Controllable penetration: Hot hid glue gels quickly, so it won't penetrate too far into porous materials like felt and harden it excessively.  Actually, hot hide glue has a two stage curing process.  First, it turns from a liquid to a gelatin as soon as it cools slightly.  Then over a longer period of time, it turns hard and rigid as the water content leaves the glue.

Sizing properties: Hot hide glue contains water, and this moisture causes bushing cloth to conform to shape when pressed into key mortises, flange bushings, etc. It also swells knurled hammer shanks for a tight fit in hammer bore holes.  When the glue later turns hard, the parts are locked into the desired shape.  Thus, precision key bushing cauls and hot hide glue yield bushings that require little or no easing.

Adjustable gelling time: The time between application of the liquid glue and when it gels can be adjusted by the addition of urea, available from fertilizer stores or from Pianotek Supply Company.  This allows you to tailor the working properties to the specific job and environment.  For instance, if the work area is cold, the glue may begin to gel before you've assembled the parts, bushing cloth, etc.

High strength: Hot hide glue makes wood joints stronger than the wood itself, and because it dries rigid it will not creep under load.

Indefinite shelf life of dried glue: As long as they are kept dry, dried glue crystals will last virtually forever, unlike liquid gludes which deteriorate badly with age.

Simple to mix and use: With the right glue pot, hot hide glue is a pleasure to use.  With the wrong setup, it can be a nuisance.

Make the world's best hot hide glue pot for under $20.00!
See http://spurlocktools.com/id57_hot_hide_glue.html