Testing liquid hide glue
Bill Tindall, E.Tn.
To make a strong bond the glue must be tough, not brittle. High molecular weight polymers are inherently tough because the molecules are extremely long and entangled. Hide glue is tough because the protein is high molecular weight. Degraded means the molecular weight has lessened, lessening entanglement, leading to brittle.
Dry a thin film and flex it. If brittle its dead.
To spin a thread from a polymer solution the polymer must be high molecular weight. Rub hide glue between finger and thumb, then separate and try to pull strands of "fiber". If you can't pull long strands the glue molecular weight had lessened. Test with a fresh bottle to get calibrated.