To slow the set of hot hide glue, mix in urea which comes in the form
of pellets and should be available from your hot glue crystals supplier.

Regards,
Jon Page

 [ See, for example, this tech note at Pianotek Supply Co.:
 [ http://www.pianoteksupply.com/assets/pdf/instructions/Hide%20Glue%20Technical.pdf 
 [ -- Robbie