constituent of animal hides. Collagen, hide glue and gelatin are very closely related as to protein and chemical composition. An approximate chemical composition for glue is:
Total 100 %
The molecular weight of hide glue has a wide range from 20,000 - 250,000. The higher the gel strength, the higher the molecular weight.