Here is a bit of Information on Hides Glue, it's different Grades, and the Properties of Hide Glue
Hide Glue is graded on a basis of its gel strength, a measure of how many grams of force it requires to depress a 1/2” plunger 4mm. into a 12.5% protein solution of the glue at 10° C. Glue is manufactured in standard grades from 32 to 512 grams. 192 gram strength is the most commonly used for woodworking; 251 is the highest normally used for instrument building; 135 is the lowest used for general woodwork.
The higher the gram strength, the stronger the cured glue, and the shorter the working or gel time. The lowest grades are considered strong enough for woodworking: “stronger than the wood itself.” (Personally, I don’t think any glue deserves that sort of comment. It may be that strong only for certain tests; it is not the same as the wood itself.) It is never a good idea to dilute too strong a glue to obtain lower viscosity or longer working time when it is possible to use a lower grade of glue. Glues of different gram strengths may be mixed to get an intermediate. I like 192 gram “high clarity” because it’s transparent and doesn't gel too fast.
Now, here's an important note. The higher gram strength glues have higher molecular weight, which (I'm guessing here) may have slightly lower electrochemical adhesion, i.e., they may not stick as well. Because the very high gram strength glues require up to twice the water to reach workable consistency, there will be less glue actually in the joint after it is clamped and the water evaporates. This may be why the very high strength glues (300g. to 500g.) are often described in the literature as "too strong for woodworking." Additionally, they gel too fast for many applications.
Commercial liquid hide glues have gel suppressants & remain liquid at room temperature. They are not quite as useful for most work: slightly greater tendency for the joint to “creep,” lower moisture & heat resistance, short shelf life, no initial tack. Many Bowyers use liquid hide glue, but they are (or should be) careful to test each batch, in addition to watching the expiration date printed on the bottle.
When a hide glue joint is clamped, thick liquid glue should squeeze out all around. If the squeeze-out is “crumbly” when rubbed with a finger, or has the texture of cottage cheese, the joint may not have been clamped in time. Excess glue should be washed off finished surfaces to avoid chipping the finish as the glue dries and shrinks.
Hide glue cures entirely by evaporation. Hide glue is about 2/3 water, and it is important to pay attention to the amount of water absorbed into the work piece. Maple necks can take on quite a “back bow” as a result of water absorption. Joints will swell when glued and must be allowed to dry completely before leveling to avoid “sunken” areas around the glue joint. Old hide glue joints can be restored by adding more hot hide glue. The heat and moisture will reactivate the old glue on the joint surfaces, and the new joint may be nearly as strong as the original.
Hide glue sticks to surfaces by electrochemical attraction, or specific adhesion. Mechanical bonds, like little “fingers,” may help modern adhesives such as epoxy because of their very high cohesive strength, but with hide glue’s low cohesive strength, roughening joint surfaces will not help adhesion. It is better to be concerned about the electrochemical properties of the wood.
